Abstract: The density of I/O circuits on input and output modules in a programmable controller is doubled, while the number of optically coupled circuits is reduced, by transmitting data serially through one of the optically coupled circuits. Optically coupled circuits are also used for address and test signals and error-checking circuitry is provided to sense failures of the optically coupled circuits. The optically coupled circuits on each module are connected between two universal large scale integrated (LSI) circuits. The universal circuit can be operated on either the processor side or the machine side of either an input module or an output module by selecting a corresponding one of its four modes of operation. The multiplexing electronics on the input and output modules, aside from the optically coupled circuits, is integrated into just two components, and only one universal component is required for replacement of either of these two.