My 3150-MCM block IDs are incrementing until I connect the communications cable. Once the cable is connected, the block ID stops incrementing.

Verify the cable matches the diagram on page 50 of the user manual. If pins 7 and 8 are not jumpered, and there is communications on pins 1 or 9, your backplane transfers will fail. The error status data will also show an error code of 255.