This post is part of the Rack Box Project series.

Get more digital outputs on your I/O device with a multiplexer, 3 outputs turned into 7.

Table of contents

Details

A common problem, at least for me, when using I/O devices is running out of ports. When I was using the parallel I/O device in my rack box project, I ran out of outputs. I solved that by making this output extender. It’s quite simple; it uses a multiplexer circuit to multiplex 3 outputs and turn them into 7. It does have some limitations though…

74HC138 multiplexer IC

I used the 3-line to 8-line decoder/demultiplexer circuit 74HC138. It has, as implied by the name, 3 inputs and 8 outputs. The outputs are inverted, meaning they are low when turned on. That was perfect for my purpose, since all my modules had open collector outputs, which operates the same way.

There are however some limitations to this approach; multiplexing the outputs means that only one of the multiplexer outputs can be on at any given time. For some applications this may be a problem, but it wasn’t for me; since I only used the outputs to send signal pulses to other modules.

If I wanted to pulse an output on the multiplexer I needed to turn on 1, 2 or 3 of the outputs on my parallel I/O device. The outputs on the 74HC138 multiplexer is rated for 25 mA, which was more than enough for my needs.

Green LED shows system in standby, while the blue LED is server time-out alarm.

Output extender mounted in the rack box

Function table

74HC138 function table

This is function table for the 74HC138 multiplexer, it explains the relation between the inputs and outputs. Looking at it we can see that in order to turn off all outputs we need to set Enable 3 low. So you can either:

  • Use E3 and the inputs to get 8 outputs, that gives a input/output ratio of 2.
  • Or use only the inputs and get 7 outputs, which gives a input/output ratio of 2.33.

So that was what I did; I used only the three inputs, set Enable 3 constantly high and disregarded output 1.

Binary values

Out In 1 In 2 In 3 Binary value
1 1 0 0 1
2 0 2 0 2
3 1 2 0 3
4 0 0 4 4
5 1 0 4 5
6 0 2 4 6
7 1 2 4 7

D-Sub 15-pin

  1. 0V
  2. 5V
  3. Input 1
  4. Input 2
  5. Input 3
  6. Output 1
  7. Output 2
  8. Output 3
  9. N/A
  10. Server time-out alarm LED (Main monitoring unit)
  11. Status-LED +
  12. Output 7
  13. Output 6
  14. Output 5
  15. Output 4

Parts list

  • 1 × D-sub soldering cups, 15 pin male
  • 1 × Demultiplexer, 3 -> 8, 74HC138
  • 1 × DIL socket, 16-pin, 7.62mm
  • 1 × Enclosure, plastic (1551), 50x50x20mm
  • 1 × LED 5mm clear, Blue, 4.9V, 20mA, 350mcd, 12°
  • 1 × LED 5mm coloured clear, Green, 2.1V, 20mA, 30mcd, 10°
  • 1 × LED holder 5mm, RTC51, black plastic
  • 1 × LED lens 5mm, CLF 280, Blue
  • 5 × Resistor, carbon film, 0.25W, 330 Ω, 5%
  • 3 × Resistor, carbon film, 0.25W, 10 kΩ, 5%

Last commit 2024-04-05, with message: Tag cleanup.

Rack Box Project series

  1. Parallel port I/O module
  2. Power supply and fuse monitoring module, AVR
  3. Monitored fuse box, 6 channels
  4. Stack lights and horn controller — with AVR
  5. Mute and light controller for the Rack box — AVR module
  6. Monitored fuse box, 4 channels
  7. Module heartbeat monitor, 6 inputs — AVR
  8. Controller for lights and relays — AVR driven
  9. Emergency power off controller — controlled by 555 timers
  10. Fan controller with LCD — AVR powered
  11. Sound alarm control unit — AVR module
  12. Multiplexer output extender
  13. Multi-purpose AVR module
  14. Electric heater and timer controller — AVR
  15. Module heartbeat monitor, 15 inputs — LCD and AVR
  16. Serial port I/O module with 11 inputs — AVR
  17. Serial port I/O module with 9 in and outputs — AVR
  18. Serial interface for emergency power off — AVR
  19. Status panel for the Rack box project
  20. Intruder alarm system controller — AVR
  21. Serial port I/O module with 15 inputs — AVR
  22. Serial interface module, with analog and digital I/O — AVR
  23. The rack box project — an overview