Datasheet

Identifying Characteristics of Ports and Cables 63
Parallel
For many years, the most popular type of interface available on computers was the parallel
interface. Parallel communications take the interstate approach to data communications.
Normally, interstate travel is faster than driving on city roads. This is the case mainly
because you can fit multiple cars going the same direction on the same highway by using
multiple lanes. On the return trip, you take a similar path, but on a completely separate
road. The parallel interface (an example is shown at the top of Figure 1.36) transfers data
8 bits at a time over eight separate transmit wires inside a parallel cable (one bit per wire).
Normal parallel interfaces use a DB-25 female connector on the computer to transfer data
to peripherals. Parallel was faster than the original serial technology, which was also once
used for printers in electrically noisy environments or at greater distances from the com-
puter, but the advent of USB has brought serial, fast serial, back to the limelight.
The most common use of the parallel interface is printer communication. There are
three major types: standard, bidirectional, and enhanced parallel ports. Let’s look at the
differences between the three.
Standard Parallel Ports
The standard parallel port only transmits data out of the computer. It cannot receive data
(except for a single wire carrying a Ready signal). This parallel port came with the original
IBM PC, XT, and AT. It can transmit data at 150KBps and is commonly used to transmit
data to printers. This technology also had a maximum transmission distance of 10 feet.
Bidirectional Parallel Ports
As its name suggests, the bidirectional parallel port has one important advantage over
a standard parallel port: it can both transmit and receive data. These parallel ports are
capable of interfacing with such devices as external CD-ROM drives and external paral-
lel port backup drives (Zip, Jaz, and tape drives). Most computers made since 1994 have
a bidirectional parallel port.
In order for bidirectional communication to occur properly, the cable must
support bidirectional communication as well.
Enhanced Parallel Ports
As more people began using parallel ports to interface with devices other than printers,
they started to notice that the available speed wasn’t good enough. Double-speed CD-ROM
drives had a transfer rate of 300KBps, but the parallel port could transfer data at only
150KBps, thus limiting the speed at which a computer could retrieve data from an external
device. To solve that problem, the Institute of Electrical and Electronics Engineers (IEEE)
came up with a standard for enhanced parallel ports called IEEE 1284. The IEEE 1284
standard provides for greater data transfer speeds and the ability to send memory addresses
as well as data through a parallel port. This standard allows the parallel port to theoreti-
cally act as an extension to the main bus. In addition, these ports are backward compatible
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