PROTEUS Wallstand Longitudinal Detent Problem

PROTEUS OTS longitudinal locks didn't come on at the Wallstand longitudinal SID positions with the DETENT button pressed. PROTEUS software on one system is 2.08 and the other is 2.09, System I/F Board firmware on both systems is V1.5. See long description: Both sites have a non-tilt Wall Stand. The OTS detents at lateral center, the OTS UIF has a green Exposure OK indication, the collimator is in auto mode and sizes to the cassette. The SID code LED indicators on the System I/F Board agree with the codes displayed on the OTS I/O Board indicating the correct magnet code for each SID position is being read by the System I/F Board. On the System I/F Board, if U17 pin 11 (DETENT_VALID) is jumpered to pin 12 (P21V), the OTS locks come on, indicating that there is not a connection problem. It appears that the CPU on the System I/F Board is not asserting the "P17" line to drive U17 to output the DETENT_VALID signal.