TM 1-1520-238-103-68Table 3-23.RT-1296/APX-100(V)1(IFF) and RT-1557/APX-100(V)1(IFF) Control and IndicatorFunctions – continuedControl/Indicator FunctionMODE 4 CODESelectorSelects condition of code changer in remote computer.ASelects mode 4 code setting for present period.BSelects mode 4 code setting for succeeding period.HOLDRetains mode 4 code setting when power is removed from transponder.MODE 4TEST/ON/OUTDetermines whether Mode 4 is on, off, or in BIT operation.MODE 4 AUDIO/LIGHT/OUTEnables or disables audio and visual Mode 4 indicators. Visual indicators signify valid replies.Audible signal indicates the interrogation and reply computers are set to opposite codes.RAD TEST/OUTEnables TEST mode.STATUS ALTIndicates that BIT or MON failure is due to altitude digitizer.STATUS KITIndicates that BIT or MON failure is due to external computer.STATUS ANTIndicates that BIT or MON failure is due to high voltage standing wave ratio (VSWR) inantenna.MODE 4 REPLYIndicates that a Mode 4 reply is generated.IDENT/OUT/MICControls transmission of I/P pulse.MODE 1 SelectorbuttonsSelects Mode 1 code to be transmitted.MODE 2 SelectorbuttonsSelects Mode 2 code to be transmitted.MODE 3/A SelectorbuttonsSelects Mode 3/A code to be transmitted.3.17.3 Operation.CAUTIONDue to possible operating problems ofthe (IFF), usage of the UHF radio on fre-quency ranges 341.325 thru 345.325 isprohibited while operating the IFF inMODE 4.a. Starting Procedures.If the MODE 2 code hasnot been set previously, loosen two screws which hold theMODE 2numeral cover, and slide the cover upward to ex-pose numerals of MODE 2 code switches. Set theseswitches to code assigned to helicopter. Slide cover downand tighten screws.1. MASTER control switch – STBY. Allow 2 min-utes for warm up.
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