I got a device with a 16-pin connector. One of the functionality tests consists of shorting every pair of pins (the first with the second, the third with the fourth, etc.) and measuring the voltage between A and B (B is grounded). The device itself has multiple states. Every state has 2 measurements: one unshorted and one shorted.

The test goes like this:

  1. Go to next state.
  2. Measure voltage A-B
  3. Short pins consistent with that state
  4. Measure voltage A-B
  5. Remove short
  6. Goto 1.

So the setup looks like this:

Description of 6 pins, 3 shorting switches and measuring points A and B

Where the connectors X1-1 through X1-16 (not all are drawn) are available at the exterior of the device and the switches are added only during test. What they are connected to and how A-B is determined is quite irrelevant.

The important part is I want to automate this process. The current switches are hand-toggled. The easy solution is replacing every switch with a non-latching relay. However, this would require 8 relays and 8 I/O on whatever switches them. The current going through them is negligible and the most important part is the resistance is as low as possible.

I'm looking for a solution requiring less I/O. At all times only one relay will be active at a time, so I'm hoping a demultiplexer or something similar might make it less I/O consuming. Even better alternatives are welcome as well. Resistance between pins should be as low as possible, say below 0.5 Ohm.

Driving secondary test units with a low I/O solution like I2C is unacceptable unfortunately.

  • \$\begingroup\$ What voltages are present on the wires? What current flows when you short them? \$\endgroup\$ – Jack Creasey Dec 21 '16 at 22:47
  • \$\begingroup\$ @JackCreasey Next to no current, X1 is a digital signal bus. A handful of mA at best. Voltage on the wires is 5Vdc maximum. \$\endgroup\$ – Mast Dec 21 '16 at 22:54
  • \$\begingroup\$ So your cable is Dig1 - Gnd1 - Dig2 - Gnd2 - Dig3 - Gnd3 ..... 8 signals and 8 Gnds. You want to check open circuit signal/short to adjacent, what about faults from Dig2 - Gnd1? If all you have is 5 V on the cable, why use relays? What are the 'states'? \$\endgroup\$ – Jack Creasey Dec 21 '16 at 23:51
  • \$\begingroup\$ No, I have 16 pins. Whatever is on those pins is irrelevant at the current tests, but they'll never exceed 5V relative to each other or GND. Those states simply means the wiring of the device under test can be different relative to other states. The wiring should be largely irrelevant. All I need is to automate connecting some pins to each other where I only need one connection at a time. Look at it like a moving jumper. \$\endgroup\$ – Mast Dec 21 '16 at 23:57
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    \$\begingroup\$ Agree with the demux IC. More common is the CD4051. Use 3 IO bits to select one of 8 relays to drive. If you need a state where all relays are off, then use one more IO to drive INH high. Then use each demux output to drive each relay driver. Also, you can use a relay IC instead of 8 transistors+resistors+diode clamp. ULN2003 is very popular and old but only has 7 outputs; can find similar with 8 drivers. The whole solution can be done in 2 cheap old ICs and 3 digital outputs. \$\endgroup\$ – Vince Patron Dec 22 '16 at 0:41

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