Ah, ok, I was looking in the Diagnostics Manual, which just says "serial I/O failure", and to check either the CPU or I/O board. Could have been referring to either the uPD7201 or the SCN2652. Although I'm pretty confident that the 7201 is used for the RS232 ports (so the SCN2652 is used for RS422 cluster communications, which makes some sense).
