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?
Best Answer
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.