I am designing a system based on this application note of TI. The overall design goal is to have multiple shift registers, connected via RS-422 and ethernet cables over fairly long distances(up to 5 meters between each node). Each node should convert the differential RS-422 signal to SPI for the shift register and after that back to differential the RS-422 for daisy-chaining nodes. Each node can have a single power supply or - if spatial possible - share a power bus with neighbouring nodes.

My plan is to have MOSI,MISO and CLK as differential signals in a twisted pair, since they will have the highest data rate(not specified yet). Using standard ethernet cables this setup will use 3 of 4 twisted pairs. This leaves me with one twisted pair.

For clean initial conditions I would like to have STROBE/LATCH and OUTPUT_ENABLE for clean operations as control signals. Is there any problem using the remaining twisted pair non-differential for the control signals?

  • \$\begingroup\$ You'll have 5m of the two signals tightly twisted together, so crosstalk will be severe unless the edge rate of the signals is very low. \$\endgroup\$ Oct 6, 2021 at 2:48
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    \$\begingroup\$ What's providing the ground reference among the nodes? This would also be carrying the return current for any single-ended signals. \$\endgroup\$
    – Dave Tweed
    Oct 6, 2021 at 3:23
  • \$\begingroup\$ Dave: Thats a good call! Thanks - haven't thought of that. In this scenario the ground reference would be provided by the different power supplies or the shared power bus, which would create a hell of a ground loop - not good. I guess I need to have one ground wire in the ethernet cable and do some circuit magic at each node for my control signals \$\endgroup\$ Oct 6, 2021 at 3:29
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    \$\begingroup\$ SPI may not work effectively when slave responds to master due to accumulated delays. \$\endgroup\$
    – Andy aka
    Oct 6, 2021 at 7:08
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    \$\begingroup\$ For 5m distance if everything is well done, you could push max. 25Mbit/s. Not to mention the complexity and cost of such circuit. It would be economically better to use a separate MCU for each node and some standard communication like CAN, RS485,... \$\endgroup\$ Oct 6, 2021 at 7:30


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