History of Parallel

When IBM introduced the PC, in 1981, the parallel printer port was included as an alternative to the slower serial port as a means for driving the latest high performance dot matrix printers. The parallel port had the capability to transfer 8 bits of data at time whereas the serial port transmitted one bit at a time. When the PC was introduced, dot matrix printers were the main peripheral that used the parallel port. As technology progressed and the need for greater external connectivity increased, the parallel port became the means by which you could connect higher performance peripherals. These peripherals now range from printer sharing devices, portable disk drives and tape backup to local area network adapters and CD ROM players.

The problems faced by developers and customers of these peripherals fall into three categories.

First, although the performance of the PC has increased dramatically, there has been virtually no change in the parallel port performance or architecture. The maximum data transfer rate achievable with this architecture is around 150 kilobytes per second and is extremely software intensive.

Second, there is no standard for the electrical interface. This causes many problems when attempting to guarantee operation across various platforms.

Finally, the lack of design standards forced a distance limitation of only 6 feet for external cables.

In 1991 there was a meeting of printer manufacturers to start discussions on developing a new standard for the intelligent control of printers over a network. These manufacturers, which included Lexmark, IBM, Texas Instruments and others, formed the Network Printing Alliance. The NPA defined a set of parameters that, when implemented in the printer and host, will allow for the complete control of printer applications and jobs.

While this work was in progress it became apparent that to fully implement this standard would require a high performance bi-directional connection to the PC. The usual means of connection, the ordinary PC parallel port, did not have the capabilities required to meet the full requirements or abilities of this standard.

The NPA submitted a proposal to the IEEE for the creation of a committee to develop a new standard for a high speed bi-directional parallel port for the PC. It was a requirement that this new standard would remain fully compatible with the original parallel port software and peripherals, but would increase the data rate capability to greater than 1M bytes per second, both in and out of the computer. This committee became the IEEE 1284 committee.

The IEEE 1284 standard, "Standard Signalling Method for a Bi-directional Parallel Peripheral Interface for Personal Computers", was approved for final release in March of 1994.

THE PARALLEL PORT -- AN OVERVIEW

The parallel port, as implemented on the PC, consists of a connector with 17 signal lines and 8 ground lines. The signal lines are divided into three groups:

Control (4 lines)

Status (5 lines)

Data (8 lines)

As originally designed, the Control lines are used as interface control and handshaking signals from the PC to the printer. The Status lines are used for handshake signals and as status indicators for such things as paper empty, busy indication and interface or peripheral errors. The data lines are used to provide data from the PC to the printer, in that direction only. Later implementations of the parallel port allowed for data to be driven from the peripheral to the PC.