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Digital output extender with multiplexer

From CaveLab
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This module was part of the rack box project — which was terminated when I moved out of my first apartment.
4EXM installed in the Rack box

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


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 (duh) 3 outputs and turn them into 7. It does have some limitations though...

Xp mj-8050 dmux.jpg

I used the 3-line to 8-line decoder/demultiplexer circuit 74HC138. It has, as implied by the name, 3 inputs and 8 outputs. And the outputs are inverted, which means that 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. So 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.

Function table

Function-table-74hc138n-qxowzq.png This is function table for the 74HC138 multiplexer, it explains the relation between the inputs and the 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

4EXM Parallel 1 Parallel 2 Parallel 3 Binary value
1 32 0 0 32
2 0 64 0 64
3 32 64 0 96
4 0 0 128 128
5 32 0 128 160
6 0 64 128 192
7 32 64 128 224

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

Qty. Part
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%