Abstract: A programmable controller suitable for operating a machine to carry out programmed functions includes a plurality of program processors. Each of the program processors being operable to execute simultaneously a different user control program that directs the operation of the machine to perform specific functions. Each of the processor means is contained within a separate module which also includes a memory for storing the user control programs that are to be executed by that processor means. A mechanism is also provided to control the sequence in which the user control programs are executed and which of the processor means executes a given control program. At least one input/output interface circuit controls the gathering of data from various external sensors and in response to output data received from the processor means, controls the operation of actuator devices on the machine. The input/output data regarding state of the sensors and actuator devices are stored in a memory within the interface circuit.