Parallel Port Interfacing

What is a Parallel Port

The parallel port has been replaced in laptops and many today's computers with the USB. But on desktop computers that are a few years old there is often still a parallel port. These are useful for interfacing projects as they allow for a simple way to send data out of the computer and input back in. The parallel port also has grounds that we will need to use as part of our interface.

For more information about the parallel port read the Blue Book pages 380-384. Also see the Wikipedia entry.


A pinout is a "cross-reference between the contacts, or pins, of an electrical connector or electronic component, and their functions" (Wikipedia). This provides us with the information we need when we want to understand how we can build a project which will connect to the parallel port. 

The diagram below shows the parallel port with the pins numbered. The pins appear as they would if you were looking at the port on the back of the computer. Please note that getting the left / right orientation is crucial to getting your parallel port cable built properly.

Note: This appears as if you are looking at the connector on the back of the computer.

What Connections To Make

You will need to make connections to the following pins for your parallel cable - Data 0-7 for outputs, Ack / Busy / Paper / Select as inputs, Error and two Ground pins. Label the other end of the cable when you make the connection. The data connections can just be labelled D0 - D7.

Pin No (DB25)Signal nameDirectionRegister - bitInverted

Note: The inverted column indicates that a value of 0 is indicated by +5V and 1 is indicated by a ground. Table from Wikipedia.

Constructing the Parallel Port Cable

Obtain a D-Sub solder connector and two 110 cm lengths of unshielded twisted pair cable from your teacher. Get some masking tape and a pen to label your connections. Start by stripping back about 4 cm of the blue casing on both ends. Cut off the white fiber. Strip your wires about 5mm on both ends.

Setup your solder station. Please review the soldering page and safety notes here before starting. Solder each connection and label the other end of the cable.

White Wires

Some of the cables have white with a strip of colour. Others might have four white wires - how can you figure out which is which? When you first open the cable up you will find that there is one white twisted with each colour - so if you connect the white wire twisted with orange on the d-sub connector side then you simply label the white twisted with orange on the other side.


  1. Do not strip the wire too much (about 5 mm at the most) where you will be making the connection otherwise you might have bare leads that can short each other. 
  2. Do not apply too much solder otherwise you may create a short between two pins.
  3. Label the other end of your connection as soon as you complete soldering it.
  4. Make sure you have the correct left / right orientation as you create your cable.

More Information

Read the Blue Book pages 380-384 for more information.