Now the module is inserted at the 2nd crate from the top of 1Y2 (LSC analog rack). It is next to the DCPD whitening module.
I found the backplane cable for the Common Mode servo module.
I traced a cable form XY220 at the right most module on the crate where iscaux2 is living.
This cable was connected to the upper backplane connector.
Switching of the module is tested. All the switches and gain control are doing their job.
It was found that the offset and slow readback are not responding.
I checked the schematic of the CM servo module (D040180).
It seems that there is another cable for the offset and read back voltages.