Information Technology Reference
In-Depth Information
16
Parallel Port
16.1 Introduction
This chapter discusses parallel communications. The Centronics printer interface transmits
eight bits of data at a time to an external device, normally a printer. A 25-pin D-type connec-
tor is used to connect to the PC and a 36-pin Centronics interface connector normally con-
nects to the printer. This interface is not normally used for other types of interfacing as the
standard interface only transmits data over the data lines in one direction, that is, from the PC
to the external device. Some interface devices overcome this problem by using four of the
input handshaking lines to input data and then multiplexing using an output handshaking line
to multiplex them to produce eight output bits.
As technology has improved there is a great need for a bidirectional parallel port to con-
nect to devices such as tape backup drives, CD-ROMs, and so on. The Centronics interface
unfortunately lacks speed (150 kbps), has limited length of lines (2 m) and very few computer
manufacturers comply with an electrical standard.
Thus, in 1991, several manufacturers (including IBM and Texas Instruments) formed a
group called NPA (National Printing Alliance). Their original objective was to develop a
standard for controlling printers over a network. To achieve this a bi-directional standard was
developed which was compatible with existing software. This standard was submitted to the
IEEE so that they could standardise it. The committee that the IEEE set up was known as the
IEEE 1284 committee and the standard they produced is known as the IEEE 1284-1994
Standard (as it was released in 1994).
With this standard all parallel ports use a bidirectional link in either a compatible, nibble
or byte mode. These modes are relatively slow as the software must monitor the handshaking
lines (up to 100 kbps). To allow high-speed the EPP (enhanced parallel port) and ECP (ex-
tended capabilities port protocol) modes which allows high-speed data transfer using auto-
matic hardware handshaking. In addition to the previous three modes, EPP and ECP are be-
ing implemented on the latest I/O controllers by most of the Super I/O chip manufacturers.
These modes use hardware to assist in the data transfer. For example, in EPP mode, a byte of
data can be transferred to the peripheral by a simple OUT instruction. The I/O controller
handles all the handshaking and data transfer to the peripheral.
16.2 PC connections
Figure 16.1 shows the pin connections on the PC connector. The data lines (D0-D7) output
data from the PC and each of the data lines has an associated ground line (GND).
