A method and apparatus for encoding data to be transmitted through a RS-422 standard serial interface such that the encoded data approximates one of the Control-L, Control-S wired or Control-S infrared protocols used in some audio and video equipment. Each bit of a data byte to be transmitted to the audio/video equipment is converted into one or more expansion bytes and transmitted at a baud rate such that the encoded bits approximate the shape and size of a bit in the data format of the selected protocol. In the case of Control-L protocol, the receiving port of the RS-422 is coupled directly to the serial interface port of the controlled device while the transmitting port is coupled through a diode to the serial interface port of the controlled device. For Control-S wired protocol, the transmit port of the RS-422 is coupled by an uninterrupted wire to the serial interface port of the controlled device. In Control-S infrared protocol, the transmit port of the RS-422 is coupled to an infrared light emitting diode which translates the data output from the RS-422 into infrared signals which are picked up by an infrared receiver at the controlled device.