I'd be inclined to make the RS485/422 work. The interface chips are real small and don't require external components except a terminating resistor.

Depending upont the frequency of data and your processor load, you might want the simplicity of a loop. Every node listens in one direction, processes any packets addressed to itself, and passes everything else in the opposite direction. You obviously have to have a handle of the communication density for this system to avoid everyone's buffers from overloading. But it works well in exception handling monitors.

If you do, make sure that you hand-off traffic at high priority. If you stop to process your packets and ignore the loop, everyone suffers. Also be aware that one cable break or disconnect or system lockup affects the whole system. For this reason its not often chosen.

The other challenge is a contention system that most will suggest. That's more the norm because embedded system programmers are attracted to the challenges. :) And if one unit fails, the rest can continue. Its also much easier to initialize than the loop which must await a loop-traversal message arrival before running.

aka J