Abstract: An automatic circuit tester (10) employs a scanner (20) embodied in a group of interconnected scanner boards (46, 47) that provide switching by means of mechanical relays. The scanner boards plug simultaneously into respective instrument boards (44) and into a common scanner bus (50) separate from an instrument bus (38) that carries the signals that control the instruments on the instrument boards (44). The scanner bus provides a common pathway for signals to travel between instruments or system pins to which one scanner board is connected and those to which another is connected. It also provides scanner-control paths so that the instrument boards to which the scanner boards are connected to not need to provide such paths and thus do not need to be custom-designed for the particular tester in which they are used.