Front of the parallel I/O module, with switches and LEDs.
Front of the parallel I/O module, with switches and LEDs.

With this unit a regular computer parallel (printer) port will give you 5 inputs and 8 outputs to use for something fun. And it’s very simply to build; one transistor and one resistor per output, and a single resistor for each inputs. The parallel port is pretty easy to access in most programming languages. The module as a 36 pin centronics connector, making it easy to connect to the computer using a regular printer cable.

Table of content

I/O

Inputs

The parallel port has 5 status pins that can be used as inputs by pulling them low using a 120 ohm resistor to ground. Some experimentation might be needed as not all the status pins have the same behavior with regards to high and low state.

Outputs

The parallel port also has 8 data pin with TTL-level, meaning active 5V and inactive 0V. Using an NPN transistor with a 4.7K resistor will give you a open collector output. Each output has a LED indicator.

Terminal connectors

Backside of parallel I/O module, with terminal block and centronics connector.
Backside of parallel I/O module, with terminal block and centronics connector.
  1. Output 1
  2. Output 2
  3. Output 3
  4. Output 4
  5. Output 5
  6. Output 6
  7. Output 7
  8. Output 8
  9. Input 1
  10. Input 2
  11. Input 3
  12. Input 4
  13. Input 5
  14. When-on (active when module is turned on)
  15. +12V
  16. 0V

Parallel port

DB-25 pin-out

Pin out for a computer parallel (printer) port.
Pin out for a computer parallel (printer) port.

The data pins (D0-D7) are outputs, and the status pins (S3-S7) inputs. The green pins (18-25) are ground. See table below for details.

DB-25 to Centronics

Description DB25 pin Centronics pin
Strobe 1 1
Data bit 0 2 2
Data bit 1 3 3
Data bit 2 4 4
Data bit 3 5 5
Data bit 4 6 6
Data bit 5 7 7
Data bit 6 8 8
Data bit 7 9 9
Acknowledge 10 10
Busy 11 11
Paper out 12 12
Select 13 13
Autofeed 14 14
Error 15 32
Reset 16 31
Select 17 36
Signal ground 18 33
Signal ground 19 19 + 20
Signal ground 20 21 + 22
Signal ground 21 23 + 24
Signal ground 22 25 + 26
Signal ground 23 27
Signal ground 24 28 + 29
Signal ground 25 16 + 30
Shield Cover Cover + 17

Please leave a comment or tweet if you have any questions or feedback on this project.

Schematic drawing

Schematic drawing for parallel I/O module.
Schematic drawing for parallel I/O module.

Image gallery

Parts list

Project: I/O module driven by a computer parallel port by Thomas Jensen is licensed under CC BY-SA 4.0 with attribution required.

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  •   Created Jan 01, 2002
  •   Last modified 1 day ago

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