Abstract: A switching system (1) connects with a host computer (6) via a GPIB bus (5). A controller (3) in a chassis (2) slot transmits control signals to up to twelve switch modules (4). A passive backplane (20) is used to address the modules (4) according to voltage levels sensed on pins (A15-A18) in the connectors. The controller (3) uses the backplane to provide a 16-bit data bus (21), a module address bus (22), a register address bus (23), a control bus (24), an auxiliary bus (24) and a power supply bus (25).