The PS 578 is the control unit for use with MST-67A Enhanced Mode S Transponder (-2001/2101 versions) and contains all controls required for transponder operation including TEST, STANDBY, ON (transponder), ALTITUDE, VFR, and IDENT.The PS 578A will also display the selected 4096 ATC code and currentmode of operation in the display window.Version071-01618-0104 has a black face with an amber display and ON/OFF switch. Compatible with single transponder operation.