Back-driving is the process of forcing a node to a particular state when other devices on the node may be driving to the opposite state. Disabling the output is much preferred over back-driving, because in some cases back-driving can damage the device. If the outputs on other devices can’t be disabled and they connect to outputs on the DUT, then it is impossible to discern which device is responsible for a particular signal. This reduces pin coverage on the DUT and also prevents a complete test on the surrounding devices when they are tested.
To minimise the potential for device damage from back-driving during digital in-circuit testing, each test is time limited based on an analysis of test conditions. The conditions considered include for example, the number of overdriven outputs on a given device, the frequency at which the test patterns are applied, the device technology and the packaging of the overdriven part.