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.
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