Datasheet

64 Chapter 1
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Personal Computer System Components
with the standard and bidirectional ports and support cable lengths of 4.5 meters, which is
almost 15 feet.
There are two implementations of IEEE 1284: EPP parallel ports and ECP parallel ports.
An enhanced parallel port (EPP) increases bidirectional throughput from 150KBps to any-
where from 600KBps to 1.5MBps. An enhanced capabilities port (ECP) is designed to trans-
fer data at even higher speeds, around 2MBps. ECP uses direct memory access (DMA) and
buffering to increase printing performance over EPP.
The cable must also have full support for IEEE 1284 in order for proper
communications to occur in both directions and at rated speeds.
Parallel Interfaces and Cables
Most parallel interfaces use a DB-25 female connector, as shown earlier in this chapter.
Most parallel cables use a DB-25 male connector on one end and either a DB-25 male
connector or Centronics-36 connector on the other. The original printer cables typically
used the DB-25M–to–Centronics-36 configuration. Inside a parallel cable, eight wires
are used for transmitting data, so one byte can be transmitted at a time. Figure 1.45 shows
an example of a typical parallel cable (in this case, a printer cable).
If a printer today uses a parallel port through which to connect to the computer, a
possible interface on the printer is known as a mini-Centronics. Figure 1.46 shows the
component end of a mini-Centronics cable. The mini-Centronics did not enjoy the success
expected due to design issues regarding attachment reliability. Again, however, nothing is
more popular today for printer connectivity than USB, so efforts to perpetuate the use of
and improve the mini-Centronics were abandoned.
FIGURE 1.45 A typical parallel cable
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