I opened DBB shutter for HPO to make some measurement. After 30 seconds or so, the DBB moved to "interlock" mode. Resetting the interlock by going to "stand by" and then to "manual" immediately brings the system back to "interlock" mode.
This is not triggered by the oscillation monitor (so called "software interlock" in T0900576), as the oscillation monitor output (H1:PSL-DBB_INTERLOCK_LP_OUT) never goes above threshold (currently set to 1000). It should be so-called "hardware interlock". However, PSL documentation is kind of convoluted I have a hard time finding what closed the shutter when there's a "hardware" interlock trigger, is it DBB frontend or is there a parallel hardware circuit that closes the shutter regardless of the DBB frontend?
Anyway, according to Jason this has been a problem for a while and then it went away on the day of power outage.
Filed FRS #6330. I asked Fil to take a look at the AA and AI chassis associated with the DBB during the next maintenance window (10/4/2016). If those check out the problem is likely in the DBB control box that lives in the PSL rack in the LVEA.