The test methods used on digital components are referred to as inhibiting and disabling for the isolation of non bussed and bussed components respectively. These methods of test are combined with the backdriving of known signal levels for very short periods of time. The backdriving capability of the tester to limit current, voltage, and the duration of the backdrive signal, minimizes possible damage to components.

Backdriving is a technique in which a current pulse, of short duration, is applied to an output of an integrated circuit to force the output transistors to a known logic level.

Problems in testing analog components are created when a component cannot be isolated, that is, a sensing probe cannot make contact with the circuit because of the hardware layout. When this situation occurs, it is essential that accessible test points be provided on the board in the form of blank pads. This must be factored into the final layout of the printed circuit artwork.

Problems occur when other components in the circuit prevent the measurement instruments in the tester from testing a single specific component. To isolate non-bussed components, the test system software must be able to inhibit and back drive the outputs of other devices connected to the component under test.

When possible, provide test pads for all unused gates. Free running gates may cause instability during in-circuit testing and actual operation. The test pads will provide a means to inhibit the spurious signals.

Another method is to put the driving components outputs into "known state," preferably output "high." It requires less current to drive a "high" to a "low" than a "low" to a "high" for most logic families. This can be accomplished by providing the proper signals to the device and interrogating the device outputs until they are stable. This method is more complex than using the reset or clear functions and will often require additional and custom programming of the software for the tester.

This is typically done by tri-stating the outputs of bussed components via the control function of the component. The use of pullup or pulldown resistors is a simple way to provide access to the control functions. It is imperative that control inputs are not tied directly to ground or power sources.

After the components on a buss are disabled, a test of the buss is performed. Each component is then placed back on the buss and individually tested. The disable testability requirement is essential to the successful testing of microprocessors, memories, and most LSI, VLSI, and ASIC devices.