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DIY 20 channel scanner card / MUX for any DMM

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trtr6842:

--- Quote from: tszaboo on November 29, 2023, 06:23:48 pm ---Cool project. Few tricks, when I made a project like this:
The DMM has a "VM complete" and a "Trig" coax on the back. If you connect that to your relay board, you can do many things with it. The VM complete can be used to set the relays to the next channel, and then the Trig can be triggered with a delay. So the next measurement can be taken without any communication to set it up on the relay board.
The other trick with a 34401A is the serial port. You can connect it to something other than a computer's USB port.
The relay's thermal EMF can be reduced if you use relays that are latching. Because the coils don't heat up internally.

--- End quote ---

These scanner cards wouldn't be able to use those signals directly, since they are just controlled via SPI.  But whatever host controller is running the SPI interface could easily use those signals!

For my applications I've always connected the test PC directly to the DMM, but I could see some applications where instead connecting it to a MCU or SBC could be helpful.

I considered using latching relays, but the miniature ones were not stocked, and they required a full-bride coil drive circuit. Then significant effort is required to ensure thay stay in a known state.  If they are used in a binary tree, then it's not critical, but then you introduce a lot more series contacts.  In a parallel architecture, suddently it becomes safety-critical to ensure that all relays get properly reset before turning on a different channel.  That would definitely require an MCU on every card to safely control.  If I ever need a metrology-grade MUX I would definitely go that route though, but for this project I kept it more suitable for general purpose use, and I was able to keep the costs down.

Doctorandus_P:
Are you aware of these other scan cards:

https://www.eevblog.com/forum/circuit-studio/example-project-relay-scan-card-for-k2000-dmm/

https://www.eevblog.com/forum/circuit-studio/example-project-20-channel-solid-state-scan-card-for-k2000-dmm/

They have full schematic and PCB layout, and I think I once imported one of them into KiCad too.

trtr6842:

--- Quote from: Doctorandus_P on November 29, 2023, 07:32:36 pm ---Are you aware of these other scan cards:

https://www.eevblog.com/forum/circuit-studio/example-project-relay-scan-card-for-k2000-dmm/

https://www.eevblog.com/forum/circuit-studio/example-project-20-channel-solid-state-scan-card-for-k2000-dmm/

They have full schematic and PCB layout, and I think I once imported one of them into KiCad too.

--- End quote ---

I have seen those!  For my specific applications they were missing a few key features.  I needed to use these in fairly high-noise environments, so ensuring that all relays are commanded safely even with SPI control line glitches was important.  Also, using an IDC header for the inputs instead of screw terminals made installation in my ATE rack units simpler since they could be connected directly to a panel mount DB-25 connector with just ribbon cable.

fenugrec:
Very nice, thanks for sharing.
I wonder if a 1-to-many decoder like e.g. 74*4514 (4-to-16, latching) or 74*138 (3-to-8) could've simplified your design ? That is of course if you trust those to only assert one line at a time.
But then I notice you also enforce 'only 1 relay' for the whole stack, not just one card, so it may not help much at all.

trtr6842:

--- Quote from: fenugrec on January 07, 2024, 03:51:11 am ---Very nice, thanks for sharing.
I wonder if a 1-to-many decoder like e.g. 74*4514 (4-to-16, latching) or 74*138 (3-to-8) could've simplified your design ? That is of course if you trust those to only assert one line at a time.
But then I notice you also enforce 'only 1 relay' for the whole stack, not just one card, so it may not help much at all.

--- End quote ---

I did not consider those, as I've never used them before.  At first glance, I think I could have replaced two of the shift registers with these.  Instead of using the outputs of the 3 shift registers directly, I could decode the output of just one shift register.  I still think I would need the D-flops to check the decoded command bits before committing them to the relays, since that is required to prevent conflicts between multiple cards.  I probably could have removed the card's local summing node, as the decoders would guarantee that only one command is active locally, and then I could leave the multi-card summing node and checks as-is.

Thanks for bringing that up!  I may use those for another revision.

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