The GPIB interface implements a standardized form of serial remote control and provides the necessary hardware to connect an instrument to the GPIB, also referred to as the 488-bus. Functional Purpose The GPIB interface allows remote programming of the synthesizer via the GPIB (General Purpose Interface Bus) in accordance with the IEEE Std 488-1978. Three basic functions are accessible through appropriate commands: frequency, output level and Local/Remote mode. In the “LISTEN ONLY” mode all valid commands are accepted without regard to the device address, but in the “ADDR’D” mode only commands preceded by the selectable “LISTEN” address.