Here are the two photos of the Detector System. The first shows the two detector boards, with signals from the opto-transistors entering at the bottom and detected signals exiting over the top of the boards. Note the vertical position. Originally these boards were mounted flat, but once it was found that voltage conversion was necessary, the detector system was "out of real estate" and, much like municipal planning, had to opt to go upward.
The second shows the 12-volt detected signals coming over the boards and down into two 8-channel voltage converters. The green LEDs show that the voltage converter boards are powered up. The red LEDs show that the input signal and output signal for that particular detector position (boundary between two railroad electrical/track blocks) are both active. This is verified by running the test program listed previously.
Exit the voltage converters, each signal goes through an 8.2 k-ohm current limiting resistor and into the ribbon cable to RasPi IO-Pi detector board channel 23.