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The CS signal may not need a differential driver since it is static during the SPI transfer. So you would need 2 differnetial transmitters and 1 receiver on the source end and two recievers and 1 transmitter on the destination side for bidirectional transfers. The CS can be just a regular buffer chip, like a 74HC125.
Tim Barr
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It is true that the CS signal is not high frequency, but if any signal is driven by a logic gate, then there can be at most, a 1V difference in the GND levels of the two devices.
Since your units are separated (and most likely powered by different power supplies that are plugged into different outlets), the possibility of offsets and common-mode noise is high. Most 485 devices allow at least 5V between the GNDs.
The differential nature of the signaling greatly reduces induced noise as well.
Charles Linquist
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