[Halld-pid] Start Counter puzzle solved

[Beni Zihlmann]

Hi All,

I think I got to the bottom of the Start Counter puzzle regarding the 
rotation
of the geometry of about 90 degree after re-installation of the Start 
Counter
around the target in fall 2022. In the process I also found out why we 
always
had an offset of pretty much 1 counter in phi from the start.

1) Counter #1 at phi 0 is NOT connected to the first sector of a readout PCB
     that is mounted on the ring but rather the third sector of a 
readout PCB.
     And in the past (pre CPP/NPP) this was "read out" by the last of 10 
break
     out PCB-boards. So In other words Counter #1 ended up to be Counter #30
     or in other words at 360

2) The signal cables that break off from the PCB-readout board to the 
patch panel
     are only labeled on the end that connects to the patch panel. On 
the PCB-readout
     board side these cables are soldered to the board and are NOT labeled.
     Therefore, when these cards were disconnected from the Start 
Counter ring and
     the LV cables were also unplugged from these PCB boards the 
correspondence
     between LV cable/PCB-readout board/PaddleNumber got lost.

3) When the PCB-readout boards and LV cables were reconnected the procedure
     followed the numbering scheme  found on the aluminum ring-hub of 
the Start
     Counter. There you find clear numbers written from 1 to 10 starting 
at 12 o'clock.
     Now the PCB boards have NO number but the three signal cables are 
labeled and
     natural thing to do was to connect the board with the three cables 
labeled 1-3
     to label number 1 at 12 o-clock. The LV cable is labeled and 
correctly corresponding
     to the number written on the aluminum ring. I guess that is the 
reason these
     numbers are on the aluminum ring in the first place to make sure 
the LV cables
     meet the correct location. However, now all the PCB-readout boards 
are shifted by
     two slots clock wise looking in beam direction. This is pretty much 
a shift of 90 degrees
     and is what Simon found in his analysis.

4) the good news is that the SiPM LV was always correct for each sensor 
and never
     changed.

sources:
https://halldweb.jlab.org/wiki/index.php/Start_Counter_Electronics_Channel_Maps_and_Cabling
https://halldweb.jlab.org/wiki/index.php/Start_Counter_Silicon_Photomultiplier_Read-Out

cheers,
Beni