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operated in any one of four master modes, each of which
may be selected by the operator at the control panel.
Five coding modes are available to the operator. The
first three modes may be used independently or in combination.
MODE 1 provides 32 possible code combinations,
any one of which may be selected in flight.
MODE 2 provides 4096 possible code combinations,
but only one is available and is normally preset before
takeoff. MODE 3/A provides 4096 possible codes, any
one of which maybe selected in flight. MODE 4 is an
external computer mode and can be selected to provide
any one of many classified operational codes for security
identification. Power to operate the IFF system is
provided from the dc emergency bus through the IFF
circuit breaker on the pilot overhead circuit breaker
panel.
3.17.1 Antenna. Two blade antennas (fig 3-1) are
used to receive interrogating signals and to transmit
reply signals. The upper IFF antenna is installed on top
of the fuselage area aft of the canopy, and the lower IFF
antenna is installed on the bottom of the fuselage as an
integral part of the UHF-AM antenna. Some helicopters
have the upper IFF antenna installed on the work
platform forward of the main rotor mast and the lower
IFF antenna installed on the bottom of the fuselage aft
of the tail boom jack pad.
3.17.2 Controls and Functions. All operating and
mode code select switches for transceiver operation are
on identical control panels (fig 3-26) for the
RT-1296/APX-100(V) and the RT-1557/APX-100(V).
Control and i n d i c a t o r f u n c t i o n s o f t h e
RT-1296/APX-100(V) and the RT-1557/APX-100(V)
transponder are described in table 3-23.
3-66
TM 1-1520-238-10
Figure 3-26. Control Panel RT-1296/APX-100(V) 1(IFF) and RT-1557/APX-100(V) 1(IFF)
Table 3-23. RT-1296/APX-100(V) 1(IFF) and RT-1557/APX-100(V) 1(IFF) Control and Indicator
3-67
CAUTION
TM 1-1520-230-10
Table 3-23. RT-1296/APX-100(V)1(IFF) and RT-1557/APX-100(V)1(IFF) Control and Indicator
Functions - continued
Control/Indicator Function
MODE 4 CODE Selects condition of code changer in remote computer.
Selector
A Selects mode 4 code setting for present period.
B Selects mode 4 code setting for succeeding period.
HOLD Retains mode 4 code setting when power is removed from transponder.
MODE 4 Determines whether Mode 4 is on, off, or in BIT operation.
TEST/ON/OUT
MODE 4 AUDIO/ Enables or disables audio and visual Mode 4 indicators. Visual indicators signify valid
LIGHT/OUT replies. Audible signal indicates the interrogation and reply computers are set to
opposite codes.
RAD TEST/OUT Enables TEST mode.
STATUS ALT Indicates that BIT or MON failure is due to altitude digitizer.
STATUS KIT Indicates that BIT or MON failure is due to external computer.
STATUS ANT Indicates that BIT or MON failure is due to high voltage standing wave ratio (VSWR) in
antenna.
MODE 4 REPLY Indicates that a Mode 4 reply is generated.
IDENT/OUT/MIC Controls transmission of I/P pulse.
MODE 1 Selector Selects Mode 1 code to be transmitted.
buttons
MODE 2 Selector Selects Mode 2 code to be transmitted.
buttons
MODE 3/A Selector Selects Mode 3/A code to be transmitted.
buttons
3.17.3 Operation. a. Starting Procedures. If the MODE 2 code has
not been set previously, loosen two screws which hold
the MODE 2 numeral cover, and slide the cover upward
to expose numerals of MODE 2 code switches. Set
these switches to code assigned to helicopter. Slide cov-
Due to possible operating problems er down and tighten screws.
of the (IFF), usage of the UHF radio
on frequency ranges 341.325 thru
345.325 is prohibited while operating 1.
the IFF in MODE 4.
MASTER control switch - STBY. Allow 2
minutes for warm up.
3 - 6 8
2.
3.
MODES 1 and/or 3A code selector buttons -
Press and release until desired code shows.
TEST. TEST/MON, and REPLY indicators -
PRESS-TO-TEST. “If MODE 1 is to be used,
check as follows:
a.
b.
c.
MASTER control switch - NORM.
M-1 switch - Hold at TEST. Observe that
no indicators are on.
M-1 switch - Return to ON. If MODE 2,
3/A or C are to be used, check as follows:
(1) M-2, M-3/A, and M-C switches - Re.
peat steps (3b) and (3c).
NOTE
Do not make anv checks (1) near a radar
site; (2) with MASTER control switch in
EMER, or (3) with M-3/A codes 7600 or
7700 without first obtaining authorization
from the interrogating stations.
4. MASTER control switch - NORM. If MODE
4 is to be used, check as follows:
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