PARALLEL PORT

6/15/96

The PC Parallel Port is an 8 bit wide external interface designed by IBM for the original IBM PC. The Parallel port is by far the fastest "normal" data port on most standard PCs allowing transfer speeds of nearly 1MegaBitPer Second over distances of 10 meters (30 feet). The "Standard Parallel Port" (SPP) is often unidirectional, inputting data in 4 bit wide "nibble" mode over "status" lines. In theory, the PC allows up to 4 parallel ports although only 3 standard addresses 3BC, 278 and 378 are in general use. IEEE standard 1284 codifies industry conventions of 15 years and defines Enhanced and Extended capabilities -- EPP and ECP. Port 3BC may not be supported in all modes. Search order varies between BIOSes. Older BIOSes search 3BC, 378, 278 in that order. Some new BIOSes search only 378 and 278.

The published schematic of the original IBM PC parallel port contained an error. As a result, many early parallel ports could not generate interrupts. This, in turn, led most PC operating systems to use polled rather than interrupt driven IO for printing.

The Enhanced Parallel Port (EPP) is a fully bidirectional 8 bit IEEE 1284 compliant port coming into use for communication with external non-printing devices such as scanners, tape and disk drives. The Extended Capability Port (ECP) is an extension to EPP defined by Microsoft and Hewlett-Packard that permits, among other things, Run Length Limited Compression of the data stream. Some PCs default programmable ports to 3BC however, EPP, and ECP are alleged to need additional contiguous port addresses which are available at other addresses, but not above 3BC. EPP/EPP drivers are available for WIN9x and above and possibly for OS/2. For MSDOS/WIN3 a driver would be required but none are known to be available. ECP/EPP devices usually require that all ground wires in the cable be attached (not always the case in inexpensive cables), and may require shielding in the cable, that thumbscrews at the connectors be used, etc.

Additional parallel port addresses purported to be used occasionally are 268, 27C and 26C, but these are uncommon. One Dell owner reports that the motherboard port has AT and PS/2 modes. At a guess, "AT" = SPP and "PS/2" = ECP.

A special cable may be required for two way communications over parallel ports. This cable crosses over wires and has two DB25 male connectors. It is typically called a "Lap-Link" or Direct Cable Connect Cable. A wiring diagram is given in the DOS6 Help file under INTERLINK.

http://web.archive.org/web/20001201200500/http://home.rmi.net/~hisys/parport.html

last change -- Spelling: 12Oct02