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To use SPI over a 5m long cable we plan to use LVDS RS-644 which calls for a twisted-pair cable with 100ohm characteristic impedance.

Evaluating our cable options we looked into Cat5 cable (originally for ethernet but perfect for LVDS), but unfortunately we need at least 5 pairs: 4 pairs for SPI (reception, transmission, chip select and clock) plus two wires for symetric power whereas Cat4 provides only 4 pairs.

We seem to come across all kinds of high quality 100ohm cables with more than 4 twisted pairs but all seem to be specialty products. Ethernet cables (only 4 pairs) are available with ease but the rest are much more difficult to source.

Any suggestions on other cable standards that could facilitate sourcing them in short lengths but still be specified as 100ohm for high-speed data links?

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You could probably go the same route that Power over Ethernet (PoE) goes: Have the supply voltage be a DC offset on pairs of the cable, and couple in your receiver through transformers with a center tap on the biased side.

5m sounds a bit much for LV differential, but that really depends on your equalizing abilities.

Also, at some point, the overhead you're incurring by extensive cabling and dedicated differential transceivers might be more than it's worth – assuming your application could withstand the latency, have you looked into using one microcontroller on each end of your 5m distance, translating the unidirectional signals, clock and chip select to a bidirectional bus? I'm thinking about a really minimal system based on USB (that's the cheapest long-distance bus that comes to mind). Or, CAN, which is actually designed for this kind of problems. For both, there's cost-efficient MCUs that come with integrated PHYs.

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  • \$\begingroup\$ Since I have an ADC with an SPI interface at one end, using one or two 4chan transceivers and going LVDS seemed a convenient solution. I am puzzled about your comment on 5m being too long for LVDS, isn't RS-644 supposed to be good for 50m at 50Mbps? (see e.g. here interfacebus.com/Design_Connector_RS644.html). \$\endgroup\$ – Eric T Aug 3 '17 at 10:43
  • \$\begingroup\$ Your suggestion to use USB or CAN is also a good one. I want to avoid getting stuck developing substantial code for a MCU though and will look for rapid development solutions for USB and CAN. \$\endgroup\$ – Eric T Aug 3 '17 at 10:45

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