In SX Microcontrollers, SX/B Compiler and SX-Key Tool, mgreen wrote:

You will have problems with high speed and distances more than a few inches.  If you use RS485, you will have better signal levels on one particular line, but the added delays through the driver/receiver will slow you down.  SPI is already a serial protocol.  An RJ-45 jack provides the mechanical and electrical connections, but you will want twisted pairs for the clock, MOSI and MISO, and SS signals with the 2nd wire of the twisted pair being ground.  You could carry Vdd and ground in a 5th twisted pair.  This isn't a standard Internet cable since it needs 5 pairs, but only 6 signal connections.  If you can combine MOSI and MISO onto one line (if the devices are half-duplex), you can manage with only 4 twisted pairs.  As pjv suggested, you'll have to experiment to see how high the speed can go reliably.  If you need to push it, you may need to incorporate some kind of error checking (like a CRC or checksum) to protect against the occasional errors.

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