J. Kissel, F. Clara,

In the background, I'm still thinking about why the sign of the RMs' OSEM PD readback is negative (LHO:84462), and had the opportunity today to make some sanity checks via measurement that can provide some pillars of truth in a very the confusing journey. 

As a part of the journey in understanding the differences in PD readout configurations that we have (G2500980), I was asking around and the current state of institutional memory is that "we do NOT bias the OSEM PDs." This contradicts what I thought was the decision made in 2011 per G1100856 that "we should just bias all OSEM PDs, because the AOSEMs' need it and the BOSEMs' performance doesn't change." 

This boils down to the position of a jumper that either connects or disconnects the satamp bias voltage to the OSEM PD's cathode or anode (yes, we have in-vac cabling to the PD + satamp connections that create both positive or negative reverse bias transimpedance configurations when the jumper connects the bias to the cathode or anode, respectively).
    - D0900900 / D0901284 :: UK 4CH SatAmp :: Negative Reverse Bias config when jumper connects -10V bias to anode.
    - D1002818 / D080276 :: US 8CH SatAmp :: Positive Reverse Bias config when jumper connects +10V bias to cathode.
    - D1900089 / D1900217 :: US 4CH SatAmp :: Negative Reverse Bias config when jumper connects -10V bias to anode.

The confusion lies in the language. Any one of the following equivalent notations / abbreviations are used throughout OSEM drawing and presentation literature, and it's *definitely* inconsistent:
    - biasing the PD = "photo-conductive" mode = pc mode = pc = AOSEMs = "LIGO OSEMs" = L
    - zero bias = "photo-voltaic" mode = pv mode = pv = BOSEMs = "Birmingham OSEMs" = B

The drawings for the SatAmps themselves use the "L" (for biased) and "B" (for zero bias), and that's what's printed on their PCB board.

To confirm that "L" = biased, while working on characterizing the UK satamps in prep for ECR E2400330, Fil and I did a quick sanity check that 
    (a) with the jumpers in the "L" position, that
    (b) -10V comes out on pins 12, 9, 6, 3 of the J1 "VACUUM TANK" DB25 port,
implying that, since the UK SatAmps -- if indeed all jumpered to the "L" position -- and in-vac wiring connect these pins to the OSEM PD's anode, then the OSEM PDs are in a negative reverse bias configuration.

We confirm this: that UK SatAmps with their jumpers in the "L" position put -10V on pins 12, 9, 6, and 3.
See attached picture series.

What we cannot confirm -- in any quick way -- is going out to each satamp, and seeing that the jumpers *are* in the "L" position.
The same institutional memory does think they are jumpered in the "L" position, but 10-20 years after setting some of them, folks are not sure any more.

BUT -- we *can* confirm as we have to go inside each UK SatAmp box to slowly roll out ECR E2400330, and so we will.