To test passive components, the tester places a test voltage across the component under test and measures the current. An in-circuit tester tests a digital IC using the IC's truth table. The tester sets the inputs to all combinations of 1's and 0's and looks for the correct logic values on the outputs. The tester does this with a process called backdriving, in which it applies a high-current pulse for a short period that overrides the logic value on the input. To perform an in-circuit test, you need a test fixture that connects the circuit nodes on the board under test to the tester.
Wires running from the tester interface to the probes carry test signals back and forth.
A technician places the board under test on the fixture and activates the vacuum, which pulls the board down onto the pins. To keep up with changing PCB technology, manufacturers of in-circuit testers have added features in recent years. With boundary-scan testing, you can test interconnections on PCBs without using a probe on each node. To perform this test, you need a fixture that positions a metal plate over the circuit under test. The tester measures the capacitance between the plate and each lead of the circuit under test.