This SAE Recommended Practice defines a method for implementing a bidirectional@ serial communications link over the vehicle power supply line among modules containing microcomputers. This document defines those parameters of the serial link that relate primarily to hardware and software compatibility such as interface requirements@ system protocol@ and message format that pertain to Power Line Communications (PLC) between Tractors and Trailers. This document defines a method of activating the trailer ABS Indicator Lamp that is located in the tractor.