A technique used to facilitate effective testing of the digital ICs, and their interconnections, on high-density printed circuit boards in the factory and in the field. Extra logic is included and this intercepts the logic signals at each functional pin of the logic components. The extra logic is transparent in normal operation but can be configured as a long shift register in a special test mode. The shift registers on each component are connected into a daisychain so that all the logic circuits can be accessed through a special group of test connections brought out to the edge connector. This structure is used to feed predefined information to the input pins and collect the results from the output pins, thus enabling several types of test to be performed (function and interconnect). See also back driving.