PS/2 connector MiniDIN-6 Connector

The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 mouse connector generally replaced the older DE-9 RS-232 "serial mouse" connector, while the PS/2 keyboard connector replaced the larger 5-pin/180° DIN connector used in the IBM PC/AT design. The PS/2 designs on keyboard and mouse interfaces are electrically similar and employ the same communication protocol. However, a given system's keyboard and mouse port may not be interchangeable since the two devices use a different set of commands.

Contents

1. Port availability o 1.1 Legacy port status and USB

2. 2 Color code

3. 3 Software issues

4. 4 Hardware issues

o 4.1 Hotplugging

o 4.2 Durability

o 4.3 Fault isolation

5. 5 See also

6. 6 References

7. 7 External links

The color-coded PS/2 connection ports (purple for keyboards

and green for mice)

Type Keyboard and computer mouse data connector

Data

Data signal Serial data at 10 to 16 kHz with 1 stop bit, 1

start bit, 1 parity bit

File:MiniDIN-6 Connector Pinout.svg

Female connector from the front

Pin 1 +DATA Data

Pin 2 Not connected Not connected*

Pin 3 GND Ground

Pin 4 Vcc +5 V DC at 275 mA

Pin 5 +CLK Clock

Pin 6 Not connected Not connected**

* On some computers mouse data for splitter cable

Port availability

When IBM-compatible PCs widely used the Intel 80386 and 486 processors, the connectors were also seen on some PC clones with non-standard case designs, and the PS/2 mouse connector was sometimes seen on a separate backplate on systems using a standard AT case. However, PS/2 ports only became the norm much later with the introduction of the ATX form factor during 1993–97. The design decision for identical but incompatible connectors would prove aggravating to consumers; manufacturers would later adopt a standardized color code.

Old laptops generally have a single port that supports either a keyboard or a mouse. Sometimes the port also allows one of the devices to be connected to the two normally unused pins in the connector to allow both to be connected at once through a special splitter cable.[1] This configuration is common on IBM/Lenovo Thinkpad notebooks among many others.

The PS/2 keyboard interface was electrically the same as the 5-pin AT system, and keyboards designed for one can be connected to the other with a simple wiring adapter. The PS/2 mouse interface is substantially different from RS-232 (which was generally used for mice on PCs without PS/2 ports), but nonetheless many mice were made that could operate on both with a simple wiring adapter.

PS/2 mouse and keyboard connectors have also been used in non-IBM PC-compatible computer systems, such as the DEC AlphaStation line, early IBM RS/6000 CHRP machines and SGI Indy, Indigo 2, and newer (Octane etc.) computers.[2] Various Macintosh clone computers from the late 90s featured PS/2 mouse and keyboard ports, including the Motorola StarMax and the Power Computing PowerBase[3]

[edit] Legacy port status and USB

PS/2 was considered a legacy port by the Intel/Microsoft PC 2001 specification of 2000, preferring to connect keyboards and mice via USB ports. Despite this, PS/2 ports are included on

most new motherboards.[4] These PS/2 ports cause fewer problems when KVM switching with non-Wintel systems.[citation needed] PS/2 ports may also be favored for security reasons in a corporate environment. Use of PS/2 ports for keyboard and mouse connectivity would allow USB ports to be totally disabled, preventing the connection of any USB removable disks. Also with high-end keyboards only way to provide full n-key rollover is by using PS/2, as USB has cap of 6 keys plus modifiers at the same time.[5]

Many keyboards and mice can connect via either USB or PS/2, selecting the appropriate protocol at power-on. Such devices are generally equipped with a USB connector, and ship with a simple wiring adapter to allow connection to a PS/2 port. Older PS/2-only peripherals can be connected to a USB port via an active adapter, which generally provides a pair of PS/2 ports at the cost of one USB port.

[edit] Color code

Original PS/2 connectors were black or had the same color as the connecting cable (mainly white). Later the PC 97 standard introduced a color code: the keyboard port, and the plugs on compliant keyboards, were purple; mouse ports and plugs were green. Some vendors initially used a different color code: Logitech used the color orange for the keyboard connector for a short period, but soon switched to purple.[citation needed] Today this code is still used on most PCs. The pinouts of the connectors are the same, but most computers will not recognize devices connected to the wrong port.

Color Description  Purple Keyboard  Green Mouse

Wiring inside keyboard cable varies widely. Here are some more common color codes, but the reader is cautioned that the only reliable method of determining color assignment is to confirm by measuring continuity to the connector.

Description Common Alternate Alternate Alternate Alternate Alternate Alternate+CLK   Green   Blue   White   Yellow   White   Blue   YellowData   White   Yellow   Green   Red   Green   Yellow   GrayGND   Yellow   Black   Orange   Gray   Black   White   BlackVcc   Red   Red   Blue   Brown   Red   Orange   Red

Software issues

As of 2010, version 8.0 of Microsoft's keyboard and mice drivers no longer supports PS/2 (even with USB adapters) in its supported keyboards and supported mice.[citation needed]

Hotplugging

PS/2 ports are designed to connect the digital I/O lines of the microcontroller in the external device directly to the digital lines of the microcontroller on the motherboard. They are not designed to be hot swappable. Hot swapping PS/2 devices usually does not cause damage because more modern microcontrollers tend to have more robust I/O lines built into them which are harder to damage than those of older controllers; however, hot swapping can still potentially cause damage on older machines, or machines with less robust port implementations.

If they are hot swapped, the devices must be similar enough that the driver running on the host system recognizes, and can be used with the new device. Otherwise, the new device will not function properly. While this is seldom an issue with standard keyboard devices, the host system rarely recognizes the new device attached to the PS/2 mouse port. In practice most keyboards can be hot swapped but this should be avoided.

[edit] Durability

PS/2 connectors are not designed to be plugged in and out very often, which can easily lead to bent or broken pins. PS/2 connectors only insert in one direction and must be rotated correctly before attempting connection. Most but not all connectors include an arrow or flat section which is usually aligned to the right or top of the jack before being plugged in. The exact direction may vary on older or non-ATX computers and care should be taken to avoid damage or bent pins when connecting devices. This issue is slightly alleviated in modern times with the advent of the PS/2 to USB adapter. Users can just leave the PS/2 plugged into the USB at all times and not risk damaging the pins this way.

[edit] Fault isolation

As noted, in a standard implementation both PS/2 ports are usually controlled by a single microcontroller on the motherboard. This makes design and manufacturing extremely simple and cheap. However, a rare side effect of this design is that a malfunctioning device can cause the controller to become confused, resulting in "both" devices acting erratically. The resulting problems can be difficult to troubleshoot (e.g. a bad mouse can cause problems that appear to be the fault of the keyboard).

A way to isolate the problem is to use a USB keyboard or mouse to determine which of the devices is at fault. A USB to PS/2 adapter can also be used to connect either of the devices to a USB port but the result can be undetermined because PS/2 and USB ports can use different voltages. Quality USB to PS/2 adapters have an integrated circuit that compensates the PS/2 - USB port voltage differences. Due to this difference it is recommend to avoid cheap adapters or try using a USB Keyboard or Mouse when troubleshooting PS/2 related errors.

D-subminiatureFrom Wikipedia, the free encyclopedia

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DA, DB, DC, DD, and DE sized connectors

The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems.[citation needed]

Contents

1 Description, nomenclature, and variants 2 Typical applications

o 2.1 Communications ports

o 2.2 Network ports

o 2.3 Computer video output

o 2.4 Game controller ports

o 2.5 Other

3 Wire-contact attachment types

4 Usage

5 See also

6 References

7 External links

[edit] Description, nomenclature, and variants

The DB13W3 Connector with 3 coaxial connections and ten ordinary pins

A diagram of a DE9 plug

Male 13W3 Plug

DE-15F, used for VGA, SVGA and XGA ports

A D-sub contains two or more parallel rows of pins or sockets usually surrounded by a D-shaped metal shield that provides mechanical support, ensures correct orientation, and may screen against electromagnetic interference. The part containing pin contacts is called the male connector or plug, while that containing socket contacts is called the female connector or socket. The socket's shield fits tightly inside the plug's shield. The plug also may have screws on either side of the shield that fasten into holes in the socket (although sometimes the screws are on the socket: see the DE9 pictured to the left). When screened cables are used, the shields are connected to the overall screens of the cables. This creates an electrically continuous screen covering the whole cable and connector system.

The D-sub series of connectors was invented by ITT Cannon, part of ITT Corporation, in 1952.[1]

Cannon's part-numbering system uses D as the prefix for the whole series, followed by one of A, B, C, D, or E denoting the shell size, followed by the number of pins or sockets, followed by either P (plug) or S (socket) denoting the gender of the part. Each shell size usually (see below for exceptions) corresponds to a certain number of pins or sockets: A with 15, B with 25, C with 37, D with 50, and E with 9.[2] For example, DB25 denotes a D-sub with a 25-position shell size and a 25-position contact configuration. The contacts in either row of these connectors are spaced 326/3000 of an inch apart, or approximately 0.109 inches (2.77 mm), and the rows are spaced 0.112 inches (2.84 mm) apart (the pins in the two rows are offset by half the distance between adjacent contacts in a row).[3] The suffixes M and F (for male and female) are sometimes used instead of the original P and S.

This naming pattern is not always followed, however. Because personal computers first used DB25 connectors for their serial and parallel ports, when the PC serial port began to use 9-pin connectors, they were often labeled as DB9 instead of DE9 connectors, due to an ignorance of the fact that B represented a shell size. It is now common to see DE9 connectors sold as DB9 connectors. DB9 nearly always refers to a 9-pin connector with an E size shell. The non-standard 23-pin D-sub connectors for external floppy drives and video output on most of the Amiga computers are usually labeled DB23, even though their shell size is two pins smaller than ordinary DB sockets.

There are now D-sub connectors which have the original shell sizes, but more pins, and their names follow the same pattern. For example, the DE15, usually found in VGA cables, has 15 pins in three rows, all surrounded by an E size shell. The pins are spaced at 0.090 inches (2.3 mm) horizontally and 0.078 inches (2.0 mm) vertically.[3]) The other connectors with the same pin spacing are the DE15, DA26, DB44, DC62, and DD78. Reflecting the same confusion of the letters DB with just D as mentioned above, these connectors are also often called DB15HD (or even DB15 or HD15), DB26HD, DB44HD, DB62HD, and DB78HD connectors, respectively, where HD stands for "high density". They all have three rows of pins, except the DD78, which has four. The "double density" series of D-sub connectors features even denser arrangements and consists of the DE19, DA31, DB52, DC79, and DD100. These each have four rows of pins.

Cannon also produced "hybrid" D-subs with larger contacts in place of some of the normal contacts that could be used for high-current, high-voltage, or co-axial inserts. The DB13W3 variant was commonly used for high-performance video connections; this variant provided 10 regular (#20) pins plus three coaxial contacts for the red, green, and blue video signals. Hybrid D-subs are currently being manufactured in a broad range of configurations by other companies, including Amphenol, Conec, Teledyne Reynolds, Assmann Electronics, Norcomp, Cinch, 3M, and Tyco. Some variants have current ratings up to 40A or operating voltages as high as 13,500V; others are waterproof and meet IP67 standards.

A smaller type of connector derived from the D-sub is called the microminiature D, or micro-D, which is a trademark of ITT Cannon. It is about half the length of a D-sub.

There is yet another similar family of connectors that is easy to confuse with the D-sub family. These connectors have names like HD50 and HD68, and have a D-shaped shell that is about half the width of a DB25. They are common in SCSI attachments.

The original D-sub connectors are now defined by an international standard, IEC 60807-3 / DIN 41652. The United States military also maintains another specification for D-subminiature connectors, the MIL-DTL-24308 standard.[3]

[edit] Typical applications

D-sub connectors.The connector on the left is a 9-pin male (DE9M) connector plug, and the one on the right is a 25-pin female (DB25F) socket. The hexagonal pillars (4-40 bolt) at either end of each connector have a threaded stud (not visible) that passes through flanges on the connector, fastening it to the metal panel. They also have a threaded hole that receives the jackscrews on the cable shell, to hold the plug and socket together.

[edit] Communications ports

The widest application of D-subs is for RS-232 serial communications, though the standard did not make this connector mandatory. RS-232 devices originally used the DB25, but for many applications the less common signals were omitted, allowing a DE9 to be used. The standard

specifies a male connector for terminal equipment and a female connector for modems, but many variations exist. IBM PC-compatible computers tend to have male connectors at the device and female connectors at the modems. Early Apple Macintosh models used DE9 connectors for RS-422 serial interfaces (which can operate as RS-232). Later Macintosh models use 8-pin miniature DIN connectors instead.

On PCs, 25-pin and (beginning with the IBM-PC/AT) 9-pin plugs are used for the RS-232 serial ports; 25-pin sockets are used for the parallel printer ports (instead of the Centronics socket found on the printer itself).

Many uninterruptible power supply units have a DE9F connector on them in order to signal to the attached computer via an RS-232 interface. Often these do not send data serially to the computer but instead use the handshaking control lines to indicate low battery, power failure, or other conditions. Such usage is not standardized between manufacturers and may require special cables.

[edit] Network ports

DE9 connectors were used for some token ring networks as well as other computer networks.

The Attachment Unit Interfaces that were used with 10BASE5 "thick net" in the 1980s and 1990s used DA15 connectors for connectivity between the Medium Attachment Units and (Ethernet) network interface cards, albeit with a sliding latch to lock the connectors together instead of the usual hex studs with threaded holes. (The sliding latch was intended to be quicker to engage and disengage and to work in places where jack screws could not be used for reasons of component shape.

[edit] Computer video output

A male DE9 connector.

A female 9-pin connector on an IBM compatible personal computer may be a video display output such as MDA, Hercules, CGA, or EGA (rarely VGA or others). Even though these all use the same DE9 connector, the displays cannot all be interchanged and monitors or video

interfaces may even be damaged if connected to an incompatible device using the same connector.

Later analog video (VGA and later) adapters generally replaced these connectors with DE15 high-density sockets (though some early VGA devices still used DE9 connectors). DE15 connectors are similar to DE9 connectors (see above).

Many Apple Macintosh models (beginning with the Macintosh II) used DA15 sockets for analogue RGB video out. Just prior to this, the Apple IIgs used the same connector for the same purpose, but in a non-compatible way. A digital (and thus also incompatible) RGB adapter for the Apple IIe also used a DA15F. And the Apple IIc used a DA15F for an auxiliary video port which was not RGB, but provided the necessary signals to derive RGB.

[edit] Game controller ports

Starting in the late 1970s the Atari 2600 game console used DE9 connectors without the pair of fastening screws (male on the system, female on the controller) for its game controller connectors. In the years following, various video game consoles and home computers adopted the connector for their own game ports, though they were not all interoperable. The common wirings supported digital connections for up, down, left, and right buttons along with one or two others. Some systems supported connecting a pair of analog potentiometers, or paddles, and on some computers a computer mouse or a light pen was also supported via the game port. Like joysticks, these devices were not typically interchangeable between different systems.

Systems utilizing the DE9 connector for their game port included the Atari 8-bit and ST lines; the Commodore VIC-20, 64, 128, and Amiga; the Amstrad CPC (which employed daisy-chaining when connecting two Amstrad-specific joysticks); the MSX, Sharp X68000, and FM-Towns, predominantly used in Japan; the Sega Master System and Sega Genesis; and the Panasonic 3DO. The Sinclair ZX Spectrum lacked a built-in joystick connector of any kind but aftermarket interfaces provided the ability to connect DE9 joysticks.

Many Apple II computers also used DE9 connectors for joysticks, but they had a female port on the computer and a male on the controller, used analog rather than digital sticks, and the pin-out was completely unlike that used on the aforementioned systems. DE9 connectors were not used for game ports on the Apple Macintosh, Apple III, IBM PC systems, or most newer game consoles.

DA15S connectors are used for PC joystick connectors, where each DA15 connector supports two joysticks each with two analog axes and two buttons. In other words, one DA15S "game adapter" connector has 4 analog potentiometer inputs and 4 digital switch inputs. This interface is strictly input-only, though it does provide +5V DC power. Some joysticks with more than two axes and/or more than two buttons use the signals designated for both joysticks. Conversely, Y-adapter cables are available that allow two separate joysticks to be connected to a single DA15 game adapter port; if a joystick connected to one of these Y-adapters has more than two axes or buttons, only the first two of each will work.

The IBM DA15 PC game connector has been modified to add a (usually MPU-401 compatible) MIDI interface, and this is often implemented in the game connectors on third-party sound cards, for example the Sound Blaster line from Creative Labs. The "standard" straight game adapter connector (introduced by IBM) has three ground pins and four +5V power pins, and the MIDI adaptation replaces one of the grounds and one of the +5V pins, both on the bottom row of pins, with MIDI In and MIDI Out signal pins. (There is no MIDI Thru provided.) Creative Labs introduced this adaptation.[citation needed]

[edit] Other

25-pin sockets on Macintosh computers are typically SCSI connectors (again in contrast to the Centronics C50 connector typically found on the peripheral), while older Sun hardware uses DD50 connectors for FastSCSI equipment.

The complete range of D-sub connectors also includes DA15s (one row of 7 and one of 8), DC37s (one row of 18 and one of 19), and DD50s (two rows of 17 and one of 16); these are often used in industrial products, the 15-way version being commonly used on rotary and linear encoders.

The early Macintosh and late Apple II computers used an obscure 19-pin D-sub for connecting to external floppy disk drives. The Commodore Amiga used an equally unusual 23-pin version for both its video output and connection to an external floppy disk drive.

TASCAM used DB25 connectors for their multi-track recording audio equipment (TDIF), and Logitek Audio later did the same for its broadcast consoles, though with different pinouts.[4] Roland used DB25 connectors for their multi-track recording audio equipment (R-BUS). A few patch panels have been made which have the DB25 connectors on the back with phone jacks (or even TRS jacks) on the front, however these are normally wired for TASCAM, which is more common outside of broadcasting.

In broadcast and professional video, "parallel digital" is a digital video interface that uses DB25 connectors, per the SMPTE 274M specification adopted in the late 1990s. The more common SMPTE 259M "serial digital interface" (SDI) uses BNC connectors for digital video signal transfer.

[edit] Wire-contact attachment types

A male PCB-mounting DD50 connector

There are at least five different methods used to attach wires to the contacts in D-sub connectors.

Solder-bucket (or solder-cup) contacts have a cavity into which the stripped wire is inserted and hand-soldered.

Insulation displacement contacts (IDCs) allow a ribbon cable to be forced onto sharp tines on the back of the contacts; this action pierces the insulation of all the wires simultaneously. This is a very quick means of assembly whether done by hand or automatically.

Crimp contacts are assembled by inserting a stripped wire end into a cavity in the rear of the contact, then crushing the cavity using a crimp tool, causing the cavity to grip the wire tightly at many points. The crimped contact is then inserted into the connector where it locks into place. Individual crimped pins can be removed later by inserting a special tool into the rear of the connector.

PCB pins are soldered directly to a printed circuit board and not to a wire. These connectors are frequently mounted at a right angle to the PCB, allowing a cable to be plugged into the edge of the PCB assembly.

Wire wrap connections are made by wrapping solid wire around a square post with a wire wrap tool. This type of connection is usually used in prototyping.

[edit] Usage

The 25-pin D-sub connector is occasionally used in recording studios for multi-channel analog audio and AES digital audio.

The D-sub connector family is now in decline for general usage in the computer industry, due to size and cost. For portable devices such as PDAs, MP3 players or mobile phones, the D-sub connector is usually too large to fit. In the laptop computer sector, where weight and size are crucial, many models no longer include D-subs. Even small form factor desktop PCs may find D-sub connectors too large for their value.

Because of its relative complexity (the D-shaped metal shield, the screws and nuts), D-sub connectors are now quite expensive compared to other, mostly simpler, common connectors. In the retail PC world where margins are very thin, these connectors are a natural target for removal.

The physical design of D-sub connectors is ill-suited for consumer plug-and-play applications. Thin metal pins, especially in higher-density connectors, are easily bent or broken, particularly if frequently plugged in by touch behind equipment. The need to tighten screws for a secure connection is cumbersome. Although ESD- and EMI-resistant D-sub connectors exist, the fundamental design was never intended to protect from electrostatic discharge or electromagnetic interference or facilitate very high frequency interconnections.[citation needed]

For video purposes, the DE15HD connector is in the process of being replaced by DVI and HDMI connectors. A notable exception to this replacement is on the many analog CRT monitors still in use: the analog version of the DVI connector is similar in price and more complex than the D-sub, so the shift away from D-subs is slow in this case. For the majority of other consumer applications, D-sub serial and parallel connectors have been replaced by the physically much simpler and cheaper IEEE 1394 (FireWire), SATA, USB, or modular connectors.

D-sub 9 Connector Pinout

Pinout and diagram of DE9 connector (DB9 connector), commonly used for serial ports (RS-232).

Pin SIG. Signal Name DTE (PC)

1 DCD Data Carrier Detect in

2 RXD Receive Data in

3 TXD Transmit Data out

4 DTR Data Terminal Ready out

5 GND Signal Ground -

6 DSR Data Set Ready in

7 RTS Request to Send out

8 CTS Clear to Send in

9 RI Ring Indicator in

The DTE (PC) has the male connector (shown below), and the DCE (peripheral) has the female.

Related:

Cmd-line tool to transport files over an unreliable serial link AVR microcontroller C library with UART module for serial comms

Modified Modular Jack (MMJ)From Wikipedia, the free encyclopedia

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Modified Modular Jack

MMJ socket and plug

Type RS-423 serial

Production history

Designer Digital Equipment Corporation

Pin out

Pin 1 Data terminal ready DTR

Pin 2 Transmit data Tx+

Pin 3 Transmit data ground Tx-

Pin 4 Receive data ground Rx-

Pin 5 Receive data Rx+

Pin 6 Data set ready DSR

The DECconnect Modified Modular Jack (MMJ) is a variation of the 6P6C modular connector and was developed by Digital Equipment Corporation. The main difference from the

conventional modular connector is that the hook is toward the side instead of the center of the plug, making it impossible to insert a standard modular cable plug such as those used telephone or Ethernet applications. MMJ connectors are used on Digital minicomputers, such as the PDP-11, VAX and Alpha based machines and to connect terminals, printers or serial console servers.

The six conductors of the DECconnect MMJ connector are used for the main signals in RS-423 serial communication: Tx and Rx for the data transmission and DSR and DTR for handshaking. The transmit and receive signals are differential, i.e. each signal is the voltage difference between two conductors, as opposed to a voltage on a single connector relative to a common reference. But one can in practice wire these signals to an RS-232 device, which uses the common reference method, by combining the lower voltage sides of each signal to the RS-232 signal ground line. For this type of connection there are a number of models of conversion adapter.

When connecting two DTE devices such as a computer and a printer, the Digital BC16E crossover cable is used.

System consoleFrom Wikipedia, the free encyclopedia

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Knoppix system console showing the boot process

The system console, root console or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the init system and from the system logger. It is a physical device consisting of a keyboard and a screen.

On traditional minicomputers, the console was a serial console, an RS-232 serial link to a terminal such as a DEC VT100. This terminal was usually kept in a secured room since it could be used for certain privileged functions such as halting the system or selecting which media to

boot from. Large midrange systems, e.g. those from Sun Microsystems, Hewlett-Packard and IBM, still use serial consoles. In larger installations, the console ports are attached to multiplexers or network-connected multiport serial servers that let an operator connect a terminal to any of the attached servers. Today, serial consoles are often used for accessing headless systems, usually with a terminal emulator running on a laptop. Also, routers, enterprise network switches and other telecommunication equipment have RS-232 serial console ports.

On PCs and workstations, the computer's attached keyboard and monitor have the equivalent function. Since the monitor cable carries video signals, it cannot be extended very far. Often, installations with many servers therefore use keyboard/video multiplexers (KVM switches) and possibly video amplifiers to centralize console access. In recent years, KVM/IP devices have become available that allow a remote computer to view the video output and send keyboard input via any TCP/IP network and therefore the Internet.

Some PC BIOSes, especially in servers, also support serial consoles, giving access to the BIOS through a serial port so that the simpler and cheaper serial console infrastructure can be used. Even where BIOS support is lacking, some operating systems, e.g. FreeBSD and Linux, can be configured for serial console operation either during bootup, or after startup.

It is usually possible to log in from the console. Depending on configuration, the operating system may treat a login session from the console as being more trustworthy than a login session from other sources.

RS-232From Wikipedia, the free encyclopedia

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This article is about the RS-232 standard. For RS-232 variants, see serial port.

A 25 pin connector as described in the RS-232 standard

In telecommunications, RS-232 (Recommended Standard 232) is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pin out of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997.

An RS-232 port was once a standard feature of a personal computer for connections to modems, printers, mice, data storage, un-interruptible power supplies, and other peripheral devices. However, the limited transmission speed, relatively large voltage swing, and large standard connectors motivated development of the universal serial bus which has displaced RS-232 from most of its peripheral interface roles. Many modern personal computers have no RS-232 ports and must use an external converter to connect to older peripherals. Some RS-232 devices are still found especially in industrial machines or scientific instruments.

Contents

1 Scope of the standard 2 History

3 Limitations of the standard

4 Role in modern personal computers

5 Standard details

o 5.1 Voltage levels

o 5.2 Connectors

o 5.3 Pinouts

o 5.4 Cables

6 Conventions

o 6.1 RTS/CTS handshaking

o 6.2 3-wire and 5-wire RS-232

7 Seldom used features

o 7.1 Signal rate selection

o 7.2 Loopback testing

o 7.3 Timing signals

o 7.4 Secondary channel

8 Related standards

9 Development tools

10 References

[edit] Scope of the standard

The Electronic Industries Association (EIA) standard RS-232-C[1] as of 1969 defines:

Electrical signal characteristics such as voltage levels, signaling rate, timing and slew-rate of signals, voltage withstand level, short-circuit behavior, and maximum load capacitance.

Interface mechanical characteristics, pluggable connectors and pin identification.

Functions of each circuit in the interface connector.

Standard subsets of interface circuits for selected telecom applications.

The standard does not define such elements as

character encoding (for example, ASCII, Baudot code or EBCDIC) the framing of characters in the data stream (bits per character, start/stop bits, parity)

protocols for error detection or algorithms for data compression

bit rates for transmission, although the standard says it is intended for bit rates lower than 20,000 bits per second. Many modern devices support speeds of 115,200 bit/s and above

power supply to external devices.

Details of character format and transmission bit rate are controlled by the serial port hardware, often a single integrated circuit called a UART that converts data from parallel to asynchronous start-stop serial form. Details of voltage levels, slew rate, and short-circuit behavior are typically controlled by a line driver that converts from the UART's logic levels to RS-232 compatible signal levels, and a receiver that converts from RS-232 compatible signal levels to the UART's logic levels.

[edit] History

RS-232 was first introduced in 1962.[2] The original DTEs were electromechanical teletypewriters, and the original DCEs were (usually) modems. When electronic terminals (smart and dumb) began to be used, they were often designed to be interchangeable with teletypes, and so supported RS-232. The C revision of the standard was issued in 1969 in part to accommodate the electrical characteristics of these devices.[citation needed]

Since application to devices such as computers, printers, test instruments, and so on was not considered by the standard, designers implementing an RS-232 compatible interface on their equipment often interpreted the requirements idiosyncratically. Common problems were non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of breakout boxes, patch boxes, test equipment, books, and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at a reduced voltage. Some manufacturers therefore built transmitters that supplied +5 V and -5 V and labeled them as "RS-232 compatible".[citation needed]

Later personal computers (and other devices) started to make use of the standard so that they could connect to existing equipment. For many years, an RS-232-compatible port was a standard feature for serial communications, such as modem connections, on many computers. It remained in widespread use into the late 1990s. In personal computer peripherals, it has largely been supplanted by other interface standards, such as USB. RS-232 is still used to connect older designs of peripherals, industrial equipment (such as PLCs), console ports, and special purpose equipment, such as a cash drawer for a cash register.[citation needed]

The standard has been renamed several times during its history as the sponsoring organization changed its name, and has been variously known as EIA RS-232, EIA 232, and most recently as TIA 232. The standard continued to be revised and updated by the Electronic Industries Alliance and since 1988 by the Telecommunications Industry Association (TIA).[3] Revision C was issued in a document dated August 1969. Revision D was issued in 1986. The current revision is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997. Changes since Revision C have been in timing and details intended to improve harmonization with the CCITT standard V.24, but equipment built to the current standard will interoperate with older versions.[citation needed]

Related ITU-T standards include V.24 (circuit identification) and V.28 (signal voltage and timing characteristics).[citation needed]

[edit] Limitations of the standard

Because the application of RS-232 has extended far beyond the original purpose of interconnecting a terminal with a modem, successor standards have been developed to address the limitations. Issues with the RS-232 standard include:[4]

The large voltage swings and requirement for positive and negative supplies increases power consumption of the interface and complicates power supply design. The voltage swing requirement also limits the upper speed of a compatible interface.

Single-ended signaling referred to a common signal ground limits the noise immunity and transmission distance.

Multi-drop connection among more than two devices is not defined. While multi-drop "work-arounds" have been devised, they have limitations in speed and compatibility.

Asymmetrical definitions of the two ends of the link make the assignment of the role of a newly developed device problematic; the designer must decide on either a DTE-like or DCE-like interface and which connector pin assignments to use.

The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices.

No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low power devices such as mice.

The 25-way connector recommended in the standard is large compared to current practice.

[edit] Role in modern personal computers

PCI Express x1 card with one RS-232 port

Main article: Serial port

In the book, PC 97 Hardware Design Guide,[5] Microsoft deprecated support for the RS-232 compatible serial port of the original IBM PC design. Today, RS-232 has mostly been replaced in personal computers by USB for local communications. Compared with RS-232, USB is faster, uses lower voltages, and has connectors that are simpler to connect and use. However, USB is limited by standard to no more than 5 meters of cable, thus favoring RS-232 when longer distances are needed. Both standards have software support in popular operating systems. USB is designed to make it easy for device drivers to communicate with hardware. However, there is no direct analog to the terminal programs used to let users communicate directly with serial ports. USB is more complex than the RS-232 standard because it includes a protocol for transferring data to devices. This requires more software to support the protocol used. RS-232 only standardizes the voltage of signals and the functions of the physical interface pins. Serial ports of personal computers are also sometimes used to directly control various hardware devices, such as relays or lamps, since the control lines of the interface can be easily manipulated by software. This isn't feasible with USB, which requires some form of receiver to decode the serial data.

As an alternative, USB docking ports are available which can provide connectors for a keyboard, mouse, one or more serial ports, and one or more parallel ports. Corresponding device drivers are required for each USB-connected device to allow programs to access these USB-connected devices as if they were the original directly connected peripherals. Devices that convert USB to RS-232 may not work with all software on all personal computers and may cause a reduction in bandwidth along with higher latency.

Personal computers may use a serial port to interface to devices such as uninterruptible power supplies. In some cases, serial data is not exchanged, but the control lines are used to signal conditions such as loss of power or low battery alarms.

Many fields (for example, laboratory automation, surveying) provide a continued demand for RS-232 I/O due to sustained use of very expensive but aging equipment. It is often far cheaper to continue to use RS-232 than it is to replace the equipment. Additionally, modern industrial

automation equipment, such as PLCs, VFDs, servo drives, and CNC equipment are programmable via RS-232. Some manufacturers have responded to this demand: Toshiba re-introduced the DE-9M connector on the Tecra laptop.

Serial ports with RS-232 are also commonly used to communicate to headless systems such as servers, where no monitor or keyboard is installed, during boot when operating system isn't running yet and therefore no network connection is possible. An RS-232 serial port can communicate to some embedded systems such as routers as an alternative to network mode of monitoring.

[edit] Standard details

In RS-232, user data is sent as a time-series of bits. Both synchronous and asynchronous transmissions are supported by the standard. In addition to the data circuits, the standard defines a number of control circuits used to manage the connection between the DTE and DCE. Each data or control circuit only operates in one direction, that is, signaling from a DTE to the attached DCE or the reverse. Since transmit data and receive data are separate circuits, the interface can operate in a full duplex manner, supporting concurrent data flow in both directions. The standard does not define character framing within the data stream, or character encoding.

[edit] Voltage levels

Diagrammatic oscilloscope trace of voltage levels for an uppercase ASCII "K" character (0x4b) with 1 start bit, 8 data bits, 1 stop bit

.

The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines. Valid signals are plus or minus 3 to 15 volts; the ±3 V range near zero volts is not a valid RS-232 level. The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the power supplies available within a device. RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts. The slew rate, or how fast the signal changes between levels, is also controlled.

For data transmission lines (TxD, RxD and their secondary channel equivalents) logic one is defined as a negative voltage, the signal condition is called marking, and has the functional

significance. Logic zero is positive and the signal condition is termed spacing. Control signals are logically inverted with respect to what one sees on the data transmission lines. When one of these signals is active, the voltage on the line will be between +3 to +15 volts. The inactive state for these signals is the opposite voltage condition, between −3 and −15 volts. Examples of control lines include request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR).

Because the voltage levels are higher than logic levels typically used by integrated circuits, special intervening driver circuits are required to translate logic levels. These also protect the device's internal circuitry from short circuits or transients that may appear on the RS-232 interface, and provide sufficient current to comply with the slew rate requirements for data transmission.

Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous ground loop. Use of a common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Balanced, differential, serial connections such as USB, RS-422 and RS-485 can tolerate larger ground voltage differences because of the differential signaling.[6]

Unused interface signals terminated to ground will have an undefined logic state. Where it is necessary to permanently set a control signal to a defined state, it must be connected to a voltage source that asserts the logic 1 or logic 0 level. Some devices provide test voltages on their interface connectors for this purpose.

[edit] Connectors

RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Communication Equipment (DCE); this defines at each device which wires will be sending and receiving each signal. The standard recommended but did not make mandatory the D-subminiature 25 pin connector. In general and according to the standard, terminals and computers have male connectors with DTE pin functions, and modems have female connectors with DCE pin functions. Other devices may have any combination of connector gender and pin definitions. Many terminals were manufactured with female terminals but were sold with a cable with male connectors at each end; the terminal with its cable satisfied the recommendations in the standard.

Presence of a 25 pin D-sub connector does not necessarily indicate an RS-232-C compliant interface. For example, on the original IBM PC, a male D-sub was an RS-232-C DTE port (with a non-standard current loop interface on reserved pins), but the female D-sub connector was used for a parallel Centronics printer port. Some personal computers put non-standard voltages or signals on some pins of their serial ports.

The standard specifies 20 different signal connections. Since most devices use only a few signals, smaller connectors can often be used.

[edit] Pinouts

The following table lists commonly used RS-232 signals and pin assignments.[7] See serial port for non-standard variations including the popular DE-9 connector.

Signal Origin DB-25 pin

Name Typical purpose Abbreviation DTE DCE

Data Terminal Ready

Indicates presence of DTE to DCE. DTR ● 20

Data Carrier Detect

DCE is connected to the telephone line. DCD ● 8

Data Set Ready

DCE is ready to receive commands or data. DSR ● 6

Ring Indicator

DCE has detected an incoming ring signal on the telephone line. RI ● 22

Request To Send

DTE requests the DCE prepare to receive data. RTS ● 4

Clear To Send

Indicates DCE is ready to accept data. CTS ● 5

Transmitted Data

Carries data from DTE to DCE. TxD ● 2

Received Data

Carries data from DCE to DTE. RxD ● 3

Common Ground

GND common 7

Protective Ground

PG common 1

The signals are named from the standpoint of the DTE. The ground signal is a common return for the other connections. The DB-25 connector includes a second "protective ground" on pin 1.

Data can be sent over a secondary channel (when implemented by the DTE and DCE devices), which is equivalent to the primary channel. Pin assignments are described in following table:

Signal Pin

Common Ground 7 (same as primary)

Secondary Transmitted Data (STD) 14

Secondary Received Data (SRD) 16

Secondary Request To Send (SRTS) 19

Secondary Clear To Send (SCTS) 13

Secondary Carrier Detect (SDCD) 12

[edit] Cables

Main article: Serial cable

The standard does not define a maximum cable length but instead defines the maximum capacitance that a compliant drive circuit must tolerate. A widely used rule of thumb indicates that cables more than 50 feet (15 metres) long will have too much capacitance, unless special cables are used. By using low-capacitance cables, full speed communication can be maintained over larger distances up to about 1,000 feet.[8] For longer distances, other signal standards are better suited to maintain high speed.

Since the standard definitions are not always correctly applied, it is often necessary to consult documentation, test connections with a breakout box, or use trial and error to find a cable that works when interconnecting two devices. Connecting a fully standard-compliant DCE device and DTE device would use a cable that connects identical pin numbers in each connector (a so-called "straight cable"). "Gender changers" are available to solve gender mismatches between cables and connectors. Connecting devices with different types of connectors requires a cable that connects the corresponding pins according to the table above. Cables with 9 pins on one end and 25 on the other are common. Manufacturers of equipment with 8P8C connectors usually provide a cable with either a DB-25 or DE-9 connector (or sometimes interchangeable connectors so they can work with multiple devices). Poor-quality cables can cause false signals by crosstalk between data and control lines (such as Ring Indicator). If a given cable will not allow a data connection, especially if a Gender changer is in use, a Null modem may be necessary.

[edit] Conventions

For functional communication through a serial port interface, conventions of bit rate, character framing, communications protocol, character encoding, data compression, and error detection, not defined in RS 232, must be agreed to by both sending and receiving equipment. For example, consider the serial ports of the original IBM PC. This implementation used an 8250 UART using asynchronous start-stop character formatting with 7 or 8 data bits per frame, usually ASCII character coding, and data rates programmable between 75 bits per second and 115,200 bits per second. Data rates above 20,000 bits per second are out of the scope of the standard, although higher data rates are sometimes used by commercially manufactured equipment. Since most RS-232 devices do not have automatic baud rate detection, users must manually set the baud rate (and all other parameters) at both ends of the RS-232 connection.

In the particular case of the IBM PC, as with most UART chips including the 8250 UART used by the IBM PC, baud rates were programmable with arbitrary values. This allowed a PC to be connected to devices not using the rates typically used with modems. Note that due to the clock frequency which IBM chose to feed the 8250 UART in the PC, some popular baud rates are

unavailable due to the granularity of the baud rate setting. This includes the baud rate of MIDI, 31,250 bits per second, which is generally not achievable by a standard IBM PC serial port. MIDI-to-RS-232 interfaces designed for the IBM PC include baud rate translation hardware to adjust the baud rate of the MIDI data to something that the IBM PC can support, for example 19,200 or 38,400 bits per second.

[edit] RTS/CTS handshaking

Further information: Hardware flow control

In older versions of the specification, RS-232's use of the RTS and CTS lines is asymmetric: The DTE asserts RTS to indicate a desire to transmit to the DCE, and the DCE asserts CTS in response to grant permission. This allows for half-duplex modems that disable their transmitters when not required, and must transmit a synchronization preamble to the receiver when they are re-enabled. This scheme is also employed on present-day RS-232 to RS-485 converters, where the RS-232's RTS signal is used to ask the converter to take control of the RS-485 bus - a concept that doesn't otherwise exist in RS-232. There is no way for the DTE to indicate that it is unable to accept data from the DCE.

A non-standard symmetric alternative, commonly called "RTS/CTS handshaking," was developed by various equipment manufacturers. In this scheme, CTS is no longer a response to RTS; instead, CTS indicates permission from the DCE for the DTE to send data to the DCE, and RTS indicates permission from the DTE for the DCE to send data to the DTE. RTS and CTS are controlled by the DTE and DCE respectively, each independent of the other. This was eventually codified in version RS-232-E (actually TIA-232-E by that time) by defining a new signal, "RTR (Ready to Receive)," which is CCITT V.24 circuit 133. TIA-232-E and the corresponding international standards were updated to show that circuit 133, when implemented, shares the same pin as RTS (Request to Send), and that when 133 is in use, RTS is assumed by the DCE to be ON at all times.[9]

Thus, with this alternative usage, one can think of RTS asserted (positive voltage, logic 0) meaning that the DTE is indicating it is "ready to receive" from the DCE, rather than requesting permission from the DCE to send characters to the DCE.

Note that equipment using this protocol must be prepared to buffer some extra data, since a transmission may have begun just before the control line state change.

RTS/CTS handshaking is an example of hardware flow control. However, "hardware flow control" in the description of the options available on an RS-232-equipped device does not always mean RTS/CTS handshaking.

[edit] 3-wire and 5-wire RS-232

A minimal "3-wire" RS-232 connection consisting only of transmit data, receive data, and ground, is commonly used when the full facilities of RS-232 are not required. Even a two-wire connection (data and ground) can be used if the data flow is one way (for example, a digital postal scale that periodically sends a weight reading, or a GPS receiver that periodically sends position, if no configuration via RS-232 is necessary). When only hardware flow control is required in addition to two-way data, the RTS and CTS lines are added in a 5-wire version.

[edit] Seldom used features

The EIA-232 standard specifies connections for several features that are not used in most implementations. Their use requires the 25-pin connectors and cables, and of course both the DTE and DCE must support them.

[edit] Signal rate selection

The DTE or DCE can specify use of a "high" or "low" signaling rate. The rates as well as which device will select the rate must be configured in both the DTE and DCE. The prearranged device selects the high rate by setting pin 23 to ON.

[edit] Loopback testing

Many DCE devices have a loopback capability used for testing. When enabled, signals are echoed back to the sender rather than being sent on to the receiver. If supported, the DTE can signal the local DCE (the one it is connected to) to enter loopback mode by setting pin 18 to ON, or the remote DCE (the one the local DCE is connected to) to enter loopback mode by setting pin 21 to ON. The latter tests the communications link as well as both DCE's. When the DCE is in test mode it signals the DTE by setting pin 25 to ON.

A commonly used version of loopback testing doesn't involve any special capability of either end. A hardware loopback is simply a wire connecting complementary pins together in the same connector (see loopback).

Loopback testing is often performed with a specialized DTE called a bit error rate tester (or BERT).

[edit] Timing signals

Some synchronous devices provide a clock signal to synchronize data transmission, especially at higher data rates. Two timing signals are provided by the DCE on pins 15 and 17. Pin 15 is the transmitter clock, or send timing (ST); the DTE puts the next bit on the data line (pin 2) when this clock transitions from OFF to ON (so it is stable during the ON to OFF transition when the DCE registers the bit). Pin 17 is the receiver clock, or receive timing (RT); the DTE reads the next bit from the data line (pin 3) when this clock transitions from ON to OFF.

Alternatively, the DTE can provide a clock signal, called transmitter timing (TT), on pin 24 for transmitted data. Data is changed when the clock transitions from OFF to ON and read during the ON to OFF transition. TT can be used to overcome the issue where ST must traverse a cable of unknown length and delay, clock a bit out of the DTE after another unknown delay, and return it to the DCE over the same unknown cable delay. Since the relation between the transmitted bit and TT can be fixed in the DTE design, and since both signals traverse the same cable length, using TT eliminates the issue. TT may be generated by looping ST back with an appropriate phase change to align it with the transmitted data. ST loop back to TT lets the DTE use the DCE as the frequency reference, and correct the clock to data timing.

Synchronous clocking is required for such protocols as SDLC, HDLC, and X.25.

[edit] Secondary channel

There is a secondary data channel, identical in capability to the first. Five signals (plus the common ground of the primary channel) comprise the secondary channel: Secondary Transmitted Data (STD), Secondary Received Data (SRD), Secondary Request To Send (SRTS), Secondary Clear To Send (SCTS), and Secondary Carrier Detect (SDCD).

[edit] Related standards

Other serial signaling standards may not interoperate with standard-compliant RS-232 ports. For example, using the TTL levels of near +5 and 0 V puts the mark level in the undefined area of the standard. Such levels are sometimes used with NMEA 0183-compliant GPS receivers and depth finders.

A 20 mA current loop uses the absence of 20 mA current for high, and the presence of current in the loop for low; this signaling method is often used for long-distance and optically isolated links. Connection of a current-loop device to a compliant RS-232 port requires a level translator. Current-loop devices can supply voltages in excess of the withstand voltage limits of a compliant device. The original IBM PC serial port card implemented a 20 mA current-loop interface, which was never emulated by other suppliers of plug-compatible equipment.

Other serial interfaces similar to RS-232:

RS-422 (a high-speed system similar to RS-232 but with differential signaling) RS-423 (a high-speed system similar to RS-422 but with unbalanced signaling)

RS-449 (a functional and mechanical interface that used RS-422 and RS-423 signals - it never caught on like RS-232 and was withdrawn by the EIA)

RS-485 (a descendant of RS-422 that can be used as a bus in multidrop configurations)

MIL-STD-188 (a system like RS-232 but with better impedance and rise time control)

EIA-530 (a high-speed system using RS-422 or RS-423 electrical properties in an EIA-232 pinout configuration, thus combining the best of both; supersedes RS-449)

EIA/TIA-561 8 Position Non-Synchronous Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange

EIA/TIA-562 Electrical Characteristics for an Unbalanced Digital Interface (low-voltage version of EIA/TIA-232)

TIA-574 (standardizes the 9-pin D-subminiature connector pinout for use with EIA-232 electrical signalling, as originated on the IBM PC/AT)

SpaceWire (high-speed serial system designed for use on board spacecraft).

[edit] Development tools

When developing or troubleshooting systems using RS-232, close examination of hardware signals can be important to find problems. A serial line analyzer is a device similar to a logic analyzer but specialized for RS-232's voltage levels, connectors, and, where used, clock signals. The serial line analyzer can collect, store, and display the data and control signals, allowing developers to view them in detail. Some simply display the signals as waveforms; more elaborate versions include the ability to decode characters in ASCII or other common codes and to interpret common protocols used over RS-232 such as SDLC, HDLC, DDCMP, and X.25. Serial line analyzers are available as standalone units, as software and interface cables for general-purpose logic analyzers, and as programs that run in common personal computers.

Serial portFrom Wikipedia, the free encyclopedia

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A male DE-9 connector used for a serial port on a IBM PC compatible computer. (Pinout)

A male Mini DIN-8 connector used for a serial port on a Macintosh or SGI style computer.

In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time (contrast parallel port).[1] Throughout most of the history of personal computers, data transfer through serial ports connected the computer to devices such as terminals and various peripherals.

While such interfaces as Ethernet, FireWire, and USB all send data as a serial stream, the term "serial port" usually identifies hardware more or less compliant to the RS-232 standard, intended to interface with a modem or with a similar communication device.

Modern computers without serial ports may require serial-to-USB converters to allow compatibility with RS 232 serial devices. Serial ports are still used in applications such as industrial automation systems, scientific instruments, shop till systems and some industrial and consumer products. Server computers may use a serial port as a control console for diagnostics. Network equipment (such as routers and switches) often use serial console for configuration. Serial ports are still used in these areas as they are simple, cheap and their console functions are highly standardized and widespread. A serial port requires very little supporting software from the host system.

Contents

1 Hardware o 1.1 Connectors

o 1.2 Pinouts

o 1.3 Hardware abstraction

2 Common applications for serial ports

o 2.1 Historic uses

3 Settings

o 3.1 Speed

o 3.2 Data bits

o 3.3 Parity

o 3.4 Stop bits

o 3.5 Conventional notation

o 3.6 Flow control

4 "Virtual" serial ports

5 See also

6 References

[edit] Hardware

PCI Express card with one serial port

Some computers, such as the IBM PC, used an integrated circuit called a UART, that converted characters to (and from) asynchronous serial form, and automatically looked after the timing and framing of data. Very low-cost systems, such as some early home computers, would instead use the CPU to send the data through an output pin, using the so-called bit-banging technique. Before large-scale integration (LSI) UART integrated circuits were common, a minicomputer or microcomputer would have a serial port made of multiple small-scale integrated circuits to implement shift registers, logic gates, counters, and all the other logic for a serial port.

Early home computers often had proprietary serial ports with pinouts and voltage levels incompatible with RS-232. Inter-operation with RS-232 devices may be impossible as the serial port cannot withstand the voltage levels produced and may have other differences that "lock in" the user to products of a particular manufacturer.

Low-cost processors now allow higher-speed, but more complex, serial communication standards such as USB and FireWire to replace RS-232. These make it possible to connect devices that would not have operated feasibly over slower serial connections, such as mass storage, sound, and video devices.

Many personal computer motherboards still have at least one serial port, even if accessible only through a pin header. Small-form-factor systems and laptops may omit RS-232 connector ports to conserve space, but the electronics are still there. RS-232 has been standard for so long that the circuits needed to control a serial port became very cheap and often exist on a single chip, sometimes also with circuitry for a parallel port.

[edit] Connectors

While the RS-232 standard originally specified a 25-pin D-type connector, many designers of personal computers chose to implement only a subset of the full standard: they traded off compatibility with the standard against the use of less costly and more compact connectors (in particular the DE-9 version used by the original IBM PC-AT). The desire to supply serial interface cards with two ports required that IBM reduce the size of the connector to fit onto a single card back panel. A DE-9 connector also fits onto a card with a second DB-25 connector that was similarly changed from the original Centronics-style connector. Starting around the time of the introduction of the IBM PC-AT, serial ports were commonly built with a 9-pin connector to save cost and space. However, presence of a 9-pin D-subminiature connector is neither necessary nor sufficient to indicate use of a serial port, since this connector was also used for video, joysticks, and other purposes.

Some miniaturized electronics, particularly graphing calculators and hand-held amateur and two-way radio equipment, have serial ports using a jack plug connector, usually the smaller 2.5 or 3.5 mm connectors and use the most basic 3-wire interface.

Many models of Macintosh favored the related RS-422 standard, mostly using German Mini-DIN connectors, except in the earliest models. The Macintosh included a standard set of two ports for connection to a printer and a modem, but some PowerBook laptops had only one combined port to save space.

The standard specifies 20 different signal connections. Since most devices use only a few signals, smaller connectors can often be used. For example, the 9 pin DE-9 connector was used by most IBM-compatible PCs since the IBM PC AT, and has been standardized as TIA-574. More recently, modular connectors have been used. Most common are 8P8C connectors. Standard EIA/TIA 561 specifies a pin assignment, but the "Yost Serial Device Wiring Standard"[2] invented by Dave Yost (and popularized by the Unix System Administration Handbook) is common on Unix computers and newer devices from Cisco Systems. Many

devices don't use either of these standards. 10P10C connectors can be found on some devices as well. Digital Equipment Corporation defined their own DECconnect connection system which was based on the Modified Modular Jack (MMJ) connector. This is a 6 pin modular jack where the key is offset from the center position. As with the Yost standard, DECconnect uses a symmetrical pin layout which enables the direct connection between two DTEs. Another common connector is the DH10 header connector common on motherboards and add-in cards which is usually converted via a cable to the more standard 9 pin DE-9 connector (and frequently mounted on a free slot plate or other part of the housing).

[edit] Pinouts

The following table lists commonly-used RS-232 signals and pin assignments.[3]

Signal Origin DB-25 pin

DE-9

pin

TIA-561 pin

Yost pin

DEC MMJName Typical purpose Abbreviation DTE DCE

Data Terminal Ready

OOB control signal: Tells DCE that DTE is ready to be connected.

DTR ● 20 4 3 2 1

Data Carrier Detect

OOB control signal: Tells DTE that DCE is connected to telephone line.

DCD ● 8 1 27 6

Data Set Ready

OOB control signal: Tells DTE that DCE is ready to receive commands or data.

DSR ● 6 61

Ring Indicator

OOB control signal: Tells DTE that DCE has detected a ring signal on the telephone line.

RI ● 22 9 — —

Request To Send

OOB control signal: Tells DCE to prepare to accept data from DTE.

RTS ● 4 7 8 1 —

Clear To Send

OOB control signal: Acknowledges RTS and allows DTE to transmit.

CTS ● 5 8 7 8 —

Transmitted Data

Data signal: Carries data from DTE to DCE. TxD ● 2 3 6 3 2

Received Data

Data signal: Carries data from DCE to DTE. RxD ● 3 2 5 6 5

Common Ground

GND common 7 5 4 4, 5 3, 4

Protective Ground

PG common 1 — — — —

The signals are named from the standpoint of the DTE, for example, an IBM-PC compatible serial port. The ground signal is a common return for the other connections; it appears on two pins in the Yost standard but is the same signal. The DB-25 connector includes a second

"protective ground" on pin 1. Connecting this to pin 7 (signal reference ground) is a common practice but not essential.

Note that EIA/TIA 561 combines DSR and RI,[4][5] and the Yost standard combines DSR and DCD.

A converter from USB to an RS-232 compatible serial port; more than a physical transition, it requires a driver in the host system software and a built-in processor to emulate the functions of the IBM-XT compatible serial port hardware.

[edit] Hardware abstraction

Operating systems usually use a symbolic name to refer to the serial ports of a computer.

Unix-like operating systems usually label the serial port devices /dev/tty* (TTY is a common trademark-free abbreviation for teletype) where * represents a string identifying the terminal device; the syntax of that string depends on the operating system and the device. On Linux, 8250/16550 UART hardware serial ports are named /dev/ttyS*, USB adapters appear as /dev/ttyUSB* and various types of virtual serial ports do not necessarily have names starting with tty.

The Microsoft MS-DOS and Windows environments refer to serial ports as COM ports: COM1, COM2, etc.

[edit] Common applications for serial ports

The RS-232 standard is used by many specialized and custom-built devices. This list includes some of the more common devices that are connected to the serial port on a PC. Some of these such as modems and serial mice are falling into disuse while others are readily available.

Serial ports are very common on most types of microcontroller, where they can be used to communicate with a PC or other serial devices.

GPS receivers (typically NMEA 0183 at 4,800 bit/s) Bar code scanners and other point of sale devices

LED and LCD text displays

Satellite phones , low-speed satellite modems and other satellite based transceiver devices

Flat-screen (LCD and Plasma) monitors to control screen functions by external computer, other AV components or remotes

Test and measuring equipment such as digital multimeters and weighing systems

Updating Firmware on various consumer devices.

Some CNC controllers

Uninterruptible power supply

Software debuggers that run on a 2nd computer.

Industrial field buses

[edit] Historic uses

Printers Computer terminal , teletype

Dial-up modems

Older digital cameras

Networking (Macintosh AppleTalk using RS-422 at 230.4 kbit/s)

Serial mouse

Older Joysticks

Older GSM mobile phones

Since the control signals for a serial port can be easily turned on and off by a switch, some applications used the control lines of a serial port to monitor external devices, without exchanging serial data. The most important commercial application of this principle was for some models of uninterruptible power supply which used the control lines to signal "loss of power", "battery low alarm" and other status information. At least some Morse code training software used a code key connected to the serial port, to simulate actual code use. The status bits of the serial port could be sampled very rapidly and at predictable times, making it possible for the software to decipher Morse code.

[edit] Settings

Many settings are required for serial connections used for asynchronous start-stop communication, to select speed, number of data bits per character, parity, and number of stop bits per character. In modern serial ports using a UART integrated circuit, all settings are usually software-controlled; hardware from the 1980s and earlier may require setting switches or jumpers on a circuit board. One of the simplifications made in such serial bus standards as Ethernet, FireWire, and USB is that many of those parameters have fixed values so that users can not and need not change the configuration; the speed is either fixed or automatically negotiated. Often if the settings are entered incorrectly the connection will not be dropped; however, any data sent will be received on the other end as nonsense.

[edit] Speed

Serial ports use two-level (binary) signaling, so the data rate in bits per second is equal to the symbol rate in bauds. A standard series of rates is based on multiples of the rates for electromechanical teleprinters; some serial ports allow many arbitrary rates to be selected. The port speed and device speed must match. The capability to set a bit rate does not imply that a working connection will result. Not all bit rates are possible with all serial ports. Some special-purpose protocols such as MIDI for musical instrument control, use serial data rates other than the teleprinter series. Some serial port systems can automatically detect the bit rate.

The speed includes bits for framing (stop bits, parity, etc.) and so the effective data rate is lower than the bit transmission rate. For example with 8-N-1 character framing only 80% of the bits are available for data (for every eight bits of data, two more framing bits are sent).

Common bit rates include 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600 and 115200 bit/s.[6]

[edit] Data bits

The number of data bits in each character can be 5 (for Baudot code), 6 (rarely used), 7 (for true ASCII), 8 (for any kind of data, as this matches the size of a byte), or 9 (rarely used). 8 data bits are almost universally used in newer applications. 5 or 7 bits generally only make sense with older equipment such as teleprinters.

Most serial communications designs send the data bits within each byte LSB (Least Significant Bit) first. This standard is also referred to as "little endian". Also possible, but rarely used, is "big endian" or MSB (Most Significant Bit) first serial communications. (See Endianness for more about bit ordering.) The order of bits is not usually configurable, but data can be byte-swapped only before sending.

[edit] Parity

Main article: Parity bit

Parity is a method of detecting errors in transmission. When parity is used with a serial port, an extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.

Electromechanical teleprinters were arranged to print a special character when received data contained a parity error, to allow detection of messages damaged by line noise. A single parity bit does not allow implementation of error correction on each character, and communication protocols working over serial data links will have higher-level mechanisms to ensure data validity and request retransmission of data that has been incorrectly received.

The parity bit in each character can be set to none (N), odd (O), even (E), mark (M), or space (S). None means that no parity bit is sent at all. Mark parity means that the parity bit is always set to the mark signal condition (logical 1) and likewise space parity always sends the parity bit in the space signal condition. Aside from uncommon applications that use the 9th (parity) bit for some form of addressing or special signalling, mark or space parity is uncommon, as it adds no error detection information. Odd parity is more common than even, since it ensures that at least one state transition occurs in each character, which makes it more reliable. The most common parity setting, however, is "none", with error detection handled by a communication protocol.

[edit] Stop bits

Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. If slow electromechanical teleprinters are used, one-and-one half or two stop bits are required.

[edit] Conventional notation

The D/P/S (Data/Parity/Stop) conventional notation specifies the framing of a serial connection. The most common usage on microcomputers is 8/N/1 (8N1). This specifies 8 data bits, no parity, 1 stop bit. In this notation, the parity bit is not included in the data bits. 7/E/1 (7E1) means that an even parity bit is added to the seven data bits for a total of eight bits between the start and stop bits. If a receiver of a 7/E/1 stream is expecting an 8/N/1 stream, half the possible bytes will be interpreted as having the high bit set.

[edit] Flow control

Main article: Flow control

A serial port may use signals in the interface to pause and resume the transmission of data. For example, a slow printer might need to handshake with the serial port to indicate that data should be paused while the mechanism advances a line.

Common hardware handshake signals (hardware flow control) use the RS-232 RTS/CTS or DTR/DSR signal circuits. Generally, the RTS and CTS are turned off and on from alternate ends to control data flow, for instance when a buffer is almost full. DTR and DSR are usually on all the time and, per the RS-232 standard and its successors, are used to signal from each end that the other equipment is actually present and powered-up. However, manufacturers have over the years built many devices that implemented non-standard variations on the standard, for example, printers that use DTR as flow control.

Another method of flow control (software flow control) uses special characters such as XON/XOFF to control the flow of data. The XON/XOFF characters are sent by the receiver to the sender to control when the sender will send data, that is, these characters go in the opposite direction to the data being sent. The circuit starts in the "sending allowed" state. When the receiver's buffers approach capacity, the receiver sends the XOFF character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an XON character to tell the sender to resume transmission. These are non-printing characters and are interpreted as handshake signals by printers, terminals, and computer systems.

XON/XOFF flow control is an example of inband signaling, in which control information is sent over the same channel used for the data. If the XON and XOFF characters might appear in the data being sent, XON/XOFF handshaking presents difficulties, as receivers may interpret them as flow control. Such characters sent as part of the data stream must be encoded in an escape sequence to prevent this, and the receiving and sending software must generate and interpret these escape sequences. On the other hand, since no extra signal circuits are required, XON/XOFF flow control can be done on a 3 wire interface.

[edit] "Virtual" serial ports

Main article: Virtual serial port

A virtual serial port is an emulation of the standard serial port. This port is created by software which enable extra serial ports in an operating system without additional hardware installation (such as expansion cards, etc.). Unlike a physical serial port the virtual one can be assigned any name (COM255, VSP33, etc.). It is possible to create a large number of virtual serial ports in a PC. The only limitation is the amount of resources, such as operating memory and computing power, needed to emulate many serial ports at the same time.

Virtual serial ports emulate all hardware serial port functionality, including Baud rate, Data bits, Parity bits, Stop bits, etc. Additionally they allow controlling the data flow, emulating all signal lines (DTR/DSR/CTS/RTS/DCD/RI) and customizing pinout. Virtual serial ports are common with Bluetooth and are the standard way of receiving data from Bluetooth-equipped GPS modules.

Virtual serial port emulation can be useful in case there is a lack of available physical serial ports or they do not meet the current requirements. For instance, virtual serial ports can share data between several applications from one GPS device connected to a serial port. Another option is to communicate with any other serial devices via internet or LAN as if they are locally connected

to computer (Serial-over-Ethernet technology). Two computers or applications can communicate through an emulated serial port link. Virtual serial port emulators are available for Microsoft Windows and some of them run under Windows CE, Windows Mobile and Pocket PC.

Parallel portFrom Wikipedia, the free encyclopedia

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This article is about the Centronics style port. For the concept in general, see Parallel communications.

A DB-25 parallel printer port, as on IBM-PC style, and a few other types of computers.

Micro ribbon 36 pin female, such as on printers and on some computers, particularly industrial equipment and early (pre-1980s) personal computers.

A parallel port is a type of interface found on computers (personal and otherwise) for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port. The IEEE 1284 standard defines the bi-directional version of the port, which allows the transmission and reception of data bits at the same time.

Contents

1 History 2 Uses

o 2.1 Current use

3 Implementation on IBM personal computers

o 3.1 Port addresses

o 3.2 Program interface

4 Pinouts

5 Unidirectional parallel ports

6 See also

7 References

8 External links

[edit] History

The Centronics Model 101 printer was introduced in 1970 and included the first parallel interface for printers.[1] The interface was developed by Robert Howard and Prentice Robinson at Centronics. The Centronics parallel interface quickly became a de facto industry standard; manufacturers of the time tended to use various connectors on the system side, so a variety of cables were required. For example, early VAX systems used a DC-37 connector, NCR used the 36-pin micro ribbon connector, Texas Instruments used a 25-pin card edge connector and Data General used a 50-pin micro ribbon connector.

Dataproducts introduced a very different implementation of the parallel interface for their printers. It used a DC-37 connector on the host side and a 50 pin connector on the printer side—either a DD-50 (sometimes incorrectly referred to as a "DB50") or the block shaped M-50 connector; the M-50 was also referred to as Winchester.[2][3] Dataproducts parallel was available in a short-line for connections up to 50 feet (15 m) and a long-line version for connections from 50 feet (15 m) to 500 feet (150 m). The Dataproducts interface was found on many mainframe systems up through the 1990s, and many printer manufacturers offered the Dataproducts interface as an option.

IBM released the IBM Personal Computer in 1981 and included a variant of the Centronics interface— only IBM logo printers (rebranded from Epson) could be used with the IBM PC.[4] IBM standardized the parallel cable with a DB25F connector on the PC side and the Centronics connector on the printer side. Vendors soon released printers compatible with both standard Centronics and the IBM implementation.

IBM implemented an early form of bidirectional interface in 1987. HP introduced their version of bidirectional, known as Bitronics, on the LaserJet 4 in 1992. The Bitronics and Centronics interfaces were superseded by the IEEE 1284 standard in 1994.

[edit] Uses

Before the advent of USB, the parallel interface was adapted to access a number of peripheral devices other than printers. Probably one of the earliest devices to use parallel were dongles used as a hardware key form of software copy protection. Zip drives and scanners were early implementations followed by external modems, sound cards, webcams, gamepads, joysticks, external hard disk drives and CD-ROM drives. Adapters were available to run SCSI devices via parallel. Other devices such as EPROM programmers and hardware controllers could be connected parallel.

[edit] Current use

For consumers, the USB interface—and often Ethernet—has effectively replaced the parallel printer port. Many manufacturers of personal computers and laptops consider parallel to be a legacy port and no longer include the parallel interface. The guidelines for Microsoft's Windows Logo Program "strongly discourages" systems builders from including parallel ports.[5] USB-to-parallel adapters are available that can make parallel-only printers work with USB-only systems.

[edit] Implementation on IBM personal computers

[edit] Port addresses

Traditionally IBM PC systems have allocated their first three parallel ports according to the configuration in the table below.

PORT NAME Interrupt # Starting I/O Ending I/O

LPT1 IRQ 2 0x3bc 0x3bf

LPT2 IRQ 7 0x378 0x37f

LPT3 IRQ 5 0x278 0x27f

If there is an unused LPTx slot, the port addresses of the others are moved up. (For example, if a port at 0x3bc does not exist, the port at 0x378 will then become LPT1.)[6] The IRQ lines, however, remain fixed (therefore, 0x378 at LPT1 would use IRQ 7)[citation needed]. . The port addresses assigned to each LPTx slot can be determined by reading the BIOS Data Area (BDA) at 0000:0408.

Bit to Pin Mapping for the Standard Parallel Port (SPP):

Address MSB LSB

Bit: 7 6 5 4 3 2 1 0

Base (Data port) Pin: 9 8 7 6 5 4 3 2

Base+1 (Status port) Pin: ~11 10 12 13 15

Base+2 (Control port) Pin: ~17 16 ~14 ~1

~ indicates a hardware inversion of the bit.

[edit] Program interface

In versions of Windows that did not use the Windows NT kernel (as well as DOS and some other operating systems), programs could access the parallel port with simple outportb() and inportb() subroutine commands. In operating systems such as Windows NT and Unix (NetBSD, FreeBSD, Solaris, 386BSD, etc.), the microprocessor is operated in a different security ring, and access to the parallel port is inhibited, unless using the required driver. This improves security and arbitration of device contention. On Linux, inb() and outb() can be used when a process is run as root and an ioperm() command is used to allow access to its base address; alternatively, ppdev allows shared access and can be used from userspace if the appropriate permissions are set. The cross-platform library for parallel port access, libieee1284, also is available on many Linux distributions and provides an abstract interface to the parallel ports of the system. Access is handled in an open-claim-release-close sequence, which allows for concurrent access in userspace.

[edit] Pinouts

Pinouts for parallel port connectors are:

Pin No (DB25) Pin No (36 pin) Signal name Direction Register - bit Inverted

1 1 Strobe In/Out Control-0 Yes

2 2 Data0 Out Data-0 No

3 3 Data1 Out Data-1 No

4 4 Data2 Out Data-2 No

5 5 Data3 Out Data-3 No

6 6 Data4 Out Data-4 No

7 7 Data5 Out Data-5 No

8 8 Data6 Out Data-6 No

9 9 Data7 Out Data-7 No

10 10 Ack In Status-6 No

11 11 Busy In Status-7 Yes

12 12 Paper-Out In Status-5 No

13 13 Select In Status-4 No

14 14 Linefeed In/Out Control-1 Yes

15 32 Error In Status-3 No

16 31 Reset In/Out Control-2 No

17 36 Select-Printer In/Out Control-3 Yes

18-25 19-30,33,17,16 Ground - - -

Inverted lines are true on logic low. If they aren't inverted, then logic high is true.

Pin 25 on the DB25 connector might not be connected to Ground on modern computers.

[edit] Unidirectional parallel ports

In early parallel ports the data lines were unidirectional (data out only) so it was not easily possible to feed data in to the computer. However, a workaround was possible by using 4 of the 5 status lines. A circuit could be constructed to split each 8-bit byte into two 4-bit nibbles which were fed in sequentially through the status lines. Each pair of nibbles was then re-combined into an 8-bit byte. This same method (with the splitting and recombining done in software) was also used to transfer data between PCs using a laplink cable.