Handout 14

Computer Buses

Buses

• A bus is a subsystem that transfers data between computer components inside a computer or between computers.

• A bus can logically connect several peripherals over the same set of wires.

• Each bus defines its set of connectors to physically plug devices, cards or cables together.

Buses

• Early computer buses were literally parallel electrical buses with multiple connections.

• The term is now used for any physical arrangement that provides the same logical functionality as a parallel electrical bus.

• Modern computer buses can use both parallel and bit-serial connections.

Characteristics of a Bus

• A bus is characterized by the amount of information that can be transmitted at once.

• This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously.

• The term "width" is used to refer to the number of bits that a bus can transmit at once.

Characteristics of a Bus

• The bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second.

• Each time that data is sent or received is called a cycle.

• The maximum transfer speed of the bus, is the amount of data which it can transport per unit of time,

Bus Subassembly

• The address bus (sometimes called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. It is a unidirectional bus.

• The data bus transfers instructions coming from or going to the processor. It is a bi-directional bus.

• The control bus (or command bus) transports orders and synchronization signals coming from the control unit and traveling to all other hardware components. It is a bi-directional bus, as it also transmits response signals from the hardware.

I²C (Inter-Integrated Circuit)

• Is a multi-master serial computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone.

• The I²C reference design has a 7-bit address space with 16 reserved addresses (maximum of 112 nodes)

I²C

• The most common I²C bus modes are the 100 kbit/s standard mode and the 10 kbit/s low-speed mode.

I²C Configuration

• Clock (SCL)• Data (SDA) lines • Master node — node that issues the clock and

addresses slaves • Slave node — node that receives the clock line

and address.

I²C to Your PC

• USB-I2C  USB to I2C Communications Module.

• http://www.robot-electronics.co.uk/htm/usb_i2c_tech.htm

• A Source: http://www.trossenrobotics.com/devantech-usb-to-12c-adapter.aspx?feed=Froogle

• 18-Bit Analog-to-Digital Converter with I2C Interface and On-Board Reference

Computer Bus Subassembly

Bus Configuration

Bus Configuration

• Backside bus (BSB), is used to connect the CPU to CPU cache memory.

• Frontside bus (FSB), system bus or local bus, connects the microprocessor and the system memory.

• Bus bridges interconnect different bus protocols.• AGP is a high-speed point-to-point channel for

attaching a graphics card to a computer's motherboard.

ISA Bus

• Industry Standard Architecture.

• The ISA bus was developed by a team at IBM as part of the IBM PC project in 1981.

• It originated as an 8-bit system.

ISA Sockets

ISA Card

Historical background for the I/O buses

ISA Bus

PCI Bus

• PCI Local Bus (Peripheral Component Interconnect)

• Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as USB or serial, TV tuner cards and disk controllers.

• Work on PCI began at Intel's Architecture Lab circa 1990.

PCI Slots

PCI Card

Conventional hardware specifications

• These specifications represent the most common version of PCI used in normal PCs.

• 33.33 MHz clock with synchronous transfers • Peak transfer rate of 133 MB/s for 32-bit bus width

(33.33 MHz × 32 bits ÷ 8 bits/byte = 133 MB/s) • Peak transfer rate of 266 MB/s for 64-bit bus width • 32-bit or 64-bit bus width • 32-bit address space (4 gigabytes)

PCI Keying

USB

• Universal Serial Bus.

• A serial bus standard to interface devices to a host computer.

• USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve plug 'n' play capabilities by allowing hot swapping.

History

• The USB 1.0 specification model was introduced in 1996.

• USB was created by the core group of companies that consisted of Intel, Compaq, Microsoft, Digital, IBM, and Northern Telecom.

USB Speeds

• The Full Speed rate of 12 Mbit/s (1.5 MB/s) is the basic USB data rate defined by USB 1.1.

• A Low Speed rate of 1.5 Mbit/s (187.5 kB/s) is defined by USB 1.0. It is very similar to full speed operation except that each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth Human Interface Devices (HID) such as keyboards, mice, and joysticks.

• A High-Speed (USB 2.0) rate of 480 Mbit/s (60 MB/s) was introduced in 2001. All high-speed devices are capable of falling back to full-speed operation if necessary.

USB Connectors

A Non-Standard USB Device

USB

• The maximum length of a standard USB cable is 5.0 meters (16.4 ft).

• The primary reason for this limit is the maximum allowed round-trip delay of about 1500 ns.

• If a USB device does not answer to host commands within the allowed time, the host considers the command to be lost.

Device Bandwidths

• Just more than you care to know.• http://en.wikipedia.org/wiki/List_of_device_bandwidths

References

• http://computer.howstuffworks.com/pci.htm

• http://members.tripod.com/~newwave_2/page1.htm

• http://en.wikipedia.org/wiki/Bus_(computing)

• http://www.karbosguide.com/hardware/module2c1.htm