The intercom type ABMA4 provides the audio communications and pre-start alarm sounding facilities of the PSACS1 system. Intercoms are distributed along the conveyor at fixed distances apart to provide complete sound coverage along the conveyor. During a pre-start sequence the intercom sounds the pre-start alarm through its two sideways facing speakers. These same speakers and the front panel mounted microphone are used to carry out a half duplex voice conversation with other intercoms on the conveyor or a surface located BMA exchange operator.