1. pot 32x16 channels = 512 Readout a pair of rows on one end and the next pair on the other. Connect the fibers to “SiPM cassettes” in 2 row by 16 channel bundles. Those cassettes each have two connectors corresponding to 2 row by 8 fiber “sections”. These cassettes allow pretesting of the SiPMs before connecting the cassettes to the mother board. The mother board simply connects the cassettes through to readout boards. The mother board doesn’t serve much of an electrical purpose, it is more mechanical (and for light tightness). Each readout board handles 32 channels, but they do so with two connectors each of 16 channels. This allows the board to be split into two halves with symmetric functionality but separately testable with calibration pieces. The readout boards have rows of pots along the edges – two rows of 8 on each edge. These are used to adjust the bias for each of the 32 channels between 24 and 30 volts max supply. This is based on a 22k pot with 100k resistor to ground. The pots have markings so they can be crudely adjusted before probing The readout cards mount to the back of the mother board with 2x16 connectors (signal and bias). There is also a single ground connection from each readout card to the detector board at the outer end. A 2x16 fiber cassette: PCB, connectors on back, SiPMs on front How the cassettes stack with interleaved rows readout on the opposite end

2. header Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO Pots, two rows along each side of board Power IO 32x16 channels = 512 Readout a pair of rows on one end and the next pair on the other. Connect the fibers to “SiPM cassettes” in 2 row by 16 channel bundles. Those cassettes each have two connectors corresponding to 2 row by 8 fiber “sections”. These cassettes allow pretesting of the SiPMs before connecting the cassettes to the mother board. The mother board simply connects the cassettes through to readout boards. The mother board doesn’t serve much of an electrical purpose, it is more mechanical (and for light tightness). Each readout board handles 32 channels, but they do so with two connectors each of 16 channels. This allows the board to be split into two halves with symmetric functionality but separately testable with calibration pieces. The readout boards have rows of pots along the edges – two rows of 8 on each edge. These are used to adjust the bias for each of the 32 channels between 24 and 30 volts max supply. This is based on a 22k pot with 100k resistor to ground. The pots have markings so they can be crudely adjusted before probing The readout cards mount to the back of the mother board with 2x16 connectors (signal and bias). There is also a single ground connection from each readout card to the detector board at the outer end.