For the past week or two, we've been annoyed by "ISS first loop is maybe off?" message in ISC_LOCK guardian, preventing guardian to go to DC. The guardian checks PSL-ISS_LOOP_STATE_OUTPUT, and if it's 32700 it passes, otherwise it wouldn't.

However, when Sheila looked back, PSL-ISS_LOOP_STATE_OUTPUT was 32000 (NOT 32700) in most of O1.

It turns out that LOOP_STATE can only become 32700 when ISS auto locker is OFF: LOOP_STATE is 32700 (1st loop ON with integrator, manual), -32700 (1st loop ON, integrator OFF, manual) or 0 (1st loop OFF, manual). The numbers are hard coded in MEDM screen buttons.

When the ISS auto locker is ON, LOOP_STATE is dictated by the state machine C code in the front end and it only outputs 32000 (1st loop ON with integrator), -32000 (1st loop ON, transition to integrator ON) or 0 (1st loop OFF, transition to 1st loop ON).

Somebody should have changed the guardian recently when we had a problem engaging ISS auto locker just to get by. But the reality is that LOOP_STATE output is sent to DAC to control the analog board, and there's no meaningful difference between 32000 and 32700 as far as analog switching is concerned.

Since ISS auto locker is working OK with larger diffraction, we changed it so it fails when PSL-ISS_LOOP_STATE_OUTPUT is smaller than 32000. This way, even when ISS auto locker has a problem, operators can change ISS to manual mode and the guardian will still pass.