A rack adapter for a programmable controller is included in each of four remotely located I/O interface racks that are connected in series to a central processor. The rack adapter includes a rack interface processor under program control which converts serial data received from the central processor to parallel data that is written into I/O modules in its rack. The rack interface processor also converts parallel data from I/O modules to serial data, which is sent back to the central processor through a select data out circuit in proper sequence with data from the other racks. The rack interface processor initiates the decontrol of machines connected to the four racks if a major transmission fault occurs, but the rack interface processor is tolerant to transmission errors caused by electrical noise. A dual-frequency clock drives the rack interface processor, and interface circuitry which is operable at two bit-transmission rates connects the rack interface processor to the communication channel, which can range up to 10,000 feet from the central processor.