OK, I was obviously confusing the serial ports with the SV network ports on the ACM.

The issue was the SV network wasn't working properly, due to a mis-wired autopilot servo. This failed the network and made the EMS and ADAHRS units not report properly.

If anyone else is having SV Network trouble, what I learned from this is to isolate things. Unplug servos from the ACM, unplug SV network boxes. Add them back slowly until you find the culprit.