Abstract: Bus-based computer system circuit modules are developed and tested in a desk-top apparatus including a base supported for select tilting upon a cradle. The base, in turn, supports a backplane support assembly which, in turn, supports a top assembly to define a card cage having open sides. A backplane into which the circuit modules may be plugged is supported by the backplane assembly, and access to the active components of the circuit modules is provided through the open sides. The rearward face of the backplane is made accessible for wire wrapping procedures and the like by mounting the power supply for the apparatus below the backplane and providing for the tilting of the assemblage downwardly and forwardly to improve rearward face access. Both voltage and current are monitored at the backplane and the values of these parameters are displayed at digital meters mounted upon the top assembly. The latter, current monitoring is carried out using closed loop Hall effect sensors.