PC Hardware A Beginner’s Guide

Parallel Ports

Parallel ports are much more straightforward than serial ports. Because all of a charac-

ter’s data moves over a parallel link at one time, the data transmits faster than it does on a

serial connection. This is the reason the PC’s internal bus structures use the parallel for-

mat; it’s also another reason why a serial port needs a device like the UART to convert the

internal parallel data format to a serial format for transmission over a serial line.

Parallel ports on a PC are female DB-25 connectors that connect to male DB-25 con-

nectors, as shown in Figure 19-7. The PC’s parallel ports were originally designed for use

by printers. However, other devices have been adapted to them, including other types of

output devices, input devices, and storage devices, all taking advantage of the

bidirectional capabilities of most of the newer parallel ports and devices. These include

some external CD-ROMs, external tape drives, and Zip drives, as well as file transfer soft-

ware over proprietary cabling.

Parallel Port Standards

The Institute of Electrical and Electronics Engineers (IEEE) has standardized the parallel

port protocols. The IEEE standard is formally titled the “IEEE Standard Signaling

Method for a Bidirectional Parallel Peripheral Interface for Personal Computers,” but it is

Chapter 19: Ports and Connectors^491

Figure 19-7. A networking device with a parallel port and a cable with a DB-25 male connector