What is a Parallel Cable?

A parallel cable is used to connect to the parallel ports on a computer and a peripheral device like a printer or external drive. Parallel cables and the ports to which they connect are slowly becoming obsolete. Much of the purpose between connecting devices with them have been replaced by USB ports and storage devices.

History

IBM originally developed parallel ports and cables as a mean for connecting a printer to the earliest personal computers. IBM had a deal with Centronics, which was one of the leading printer manufacturers of the time and rather than using the Centronics style connector which they found too big, IBM developed a 24-pin connector to work with a 36-pin connector developed by Centronics. That connector became known as a parallel cable and became the industry standard in the wake of the PC revolution.

Specifications

The original parallel port was designed so that data could travel in just one direction from the computer to the printer. In 1987 IBM introduced the PS/2 that offered the opportunity for signals to be sent back and forth between the connected device and the computer. Each of the 25 pins on a parallel port have functions that are expressed either individually or in conjunction with a series of nearby pins.

Pins 1 Through 9

The first pin carries the strobe indicating the voltage. When data is being sent, the voltage actually drops in this pin. Pins two through nine are responsible for carrying that data that is being sent from the computer to the peripheral device and back again. These pins transmit digital signals over an analog cable.

Pins 10 through 14

Pins 10 through 14 on a parallel cable record an acknowledgment that the signal has been sent and received. Pin 11 has the specific function of receiving a small amount of voltage when the printer is transmitting that it is currently busy. When it is ready to receive more messages the voltage drops to zero. Pin 12 sends the message that the printer is out of paper.

Pins 15 to 25

Pin 15 is perhaps the most hated pin on a parallel cable because it is there to send the message that no computer wants to receive: an error message. When an error has been detected, the 15 pin on the parallel cable drops the voltage to zero. Pin 16 on a parallel port drops the voltage to zero for printer initialization when a new print job is ready to begin. Pin 17 takes the printer offline. Pins 18 to 25 are simply electrical grounds.

Benefits

The parallel port began as a necessity for one company, IBM, and it became an industry standard. Each and every PC manufacturer and every manufacturer of printers and other devices that used parallel ports used this standard until it began to be replaced by the more convenient and quicker USB port. As a result, those who own older computers or older printers do not necessarily have to upgrade.

Speculation

The speculation that the parallel port is dead has been in the air for several years. The introduction of the quicker and more convenient USB cables to connect computers with devices has heralded the death knell for the parallel port. Yet computers and printers are still routinely manufactured with these ports still available. The true death of the parallel cable will probably arrive one day, but that day does not appear to be in the near future since many people still have large collections of data on zip drives which also take advantage of the parallel connection.