RS232 serial cable layoutAlmost nothing in computer interfacing is more confusing than selecting the right RS232 serial cable. These pages are intended to provide information about the most common serial RS232 cables in normal computer use, or in more common language "How do I connect devices and computers using RS232?"

RS232 serial connector pin assignment

The RS232 connector was originally developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel. In practice, only one serial communication channel with accompanying handshaking is present. Only very few computers have been manufactured where both serial RS232 channels are implemented. Examples of this are the Sun SparcStation 10 and 20 models and the Dec Alpha Multia. Also on a number of Telebit modem models the secondary channel is present. It can be used to query the modem status while the modem is on-line and busy communicating. On personal computers, the smaller DB9 version is more commonly used today. The diagrams show the signals common to both connector types in black. The defined pins only present on the larger connector are shown in red. Note, that the protective ground is assigned to a pin at the large connector where the connector outside is used for that purpose with the DB9 connector version.

The pinout is also shown for the DEC modified modular jack. This type of connector has been used on systems built by Digital Equipment Corporation; in the early days one of the leaders in the mainframe world. Although this serial interface is differential (the receive and transmit have their own floating ground level which is not the case with regular RS232) it is possible to connect RS232 compatible devices with this interface because the voltage levels of the bit streams are in the same range. Where the definition of RS232 focussed on the connection of DTE, data terminal equipment (computers, printers, etc.) with DCE, data communication equipment (modems), MMJ was primarily defined for the connection of two DTE's directly.

RS232 DB9 pinout

DEC MMJ pinout

RS232 DB25 pinout

RS232 DB25 to DB9 converter

The original pinout for RS232 was developed for a 25 pins sub D connector. Since the introduction of the smaller serial port on the IBM-AT, 9 pins RS232 connectors are commonly used. In mixed applications, a 9 to 25 pins converter can be used to connect connectors of different sizes. As most of the computers are equipped with the DB9 serial port version, all wiring examples on this website will use that connector as a default. If you want to use the example with a DB25, simply replace the pin numbers of the connector according to the conversion table below.

RS232 DB9 to DB25 converter

DB9 - DB25 conversionDB9 DB25 Function1 8 Data carrier detect

2 3 Receive data3 2 Transmit data4 20 Data terminal ready5 7 Signal ground6 6 Data set ready7 4 Request to send8 5 Clear to send9 22 Ring indicator

RS232 serial loopback test plugs

The following RS232 connectors can be used to test a serial port on your computer. The data and handshake lines have been linked. In this way all data will be sent back immediately. The PC controls its own handshaking. The first test plug can be used to check the function of the RS232 serial port with standard terminal software. The second version can be used to test the full functionality of the RS232 serial port with Norton Diagnostics or CheckIt.

RS232 loopback test plug for terminal emulation software

DB9 DB25 Function1 + 4 + 6 6 + 8 + 20 DTR CD + DSR2 + 3 2 + 3 Tx Rx7 + 8 4 + 5 RTS CTS

RS232 loopback test plug for Norton Diagnostics and CheckIt

DB9 DB25 Function1 + 4 + 6 + 9 6 + 8 + 20 + 22 DTR CD + DSR + RI2 + 3 2 + 3 Tx Rx7 + 8 4 + 5 RTS CTS

Testing occurs in a few steps. Data is sent on the Tx line and the received information on the Rx input is then compared with the original data. The signal level on the DTR and RTS lines is also controlled by the test software and the attached inputs are read back in the software to see if these signal levels are properly returned. The second RS232 test plug has the advantage that the ring-indicator RI input line can also be tested. This input is used by modems to signal an incoming call to the attached computer.

RS232 null modem cables

The easiest way to connect two PC's is using an RS232 null modem cable. The only problem is the large variety of RS232 null modem cables available. For simple connections, a three line RS232 cable connecting the signal ground and receive and transmit lines is sufficient. Depending of the software used, some sort of handshaking may however be necessary. Use the RS232 null modem selection table to find the right null modem cable for each purpose. For a Windows 95/98/ME Direct Cable Connection, the RS232 null modem cable with loop back handshaking is a good choice.

RS232 null modem cables with handshaking can be defined in numerous ways, with loopback handshaking to each PC, or complete handshaking between the two systems. The most common null modem cable types are shown here.

Simple RS232 null modem without handshaking (Null modem explanation)

Connector 1 Connector 2 Function2 3 Rx Tx3 2 Tx Rx5 5 Signal ground

RS232 null modem with loop back handshaking (Null modem explanation)

Connector 1 Connector 2 Function2 3 Rx Tx3 2 Tx Rx5 5 Signal ground1 + 4 + 6 - DTR CD + DSR- 1 + 4 + 6 DTR CD + DSR7 + 8 - RTS CTS- 7 + 8 RTS CTS

RS232 null modem with partial handshaking (Null modem explanation)

Connector 1 Connector 2 Function1 7 + 8 RTS2 CTS2 + CD1

2 3 Rx Tx3 2 Tx Rx4 6 DTR DSR5 5 Signal ground6 4 DSR DTR7 + 8 1 RTS1 CTS1 + CD2

RS232 null modem with full handshaking (Null modem explanation)

Connector 1 Connector 2 Function2 3 Rx Tx3 2 Tx Rx4 6 DTR DSR5 5 Signal ground6 4 DSR DTR7 8 RTS CTS8 7 CTS RTS

n telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. A similar ITU-T standard is V.24.

Contents

[hide]

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 See also 10 References 11 External links

[edit] Scope of the standard

The Electronics 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 (single-ended) 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.

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: the standard requires the transmitter to use +12V and -12V, but requires the receiver to distinguish voltages as low as +3V and -3V. Some manufacturers therefore built transmitters that supplied +5V and -5V and labeled them as "RS-232 compatible."

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 PLCs), and console ports, and special purpose equipment such as a cash drawer for a cash register.

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.

[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 is gradually being 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. 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 the control pins of a serial port to interface to devices such as uninterruptible power supplies. In this case, serial data is not sent, 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. Some manufacturers have responded to this demand: Toshiba re-introduced the DE-9M connector on the Tecra laptop. Companies such as Digi specialise in RS232 I/O cards.

[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 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 would see 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 would be the opposite voltage condition, between -3 and -15 volts. Examples of control lines would 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.

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 Communications 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. 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" 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 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.[6]

Signal Origin DB-25 pin

DE-9 pin

TIA-561 pin

Yost pinName Typical purpose Abbreviation DTE DCE

Data Terminal Ready

Tells DCE that DTE is ready to be connected (optional).

DTR ● 20 4 3 2

Data Carrier Detect

Tells DTE that DCE is connected to telephone line (optional).

DCD ● 8 1 27

Data Set ReadyTells DTE that DCE is ready to receive commands or data.

DSR ● 6 61

Ring IndicatorTells DTE that DCE has detected a ring signal on the telephone line.

RI ● 22 9 -

Request To Send

Tells DCE to prepare to accept data from DTE.

RTS ● 4 7 8 1

Clear To SendAcknowledges RTS and allows DTE to transmit.

CTS ● 5 8 7 8

Transmitted Data

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

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

Common Ground

GND common 7 5 4 4, 5

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; 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.

Use of a common ground is one weakness of RS-232: if the two devices are far enough apart or on separate power systems, the ground will degrade between them and communications will fail, which is a difficult condition to trace.

Note that EIA/TIA 561 combines DSR and RI,[7][8] and the Yost standard combines DSR and DCD.

[edit] CablesMain 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.[9] 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).

[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. In the particular case of the IBM PC, baud rates were programmable with arbitrary values, so that a PC could be connected to, for example, MIDI music controllers (31,250 bits per second) or other devices not using the rates typically used with modems. Since most 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.

[edit] RTS/CTS handshaking

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: CTS indicates permission from the DCE for the DTE to send data to the DCE (and is controlled by the DCE independent of RTS), and RTS indicates permission from the DTE for the DCE to send data to the DTE. 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.[10]

Thus, with this alternative usage, one can think of RTS asserted (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.

[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 (see Bit Error Rate Test).

[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. Again, 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.

[edit] Secondary channel

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] 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)

RS232 Data Interface

 a Tutorial on Data Interface and cables

RS-232 is simple, universal, well understood and supported but it has some serious shortcomings as a data interface. The standards to 256kbps or less and line lengths of 15M (50 ft) or less but today we see high speed ports on our home PC running very high speeds and with high quality cable maxim distance has increased greatly. The rule of thumb for the length a data cable depends on speed of the data, quality of the cable.

a Tutorial

Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. RS232 (single-ended) was introduced in 1962, and despite rumors for its early demise, has remained widely used through the industry.

Independent channels are established for two-way (full-duplex) communications. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol.

The RS-232 interface presupposes a common ground between the DTE and DCE. This is a reasonable assumption when a short cable connects the DTE to the DCE, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true.

RS232 data is bi-polar.... +3 TO +12 volts indicates an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition.... Modern computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits

directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced.

The output signal level usually swings between +12V and -12V. The "dead area" between +3v and -3v is designed to absorb line noise. In the various RS-232-like definitions this dead area may vary. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. Many receivers designed for RS-232 are sensitive to differentials of 1v or less.

This can cause  problems when using pin powered widgets - line drivers, converters, modems etc. These type of units need enough voltage & current to power them self's up. Typical URART (the RS-232 I/O chip) allows up to 50ma per output pin - so if the device needs 70ma to run we would need to use at least 2 pins for power. Some devices are very efficient and only require one pin (some times the Transmit or DTR pin) to be high - in the "SPACE" state while idle.

An RS-232 port can supply only limited power to another device. The number of output lines, the type of interface driver IC, and the state of the output lines are important considerations.

The types of driver ICs used in serial ports can be divided into three general categories:

Drivers which require plus (+) and minus (-) voltage power supplies such as the 1488 series of interface integrated circuits. (Most desktop and tower PCs use this type of driver.)

Low power drivers which require one +5 volt power supply. This type of driver has an internal charge pump for voltage conversion. (Many industrial microprocessor controls use this type of driver.)

Low voltage (3.3 v) and low power drivers which meet the EIA-562 Standard. (Used on notebooks and laptops.)

Data is transmitted and received on pins 2 and 3 respectively. Data  Set Ready (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD) indicates that a good carrier is being received from the remote modem.

Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (Clear To Send - from the Data set) are used to control. In most Asynchronous situations, RTS and CTS are constantly on throughout the communication session. However where the DTE is connected to a multipoint line, RTS is used to turn carrier on the modem on and off. On a multipoint line, it's imperative that only one station is transmitting at a time (because they share the return phone pair). When a station wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds for carrier to stabilize, and then raises CTS. The DTE transmits when it sees CTS up. When the

station has finished its transmission, it drops RTS and the modem drops CTS and carrier together.

Clock signals (pins 15, 17, & 24) are only used for synchronous communications. The modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate.

Note: Transmit and receive leads (2 or 3) can be reversed depending on the use of the equipment - DCE Data Communications Equipment or a DTE Data Terminal Equipment.

Glossary of Abbreviations etc.

CTS            Clear To Send [DCE --> DTE]DCD           Data Carrier Detected (Tone from a modem) [DCE --> DTE]DCE            Data Communications Equipment eg. modemDSR            Data Set Ready [DCE --> DTE]DSRS         Data Signal Rate Selector [DCE --> DTE] (Not commonly                         used)DTE            Data Terminal Equipment eg. computer, printerDTR            Data Terminal Ready [DTE --> DCE]FG              Frame Ground (screen or chassis)NC              No ConnectionRCk            Receiver (external) Clock inputRI                Ring Indicator (ringing tone detected)RTS            Request To Send [DTE --> DCE]RxD            Received Data [DCE --> DTE]SG              Signal GroundSCTS         Secondary Clear To Send [DCE --> DTE]SDCD        Secondary Data Carrier Detected (Tone from a modem)                          [DCE  --> DTE]SRTS         Secondary Ready To Send [DTE --> DCE]SRxD         Secondary Received Data [DCE --> DTE]STxD         Secondary Transmitted Data [DTE --> DTE]TxD            Transmitted Data [DTE --> DTE]

Is Your Interface a DTE or a DCE?

One of the stickiest areas of confusion in datacom is over the terms "transmit" and "receive" as they pertain to DTE (data terminal equipment) and DCE (data communication

equipment). In synchronous communication, this confusion is particularly acute, because more signals are involved. So why is it that you sometimes send data on TD, and other times you send data on RD? Is this just a cruel form of mental torture? Not really. The secret lies in adopting the proper perspective. In data-com, the proper perspective is always from the point of view of the DTE. When you sit at a PC, terminal or workstation (DTE) and transmit data to somewhere far away, you naturally do so on the TD (transmit data) line. When your modem or CSU/DSU (DCE) receives this incoming data, it receives the data on the TD line as well. Why? Because the only perspective that counts in data-com is the perspective of the DTE. It does not matter that the DCE thinks it is receiving data; the line is still called "TD". Conversely, when the modem or CSU/DSU receives data from the outside world and sends it to the DTE, it sends it on the RD line. Why? Because from the perspective of the DTE, the data is being received! So when wondering, "Is this line TD or RD? Is it TC or RC?" Ask yourself, "What would the DTE say?"

Find out by following these steps: The point of reference for all signals is the terminal (or PC).

1 ) Measure the DC voltages between (DB25) pins 2 & 7 and between pins 3 & 7. Be sure the black lead is connected to pin 7 (Signal Ground) and the red lead to whichever pin you are measuring.

2) If the voltage on pin 2 is more negative than -3 Volts, then it is a DTE, otherwise it should be near zero volts.

3) If the voltage on pin 3  is more negative than -3 Volts, then it is a DCE.

4) If both pins 2 & 3 have a voltage of at least 3 volts, then either you are measuring incorrectly, or your device is not a standard EIA-232 device. Call technical support.

5) In general, a DTE provides a voltage on TD, RTS, & DTR, whereas a DCE provides voltage on RD, CTS, DSR, & CD.

X.21 interface on a DB 15 connector

 

also see X.21 write upalso see end of page for more info

X.21General 

Voltages: +/- 0.3Vdc

Speeds:Max. 100Kbps (X.26)

Max. 10Mbps (X.27)

 

The X.21 interface was recommended by the CCITT in 1976. It is defined as a digital signaling interface between customers (DTE) equipment and carrier's equipment (DCE). And thus primarily used for telecom equipment.

All signals are balanced. Meaning there is always a pair (+/-) for each signal, like used in RS422. The X.21 signals are the same as RS422, so please refer to RS422 for the exact details.

Pinning according to ISO 4903 

Sub-D15 Male Sub-D15 Female

 

Pin Signal abbr. DTE DCE

1 Shield   - -

2 Transmit (A)   Out In

3 Control (A)   Out In

4 Receive (A)   In Out

5 Indication (A)   In Out

6 Signal Timing (A)   In Out

7 Unassigned

8 Ground   - -

9 Transmit (B)   Out In

10 Control (B)   Out In

11 Receive (B)   In Out

12 Indication (B)   In Out

13 Signal Timing (B)   In Out

14 Unassigned

15 Unassigned

Functional DescriptionAs can be seen from the pinning specifications, the Signal Element Timing (clock) is provided by the DCE. This means that your provider (local telco office) is responsible for the correct clocking and that X.21 is a synchronous interface. Hardware handshaking is done by the Control and Indication lines. The Control is used by the DTE and the Indication is the DCE one.

Cross-cable pinning 

X.21 Cross Cable

X.21 X.21

1 1

2 4

3 5

4 2

5 3

6 7

7 6

8 8

9 11

10 12

11 9

12 10

13 14

14 13

15  

RS232D uses RJ45 type connectors (similar to telephone connectors) 

Pin No. Signal Description Abbr. DTE DCE

1 DCE Ready, Ring Indicator DSR/RI

2 Received Line Signal Detector DCD

3 DTE Ready DTR

4 Signal Ground SG

5 Received Data RxD

6 Transmitted Data TxD

7 Clear To Send CTS

8 Request To Send RTS

This is a standard 9 to 25 pin cable layout for async data on a PC AT serial cable 

Description Signal 9-pin 25-pin Source DTE or DCE

DTE DCE

Carrier Detect CD 1 8 from Modem

Receive Data RD 2 3 from Modem

Transmit Data TD 3 2 from Terminal/Computer

Data Terminal Ready

DTR 4 20 from   Terminal/Computer

Signal Ground SG 5 7 from Modem

Data Set Ready DSR 6 6 from Modem

Request to Send RTS 7 4 from   Terminal/Computer

Clear to Send CTS 8 5 from Modem

Ring Indicator RI 9 22 from Modem

This a DTE port as on the back of a PC Com Port - EIA-574 RS-232/V.24 pin out on a DB-9 pin

used for Asynchronous Data

 

 

 

25 pin D-shell connector RS232

commonly used for Async. data

PIN SIGNAL  DESCRIPTION

1   PGND Protective Ground2   TXD Transmit Data

3   RXD Receive Data4   RTS RequestTo Send5   CTS Clear To Send6   DSR Data Set Ready7   SG Signal Ground8   CD Carrier Detect20  DTR Data Terminal Ready22  RI Ring Indicator

 

Some applications require more pins than a simple async. configurations.

Pins used for Synchronous data 

jump to Other Connector pages

RS-232 Specs.SPECIFICATIONS RS232 RS423

Mode of OperationSINGLE-ENDED

SINGLE-ENDED

Total Number of Drivers and Receivers on One Line1 DRIVER1 RECVR

1 DRIVER10 RECVR

Maximum Cable Length 50 FT. 4000 FT.

Maximum Data Rate 20kb/s 100kb/s

Maximum Driver Output Voltage +/-25V +/-6V

Driver Output Signal Level (Loaded Min.) Loaded +/-5V to +/-15V +/-3.6V

Driver Output Signal Level (Unloaded Max) Unloaded +/-25V +/-6V

Driver Load Impedance (Ohms) 3k to 7k >=450

Max. Driver Current in High Z State Power On N/A N/A

Max. Driver Current in High Z State Power Off +/-6mA @ +/-2v +/-100uA

Slew Rate (Max.) 30V/uS Adjustable

Receiver Input Voltage Range +/-15V +/-12V

Receiver Input Sensitivity +/-3V +/-200mV

Receiver Input Resistance (Ohms) 3k to 7k 4k min. 

One byte of async data

Cabling considerations - you should use cabling made for RS-232 data but I have seen low speed data go over 250' on 2 pair phone cable. Level 5 cable can also be used but for best distance use a low capacitance data grade cable.

The standard maxim length is 50' but if data is async you can increase that distance to as much as 500' with a good grade of cable. 

The RS-232 signal on a single cable is impossible to screen effectively for noise. By screening the entire cable we can reduce the influence of outside noise, but internally generated noise remains a problem. As the baud rate and line length increase, the effect of capacitance between the different lines introduces serious crosstalk (this especially true on synchronous data - because of the clock lines) until a point is reached where the data itself is unreadable. Signal Crosstalk can be reduced by using low capacitance cable and shielding each pair

Using a high grade cable (individually shield low capacitance pairs) the distance can be extended to 4000'

At higher frequencies a new problem comes to light. The high frequency component of the data signal is lost as the cable gets longer resulting in a   rounded, rather than square wave signal.

The maxim distance will depend on the speed and noise level around the cable run.

On longer runs a line driver is needed. This is a simple modem used to increase the maxim distance you can run RS-232 data.

Making sense of the specifications

Selecting data cable isn't difficult, but often gets lost in the shuffle of larger system issues. Care should be taken. however, because intermittent problems caused by marginal cable can be very difficult to troubleshoot.

Beyond the obvious traits such as number of conductors and wire gauge, cable specifications include a handful of less intuitive terms.

Characteristic Impedance (Ohms): A value based on the inherent conductance, resistance, capacitance and inductance of a cable that represents the impedance of an infinitely long cable. When the cable is out to any length and terminated with this Characteristic Impedance, measurements of the cable will be identical to values obtained from the infinite length cable. That is to say that the termination of the cable with this impedance gives the cable the appearance of being infinite length, allowing no reflections of the transmitted signal. If termination is required in a system, the termination impedance value should match the Characteristic Impedance of the cable.

Shunt Capacitance (pF/ft): The amount of equivalent capacitive load of the cable, typically listed in a per foot basis One of the factors limiting total cable length is the capacitive load. Systems with long lengths benefits from using low capacitance cable.

Propagation velocity (% of c): The speed at which an electrical signal travels in the cable. The value given typically must be multiplied by the speed of light (c) to obtain units of meters per second. For example, a cable that lists a propagation velocity of 78% gives a velocity of 0.78 X 300 X 106 - 234 X 106 meters per second.

Plenum cable

Plenum rated cable is fire resistant and less toxic when burning than non-plenum rated cable. Check building and fire codes for requirements. Plenum cable is generally more expensive due to the sheathing material used.

The  specification recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance. 

It can be difficult to qualify whether shielding is required in a particular system or not, until problems arise. We recommend erring on the safe side and using shielded cable. Shielded cable is only slightly more expensive than unshielded.

There are many cables available meeting the recommendations of RS-422 and RS-485, made specifically for that application. Another choice is the same cable commonly used in the Twisted pair Ethernet cabling. This cable, commonly referred to as Category 5 cable, is defined by the ElA/TIA/ANSI 568 specification The extremely high volume of Category 5 cable used makes it widely available and very inexpensive, often less than half the price of specialty RS422/485 cabling. The cable has a maximum capacitance of 17 pF/ft (14.5 pF typical) and characteristic impedance of 100 ohms.

Category 5 cable is available as shielded twisted pair (STP) as well as unshielded twisted pair (UTP) and generally exceeds the recommendations  making it an excellent choice for RS232 systems.

RS232 - V.24/V.28 - IS2110 - X.20 bis (for Async) -

X.21 bis (for Sync)General

In this document the term RS232 will be used when refered to this serial interface. The description of RS232 is an EIA/TIA norm and is identical to CCITT V.24/V.28, X.20bis/X.21bis and ISO IS2110. The only difference is that CCITT has split the interface into its electrical description (V.28) and a mechanical part (V.24) or Asynchronous (X.20 bis) and Synchronous (X.21 bis) where the EIA/TIA describes everything under RS232.

As said before RS232 is a serial interface. It can be found in many different applications where the most common ones are modems and Personal Computers. All pinning specifications are writen for the DTE side.

All DTE-DCE cables are straight through meaning the pins are connected one on one. DTE-DTE and DCE-DCE cables are cross cables. To make a destiction between all different types of cables we have to use a naming convention.DTE - DCE: Straight CableDTE - DTE: Null-Modem CableDCE - DCE: Tail Circuit Cable

Interface Mechanical

RS232 can be found on different connectors. There are special specifications for this. The CCITT only defines a Sub-D 25 pins version where the EIA/TIA has two versions RS232C and RS232D which are resp. on a Sub-D25 and a RJ45. Next to this IBM has added a Sub-D 9 version which is found an almost all Personal Computers and is described in TIA 457.

Male Female

Pinnings  

RS232-C DescriptionCircuitEIA

CircuitCCITT

RJ45 TIA 457

1 Shield Ground AA      

7 Signal Ground AB 102 4 5

2 Transmitted Data BA 103 6 3

3 Received Data BB 104 5 2

4 Request To Send CA 105 8 7

5 Clear To Send CB 106 7 8

6 DCE Ready CC 107 1 6

20 DTE Ready CD 108.2 3 4

22 Ring Indicator CE 125 1 9

8 Received Line Signal Detector CF 109 2 1

23Data Signal Rate Select(DTE/DCE Source>

CH/CI 111/112    

24Transmit Signal Element Timing(DTE Source)

DA 113    

15Transmitter Signal Element Timing(DCE Source)

DB 114    

17Receiver Signal Element Timing(DCE Source)

DD 115    

18 Local Loopback / Quality Detector LL 141    

21 Remote Loopback RL/CG 140/110    

14 Secondary Transmitted Data SBA 118    

16 Secondary Received Data SBB 119    

19 Secondary Request To Send SCA 120    

13 Secondary Clear To Send SCB 121    

12Secondary Received Line Signal Detector/Data signal Rate Select (DCE Source)

SCF/CI 122/112    

25 Test Mode TM 142    

9 Reserved for Testing        

10 Reserved for Testing        

11 Unassigned        

Interface Electrical

All signals are measured in reference to a common ground, which is called the signal ground (AB). A positive voltage between 3 and 15 Vdc represents a logical 0 and a negative voltage between 3 and 15 Vdc represents a logical 1.This switching between positive and negative is called bipolar. The zero state is not defined in RS232 and is considered a fault condition (this happens when a device is turned off).According to the above a maximum distance of 50 ft or 15 m. can be reached at a

maximum speed of 20k bps. This is according to the official specifications, the distance can be exceeded with the use of Line Drivers.

Functional description

Description Circuit Function

Shield Ground AAAlso known as protective ground. This is the chassis ground connection between DTE and DCE.

Signal Ground ABThe reference ground between a DTE and a DCE. Has the value 0 Vdc.

Transmitted Data BA Data send by the DTE.

Received Data BB Data received by the DTE.

Request To Send CA Originated by the DTE to initiate transmission by the DCE.

Clear To Send CBSend by the DCE as a reply on the RTS after a delay in ms, which gives the DCEs enough time to energize their circuits and synchronize on basic modulation patterns.

DCE Ready CCKnown as DSR. Originated by the DCE indicating that it is basically operating (power on, and in functional mode).

DTE Ready CDKnown as DTR. Originated by the DTE to instruct the DCE to setup a connection. Actually it means that the DTE is up and running and ready to communicate.

Ring Indicator CEA signal from the DCE to the DTE that there is an incomming call (telephone is ringing). Only used on switched circuit connections.

Received Line Signal Detector

CFKnown as DCD. A signal send from DCE to its DTE to indicate that it has received a basic carrier signal from a (remote) DCE.

Data Signal Rate Select(DTE/DCE Source>

CH/CIA control signal that can be used to change the transmission speed.

Transmit Signal Element Timing(DTE Source)

DATiming signals used by the DTE for transmission, where the clock is originated by the DTE and the DCE is the slave.

Transmitter Signal Element Timing(DCE Source)

DB Timing signals used by the DTE for transmission.

Receiver Signal Element Timing(DCE Source)

DD Timing signals used by the DTE when receiving data.

Local Loopback / Quality Detector

LL  

Remote Loopback RL/CG Originated by the DCE that changes state when the analog

signal received from the (remote) DCE becomes marginal.

Test Mode TM  

Reserved for Testing

   

The secondary signals are used on some DCE's. Those units have the possibility to transmit and/or receive on a secondary channel. Those secondary channels are mostly of a lower speed than the normal ones and are mainly used for administrative functions.

Cable pinningHere are some cable pinning that might be useful. Not all applications are covered, it is just a help:

Straight DB25 Cable DB25 Null- modem or cross over cable (Async)

DB25 Tail- circuit or cross over cable cable (Sync)

DB25 to DB9 DTE - DCE cable

Pin Pin

1 1

2 2

3 3

4 4

5 5

6 6

7 7

8 8

9 9

10 10

11 11

12 12

13 13

14 14

15 15

16 16

17 17

18 18

19 19

20 20

21 21

22 22

Pin Pin

1 1

2 3

3 2

4 5

5 4

6, 8 20

7 7

20 6, 8

</DB9 Null- modem or

cross over cable

1,64

2 3

3 2

4 1,6

5 5

7 8

8 7

Pin Pin

1 1

2 3

3 2

4 8

6 20

7 7

8 4

17 24

20 6

24 17 

Pin Pin

  1

3 2

2 3

7 4

8 5

6 6

5 7

1 8

4 20

9 22 

23 23

24 24

25 25

 

This cable should be used for DTE to DCE (for instance computer to modem) connections with hardware handshaking.

(To Computer).

(To Modem).

9 PIN D-SUB FEMALE to the Computer25 PIN D-SUB MALE to the Modem

  Female Male Dir

Shield   1  

Transmit Data 3 2

Receive Data 2 3

Request to Send 7 4

Clear to Send 8 5

Data Set Ready 6 6

System Ground 5 7  

Carrier Detect 1 8

Data Terminal Ready 4 20

Ring Indicator 9 22

Nullmodem (25-25) CableUse this cable between two DTE devices (for instance two computers).

(To Computer 1).

(To Computer 2).

25 PIN D-SUB FEMALE to Computer 1.25 PIN D-SUB FEMALE to Computer 2.

  D-Sub 1 D-Sub 2  

Recieve Data 3 2 Transmit Data

Transmit Data 2 3 Receive Data

Data Terminal Ready 20 6+8 Data Set Ready + Carrier Detect

System Ground 7 7 System Ground

Data Set Ready + Carrier Detect 6+8 20 Data Terminal Ready

Request to Send 4 5 Clear to Send

Clear to Send 5 4 Request to SendNote: DSR & CD are jumpered to fool the programs to think that their online.

RS232 (25 pin) Tail Circuit Cable

Null Modem cable diagrams

Nullmodem (9p to 9p) Nullmodem (9p to 25p) Nullmodem (25p to 25p)

Cross Pinned cables for Async data.

 

Pin out for local Async Data transfer

Loopback plugs:

Serial Port Loopback (9p) Serial Port Loopback (25p)

 

jump to The Belden Cable Company's cable selection tutorial pages

jump to RS232 I/O jump to General Hardware Input/Output jump to http://www.hardwarebook.info/

(in-depth  write ups)

jump to  RS232 by CAMI Research Inc jump to Interfacing the Serial / RS232 Port jump to Introduction to Serial Communications jump to Serial Communications jump to Serial Port Basics jump to http://electrosofts.com/serial jump to Parallel port

jump to related fiber Optic cable pages

jump to Data Modems for phone lines

jump to Data Modems for fiber optics

jump to Interface converters

ARC Electronics ...800-926-0226 ext 202

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