The microprocessor receives inputs from the command decoder, operates on these inputs in accordance with a program that is stored in ROM or RAM, and then outputs the results to the interface circuitry. Because there is such a wide variety of command types and messages, most command systems are implemented using programmable micro-processors. The type of interface circuitry needed is based on the command sent by the processor. These commands include relay, pulse, level, and data commands. Relay commands activate the coils of electromagnetic relays in the central power switching unit. Pulse commands are short pulses of voltage or current that is sent by the command logic to the appropriate subsystem. A level command is exactly like a logic pulse command except that a logic level is delivered instead of a logic pulse. Data commands transfer data words to the destination subsystem.