Programmable logic controller

A programmable controller includes a processor which executes a control program to alter the state of an output image table stored in a read/write memory in response to the state of an input image table stored in the memory. An input/output scanner circuit connects directly to the read/write memory and periodically steals a memory cycle from the processor to couple the status of input and output devices with corresponding bits in the input and output image tables. The rate at which the scanner circuit operates is independent of the processor speed and is selected to accommodate the input/output interface circuitry on the programmable controller.