| The parallel port , also known as the LPT (line printer ) port, was originally designed for use with parallel printers. However, don't let the name "LPT port" fool you: the parallel port is among the most versatile of I/O ports in the system because it is also used by a variety of devices, including tape backups , external CD-ROM and optical drives , scanners , and removable-media drives such as Zip drives. Although newer devices in these categories are now designed to use USB or IEEE-1394 ports, the parallel port continues to be an important external I/O device for systems running Windows 95 and Windows NT 4.0. The parallel (LPT) port is unusual because it uses two completely different connector types: -
All IBM and compatible computers since the first IBM PC of 1981 have used the DB-25F port shown in Figure 8.1, with pins 1 “13 on the top and pins 14 “25 on the bottom. This is also referred to as the type IEEE-1284-A connector. ( IEEE-1284 is an international standard for parallel port connectors, cabling, and signaling.) Figure 8.1. Parallel devices use the Centronics port (top) for printers and some other types of parallel devices, whereas the DB-25F port (bottom) is used for the computer's parallel port. Some external devices also use a DB-25F port.  -
The port used by parallel printers of all types, however, is the same Centronics 36-pin port used since the days of the Apple II and other early microcomputers of the late 1970s, as seen in Figure 8.1. This port is also referred to as the IEEE-1284-B port. It is an edge connector with 36 connectors, 18 per side. caution  | Devices other than printers that plug into the parallel (LPT) port have two connectors: one for the cable that runs from the device to the parallel port, and another for the cable that runs from the device to the printer. Although it's theoretically possible to create a long daisy-chain of devices ending with a printer, in practice you should have no more than one device plus a printer plugged into a parallel port. If you use more than one device, you could have problems getting the devices (not to mention the printer) to work reliably. |
Some recent Hewlett-Packard LaserJet printers also use a miniature version of the Centronics connector known as the IEEE-1284-C , which is also a 36-pin edge connector. The 1284-C connector doesn't use wire clips. Accordingly, a parallel printer cable also has different connectors at each end, as seen in Figure 8.2. Figure 8.2. The ends of a typical IBM-style parallel cable. The Centronics 36-pin connector (upper left) connects to the printer; the DB-25M connector (lower right) connects to the computer's DB-25F parallel port.  Parallel cables have the pinout described in Table 8.1. Table 8.1. Parallel Port Pinout (DB-25F Connector) | Pin # | Description | I/O | Pin # | Description | I/O | | 1 | -Strobe | Out | 14 | -Auto Feed | Out | | 2 | +Data bit 0 | Out | 15 | -Error | In | | 3 | +Data bit 1 | Out | 16 | -Initialize Printer | Out | | 4 | +Data bit 2 | Out | 17 | -Select Input | Out | | 5 | +Data bit 3 | Out | 18 | -Ground (Data bit 0 Return) | In | | 6 | +Data bit 4 | Out | 19 | -Ground (Data bit 1 Return) | In | | 7 | +Data bit 5 | Out | 20 | -Ground (Data bit 2 Return) | In | | 8 | +Data bit 6 | Out | 21 | -Ground (Data bit 3 Return) | In | | 9 | +Data bit 7 | Out | 22 | -Ground (Data bit 4 Return) | In | | 10 | -Acknowledge | In | 23 | -Ground (Data bit 5 Return) | In | | 11 | +Busy | In | 24 | -Ground (Data bit 6 Return) | In | | 12 | +Paper End | In | 25 | -Ground (Data bit 7 Return) | In | | 13 | +Select | In | | | | |
