This document defines a physical and electrical interface to Project 25 mobile radios. The purpose of the interface is to allow standardized interfacing of external devices that require one or more of the functions of push-to-talk, qualified audio presence, transmit audio, and receive audio. Some examples of such external devices are, 1) fire apparatus intercom and radio control audio and keying systems, 2) headset systems, 3) ambulance audio and keying systems, and 4) airborne audio and keying systems.This document also provides definition, and methods of measurement for the transmit audio, receive audio, and push to talk interfaces for radio equipment used in the Private (Dispatch) Land Mobile Services. The definition and description of the system in which this equipment operates are given in the APCO Project 25 System and Standards Definition, TIA/EIA-TSB102-A.