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Computer Components 57

Peripheral Bus

For devices that cannot be placed conveniently inside the computer case
and attached to the expansion bus, peripheral bus standards allow
external components to communicate with the system.
The original IBM PC was equipped with a single bidirectional bus that
transmitted a single bit of data at a time and therefore was called the
serial port (Table 2-8). In addition, a unidirectional 8-bit-wide bus
became known as the parallel port; it was primarily used for con-
necting to printers. Twenty years later, most PCs are still equipped
with these ports, and they are only very gradually being dropped from
new systems.
In 1986, Apple computer developed a dramatically higher-performance
peripheral bus, which they called FireWire. This was standardized in
1995 as IEEE standard #1394. FireWire was a huge leap forward.
Like the SATA and PCI-Express standards that would come years later,
FireWire provided high bandwidth by transmitting data only a single
bit at a time but at high frequencies. This let it use a very small phys-
ical connector, which was important for small electronic peripherals.
FireWire supported Plug-n-Play capability and was also hot swappable,
meaning it did not require a computer to be reset in order to find a new
device. Finally, FireWire devices could be daisy chained allowing any
FireWire device to provide more FireWire ports. FireWire became
ubiquitous among digital video cameras and recorders.
Meanwhile, a group of seven companies lead by Intel released their
own peripheral standard in 1996, Universal Serial Bus (USB). USB is
in many ways similar to FireWire. It transmits data serially, supports
Plug-n-Play, is hot swappable, and allows daisy chaining. However, the
original USB standard was intended to be used with low-performance,
low-cost peripherals and only allowed 3 percent of the maximum band-
width of FireWire.

TABLE 2-8 Peripheral Bus Standards 4
Bus Memory Transfers Max data
width bus clock Transfers per second bandwidth
Peripheral bus (b) (MHz) per clock (MT/s) (MB/s)
Serial port (RS-232) 1 0.1152 0.1 0.01 0.001
Parallel port (IEEE-1284) 8 8.3 0.17 1.4 1.4
FireWire (IEEE-1394a) S400 1 400 1 400 50
USB 1.1 1 12 1 12 1.5
USB 2.0 1 480 1 480 60
FireWire (IEEE-1394b) S800 1 800 1 800 100
4
Ibid.