Complex electronic systems are structured into functional units, printed circuit boards and components. During the manufacturing process testing starts at the component level and is carried out progressively at PCB, functional test and finally total system level.

When the equipment is in service, the reverse process is followed. That is, at the system level the suspect unit is diagnosed and replaced. This unit is then tested and if faulty diagnosed to the faulty board. This in turn is tested to find the faulty component. The testing of functional units, boards and complex components (referred to here as the ‘box’) is carried out on Automatic Test Equipment both at the factory and field level. In each case the test problem needs to be addressed during design to ensure that adequate access is given to the internal function of the item.