Board uses 18 "ternary" mechanical switches, 9 "ternary" red-green LEDs, 27 resistors and 18 bipolar transistors (9 PNP and 9 NPN). Scheme consists of 9 identical blocks:
Lower 9 pins of TRInn elements may be inputs or outputs ("IO" on the scheme). Higher 9 pins are usually inputs ("I" on the scheme). Improper usage of that board may cause short circuit of power supply - if ternary switch wrongly will connect +5V or -5V to output pin when tested board will provide -5V or +5V to the same output. So all switches that connected to outputs MUST be in neutral position! As defense against this condition we can put additional 1K resistor before every IO.