We first found that one bit in POPX demod whitening readback that corresponds to the least significant bit of the whitening gain for the third quadrant is stuck on (attached).  Corresponding BIO cable was CAB-H1:ISC_88 (i.e. the second board in the chassis).

I got suspicious about the cable but it turns out that this was the BIO itself. We removed the field cables for POPX from the BIO chassis in the CER and used an I/O tester to see the output, and the offending bit was always stuck ON (=low). Short circuit at the connector?

This is not tragic as we can still use odd dB gains (3, 9, 15, 21, 27, 33, 39 and 45dB), but it's a good idea to check these during laser upgrade.

Filter BIO seems to be OK.