Hardware Reference
In-Depth Information
Bidirectional (8-Bit) Parallel Ports
With the introduction of the PS/2 series of machines in 1987, IBM introduced the bid-
irectional parallel port. These ports are commonly found in PC-compatible systems today
andcanbedesignatedbidirectional,PS/2type,orextendedparallelports.Thisportdesign
opened the way for true communication between the computer and the peripheral across
the parallel port. This was done by defining a few of the previously unused pins in the
parallel connector and by defining a status bit to indicate in which direction information
was traveling across the channel. This allows for true 8-bit (called
byte mode
) input.
These ports can do both 8-bit input and output using the standard eight data lines and
are considerably faster than the 4-bit ports when used with external devices. Bidirectional
portsarecapableofapproximately150KBpstransferratesonbothoutputandinput.Some
newer systems use this as their standard mode.
Enhanced Parallel Port
EPP is sometimes referred to as the
Fast Mode
parallel port. Intel, Xircom, and Zenith
Data Systems developed and announced EPP in October 1991. The first products to offer
EPPwereZenithDataSystemslaptops,XircomPocketLANadapters,andtheIntel82360
SL I/O chip. On current systems that include parallel ports, EPP is one of the modes sup-
ported.
EPP operates at almost ISA bus speed and offers a tenfold increase in the raw throughput
capability over a conventional parallel port. EPP was especially designed for parallel port
peripherals, such as LAN adapters, disk drives, and tape backups. EPP is included in the
IEEE1284ParallelPortstandard.Transferratesofupto2.77MBpsarepossiblewithEPP.
EPP version 1.7(March 1992)was the first popular version ofthe hardware specification.
With minor changes, this was later abandoned and folded into the IEEE 1284 standard.
Some technical reference materials have erroneously made reference to “EPP specifica-
tion version 1.9,” causing confusion about the EPP standard. Note that “EPP version 1.9”
technically does not exist, and any EPP specification after the original (version 1.7) is
more accurately referred to as a part of the IEEE 1284 specification.
Unfortunately, this resulted in two somewhat incompatible standards for EPP parallel
ports: the original EPP Standards Committee version 1.7 standard and the IEEE 1284
Committee standard, usually called
EPP version 1.9
. The two standards are similar
enough that peripherals can be designed to support both, but older EPP 1.7 peripherals
might not operate with EPP 1284 (EPP 1.9) ports. For this reason, some multimode ports
allow configuration in either EPP 1.7 or 1.9 mode, normally selected via the BIOS Setup.
