Test Pattern Generator

16 TEST PATTERN GENERATOR A. Rigby A programmable digital pattern generator for all of you who do not have access to specialized equipment for faultfinding in digital circuits. The operation of many digital circuits is, in principle, not simpler or more complex than that of analogue circuits, but the de- pendency of certain signals on others gives digital faultfinding a labyrinth ef- fect. Furthermore, it is often impossible to isolate a particular section of the circuit for a stand-alone test. The omission of a single control signal, whose function may not be known at all initially, may cause the whole circuit to "stall", making it im- possible to track the cause of the malfunc- tion. In synchronously operating digital cir- cuits, state changes take place only as a result of clock signal transitions. This makes the operation of this type of circuit relatively simple to follow, especially if the clock signal or signals can be gener- ated by external means. By contrast, a level cha...