A system controls access to plural devices with only four lines by grouping the devices in nodes, each node including a predetermined number of the devices and a local control circuit defining a shift register with positions respectively connected to the devices. The nodes are connected in series with a host controller, to form a system shift register. The nodes are serially addressed by a serial data message from a DATA OUT line of the host controller, which message includes M.times.N data bits followed by a strobe indicator, where N is the number of nodes and M is the number of devices at each node. All controllers are connected to a V+ line and a COMMON line and a RETURN line. The system register forms a fourth line, with one end connected to the host controller DATA OUT terminal and the other end connectable to the RETURN line.