Hardware Reference
In-Depth Information
sending data to a printer. There were times, however, when it was desirable to have a bidirectional
port—for example, when it was necessary to receive feedback from a printer, which was common
with PostScript printers. This could not be done easily with the original unidirectional ports.
Although it was never intended to be used for input, a clever scheme was devised in which four of the
signal lines could be used as a 4-bit input connection. Thus, these ports could do 8-bit (byte) output
(called
compatible mode
) and 4-bit input (called
nibble mode
). This is still common on low-end
desktop systems. Systems built after 1993 are likely to have more capable parallel ports, such as
bidirectional, EPP, and ECP.
Standard parallel ports are capable of effective transfer rates of about 150KBps output and about
50KBps input.
Bidirectional (8-Bit) Parallel Ports
With the introduction of the PS/2 series of machines in 1987, IBM introduced the bidirectional
parallel port. These ports are commonly found in PC-compatible systems today and can be designated
bidirectional, PS/2 type, or extended parallel ports. This port design 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 ports are capable of
approximately 150KBps transfer rates on both output and input. 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 EPP were Zenith Data
Systems laptops, Xircom Pocket LAN adapters, and the Intel 82360 SL I/O chip. On current systems
that include parallel ports, EPP is one of the modes supported.
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 IEEE 1284 Parallel Port
standard. Transfer rates of up to 2.77MBps are possible with EPP.
EPP version 1.7 (March 1992) was the first popular version of the 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 specification 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.
