The backplane circuitry has three main functions beyond making interconnect more efficient. There is full duplex RS-232 communication for each of the 112 sensors boards. The backplane PCB converts the communications string from digital logic levels to true RS-232 levels. The backplane PCB also contains galvanic isolators in each RS-232 communications channel to isolate each bank of devices undergoing test. A third function of the backplane PCB is digital mode switching between a 4-20mA current loop or a 0-10V analog voltage output to the test data acquisition system. This allows the client to test different versions of the device-under-test (DUT). Low loss analog multiplexers are used to insert or remove a precision resistor from across the measurement channel, effectively changing between current loop or voltage acquisition mode. The test mode is controlled through software and each control line toggles banks of 16 DUT's using circuitry on the backplane PCB. The data is acquired and compared to expected values to determine pass/fail status.

The backplane PCB was quite large, measuring 15.5" x 13.75". For structural rigidity, the PCB was fabricated using a 4 layer, 0.093" thick FR4 PCB. A polyurethane conformal coating was applied to the completed system to prevent problems due to the high humidity environment within the test chamber. The system utilizes two circuit boards of the same layout with different population schemes mounted side by side. This allowed for economy of design efforts since one board layout is used twice in the system. A series of daughter cards break out the signals from a 25-pin D-sub connector into four different connectors to allow for four units to be connected per daughter card. The daughter cards are low cost interconnect PCB's and are consumed when their connectors fail. This gives the backplane PCB a long life expectancy whereas the simpler daughter cards take the wear during high volume testing.