I have switched from an 'A Revision' unit to a newer 'B Revision' unit, I am now having problems with RS422 4wire communications. Why?

The 'B Revision' units have the transmit/receive pair swapped round on
the port pin out. Please see the 'Introduction to the MI Series' topic
in the Config Manager help file for a pin out diagram. Please note that
the pin out for the expansion pod is as it always was, and so is
different from the 'B Revision' wiring for 4wire comms.