J. Kissel,

Yesterday, I built another DB9 "shorting plug," similar to that from LHO:67465 that shorts all DB9 pins to Pin 5 without shorting to the backshell of the DB9 connector -- see pictures in the attached 2024-06-06_SEI-C2_DifferentialChanShortingPlug.pdf. Yes, the internal wiring looks sloppy, but I've confirmed that it functions as desired electrically.

While the IFO was recovering from a routine lock loss, I installed the plug into the open-and-unused "In 25-28" port of the aLIGO HAM-ISI Anti-Aliasing Chassis (D1000269) in U28 of SEI-C2 in the Corner Station CER. See pictures of where in the attached 2024-06-06_SEI-C2_ShortingPlugInstalled.pdf.

These 4 channels are otherise unused channels on the seih23 IO chassis' ADC2, which otherwise houses the HAM3-ISI system's sensors and coil driver readbacks (channels ADC2_24 thru ADC2_27). These channel inputs were originally intended to be for analog copies of the ground STS2 (which are now all T240s), created because in the early 2010s, we didn't yet trust the inter process communication (IPC) system. We don't do this, and honestly never ran the cabling -- we've been successful in mapping all ground analog signals from GND T240s into the seib1 IO chassis, reading them out with h1isiitmy front-end model, and then shipping them to h1seiproc via digital IPC, processing them into a sensor correction signal, and then sending over IPC again to each ISI, including HAM3. 
Note -- the HAM ISI wiring diagram, D1101576, inaccurately portrays what channels are unused on this AA chassis at LHO, so I also took the time to create some "as built" photos of the SEI-C2 rack, which are are now linked from the rack's S-number DCC page, S1301863 and will reach out to Dean/Arnaud to get it updated.

I do so for two reasons:
    - For the future HAM3-HAM2 SPI pathfinder system (E2400121), the pitch / yaw "two-way optical lever" QPD system will be limited by ADC noise, so I want to quantify that noise in the real LIGO IFO system down to as low a frequency as possible. Indeed, this rack, and this ADC is what may be used for the pathfinder -- during the conceptual design review, a suggestion was made to look for spare channels in the SEI system before assuming the SPI needed to buy another ADC card; see M2300216.
    - Given recent DuoTone investigations (LHO:78238 and references therein), these channels might prove as an interesting base-line for "normal" shorted ADC channels, like the shorted channels on the low-noise, 524 kHz ADC from LHO:67465.

The read back channels for these will be
    H1:IOP-SEI_H23_MADC2_TP_CH24    
    H1:IOP-SEI_H23_MADC2_TP_CH25 
    H1:IOP-SEI_H23_MADC2_TP_CH26   
    H1:IOP-SEI_H23_MADC2_TP_CH27  
They've always existed, it's just that the differential inputs are now connected to the well-defined 0V GND reference of the AA chassis (which is connected to the same 0V GND reference for the ADC) -- rather than floating. They're sampled at 65536 Hz and not stored in the frames, so only live studies will be possible.

Stay tuned for measurements.