Parallel port

Originally designed by Centronics, the port was later developed by Epson and is less commonly referred to as the Centronics interface, Centronics connector, or Centronics port. The parallel port is a 25-pin (type DB-25) computer interface commonly used to connect printers to IBM compatible computers.

Where is the parallel port on a computer?

Located on the back of the computer, the parallel port is part of the motherboard. A picture of the DB25 interface can be found below.

Note

Parallel ports are rarely used nowadays because of USB, FireWire, and other faster solutions. Parallel ports are no longer found on many new computers and laptops.

Identifying a parallel port

In the above photo, you can see the DB25 parallel port connection is easy to identify because it is the biggest connector on the back of the computer. The female connector has 25 holes and is in the shape of the letter D.

Parallel port modes

Depending on your requirements and computer resources, the parallel port can run in different modes. IEEE-1284 (Auto), Centronics mode, Nibble mode, SPP (unidirectional), bidirectional, and EPP and ECP are among these modes.

Diagram of parallel ports.

DB25 connectors had an 8-bit data bus and a maximum cable length of 15 feet. Although there are 50 foot cables, it is not recommended that these cables be used as they may result in poor data signals and connection. Here is additional information about each of the pins on this connector. Pictured above are pins 1 through 25.

The purpose of each pin is explained below.

Pin1 - Data acknowledgement when the signal is low.

Pin 2-9 - Data transfer pins.

Pin 10 - Acknowledge that the data has finished processing and when the signal is high indicates ready for more.

Pin 11 - When the signal goes high indicate that the printer has accepted the data and its being processed. Once this signal goes low and Pin 10 goes high, more data is ready to be accepted.

Pin 12 - Printer paper jam when signal is high or no signal if printer jam.

Pin 13 - When high signal printer is indicating that it is on-line and ready to print.

Pin 14 - When low signal PC has indicated that the printer inset a line feed after each line.

Pin 15 - Printer sends data to the computer telling it that an error has occurred.

Pin 16 - When low signal PC has requested that the printer initiate an internal reset.

Pin 17 - When low signal the PC has selected the printer and should in return prepare for data.

Pin 18-25 - Ground.

What is the parallel port used for?

The parallel port has been largely replaced by the USB port. The following is a list of various hardware components that were used with the parallel port.

Printer - The most-commonly used parallel port component.

Scanner - Another commonly used parallel device is a parallel port scanner. Parallel port scanners are a popular alternative to SCSI scanners because of how easy they are to install.

External drives - Parallel ports are popular with external drives such as the Iomega Zip drive, which can be removed from one computer and connected to another.

Parallel ports on Apple computers

SCSI is the interface used by Apple Macintosh computers. It is parallel, but a lot more flexible than the parallel port used by IBM compatible computers. No parallel port has ever been used by Apple computers.