In order to control large machines or plants, it is necessary to connect an electronic control 8' or Central Processing Unit (CPU) with a number of reporting, positioning, and measuring peripherals. This CPU is usually not in the immediate vicinity of the peripherals. In order to obviate the need to run separate lines from each of three connected devices to the CPU, an input/output system is connected to the CPU by a data bus for transmission of signals in both directions. An input/output system is often exposed to dust, cuttings, and/or moisture generated at the machine or installation, so the present invention features dust-and-water-resistant seals 5, 17, 26, 28, 52 at all sockets. A plug distribution box 1 with sockets 7, 8, 9 for power supply, bus, and diagnostic connections can be extended by input/output modules 2 which plug into box 1. The modules 2 in turn have sockets 21 for connection with reporting, positioning, and measuring units.