Parallel port

Updated: 03/10/2024 by Computer Hope

Less commonly called the Centronics interface, Centronics connector, or Centronics port after the company that originally designed it, the port was later developed by Epson. The parallel port is found on the back of IBM compatible computers and is a 25-pin (type DB-25) computer interface commonly used to connect printers to the computer.

Where is the parallel port on a computer?

The parallel port is found on the back of the computer and is part of the motherboard. Below is an example of the DB25 interface found on the back of the computer.

Parallel port


With the introduction of USB (universal serial bus), FireWire, and other faster solutions, parallel ports are rarely used anymore. Also, many new computers and laptops no longer have a parallel port.

Identifying a parallel port

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

Parallel port modes

The computer can have the parallel port run at different modes depending on your needs and available resources. Some of these modes include: IEEE-1284 (Auto), Centronics mode, Nibble Mode, SPP (Standard Parallel Port), bidirectional, EPP (enhanced parallel port), and ECP (extended capabilities port).

Parallel port overview

The DB25 connector 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 it can create poor connection and data signals. Below is additional information about each of the pins on this connector. Pin 1 through 25 identified in the image above.

Pin Purpose
Pin 1 -Strobe
Pin 2 +Data Bit 0
Pin 3 +Data Bit 1
Pin 4 +Data Bit 2
Pin 5 +Data Bit 3
Pin 6 +Data Bit 4
Pin 7 +Data Bit 5
Pin 8 +Data Bit 6
Pin 9 +Data Bit 7
Pin 10 -Acknowledge
Pin 11 +Busy
Pin 12 +Paper End
Pin 13 +Select
Pin 14 -Auto Feed
Pin 15 -Error
Pin 16 -Initialize Printer
Pin 17 -Select Input
Pin 18 -Data Bit 0 Return (GND)
Pin 19 -Data Bit 1 Return (GND)
Pin 20 -Data Bit 2 Return (GND)
Pin 21 -Data Bit 3 Return (GND)
Pin 22 -Data Bit 4 Return (GND)
Pin 23 -Data Bit 5 Return (GND)
Pin 24 -Data Bit 6 Return (GND)
Pin 25 -Data Bit 7 Return (GND)

Below explains each of the above purposes.

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?

Today, the parallel port has widely been replaced by the USB port. However, below is a listing of various hardware components that were used with the parallel port.

Printer - The most common use for the parallel port.

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

The Apple Macintosh computers use SCSI (small computer system interface) as its interface, which is parallel, but more flexible compared to the parallel port used with IBM compatible computers. Apple computers have never used a parallel port.

Cable, Connection, Data cable, Hardware terms, LPT, Obsolete, Parallel, Printer terms, Serial port