Information Technology Reference
In-Depth Information
Data bus
Address bus
Device 1
Device 2
Device 1
Handshaking lines
Device 2
Control lines
Computer bus
Figure 2.2
Model of a computer bus
Data bus
The data bus is responsible for passing data from one device and another. This data is either
passed in a serial manner (one bit at a time) or in parallel (several bits at a time). In a parallel
data bus, the bits are normally passed in a multiple of eight bits at a time. Typical parallel
data busses are 8 bits, 16 bits, 32 bits, 64 bits or 128 bits wide.
The bus size defines the maximum size of the bus, but the bus can be used to transmit any
number of bits which is less than the maximum size. For example, a 32-bit bus can be used
to transmit eight bits, 16 bits or 32 bits at a time. Most modern computer systems use a 64-bit
address bus, although the software which runs on the computer only uses a maximum of 32
bits at a time (known as 32-bit software).
Parallel busses are normally faster than serial busses (as they can transmit more bits in a
single operation), but require many more lines (thus requiring more wires in the cable). A
parallel data bus normally requires extra data handshaking lines to synchronise the flow of
data between devices. Serial data transmission normally uses a start and end bit sequence to
define the start and end of transmission. Figure 2.3 illustrates the differences between serial
and parallel data busses. Parallel busses are typically used for local busses, or where there are
no problems with cables with a relatively large number of wires. Typically, parallel busses
are SCSI and IDE which are used to connect to hard disk drives, and typical serial busses are
RS-232, and the USB.
Serial communications can operate at very high transmission rates; the main limiting fac-
tor is the transmission channel and the transmitter/receiver electronics. Gigabit Ethernet, for
example, uses a transmission rate of 1 Gbps (125 MB/s) over high-quality twisted-pair copper
cables, or over fibre optic cables (although this is a theoretical rate as more than one bit is
sent at a time). For a 32-bit parallel bus, this would require a clocking rate of only
31.25 MHz (which requires much lower quality connectors and cables than the equivalent
serial interface).
