Mode-S Operation; Self-Test Operation; Mode-S Transponders - Boeing 737-300 Series Maintenance Manual

(b) The desired ATC system is set with the transponder select switch. The '1' position causes the
Control Panel to send an 'ON' discrete to transponder No. 1 if the AIR/GND RELAY is in AIR
mode and the mode switch is set to XPDR. The '2' position causes the Control Panel to send an
'ON' discrete to transponder No. 2 if the AIR/GND RELAY is in AIR mode and the mode switch
is set to XPDR.
(3) If the altitude reporting function is on, then the ATC system will reply to mode-A and mode-C ground
station interrogations. If the altitude reporting function is off, then system will reply only to mode-A
interrogations. Mode-S replies are not affected by the condition of the mode switch.
(4) The identity code and mode configuration information is sent to the transponders through ARINC
busses and discrete. The 4-digit code identifies the airplane to the ground station for mode-A and
mode-C interrogations.
(5) When the airplane leaves the ground, the AIR/GND relay is set to AIR. This sends a discrete signal to
the Control Panel. The Control Panel then sends an 'ON' discrete to the selected transponder. At this
point, the system will respond to ground station interrogations.
D. Mode-S Operation
(1) Mode-S functions operate when the mode-S programming plug is configured with a unique mode-S
address. If all the programming pins are grounded (they have a bit value of one), then there is no
unique address and the mode-S functions do not operate. The mode-S code is hard wired on mode-S.
The identity code set on the ATC Control Panel has no effect on the mode-S address code of the
airplane. Mode-S functions in the airplane are automatic and are not configured by the pilot.
E. Self-Test Operation
(1) Both ATC transponders have Built-In Test Equipment (BITE). A BITE test can be started if the
transponder to be tested has power. The BITE test is started when the TEST switch on the transponder
face is pushed. The BITE test does the following:
(a) Does a checksum of internal ROM contents.
(b) Tests the transponder RAM.
(c) Simulates noise sources and monitors the results.
(d) Tests the antenna impedance.
(e) Simulates a Mode-S All-Call interrogation and reply (Mode-S transponder only).
(f) Samples the internal power supply monitor.
(g) Checks the operation of the serial busses connected to the transponder

(2) Mode-S Transponders:

The green LRU STATUS indicator light will come on if no errors were detected during the BITE test.
It will illuminate red if any part of the BITE test fails.
(3) The BITE monitor continuously tests transponder functions while the ATC system operates normally
(not in test mode). If a failure is detected, the red CTL indicator light will come on and a
TRANSPONDER FAIL DISCRETE will be sent to the Control Panel. If the altitude input from the
ADC is in error, the red ALT indicator light will come on.
EFFECTIVITY
PW245
Use and disclosure shall be in accordance with the proprietary legend located on the cover of this document
GENERAL AEROSPACE
Boeing 737-300
MAINTENANCE MANUAL
34-53-00GA
DRAWING # GA-BC17-ATC-MM
AUG 15/10
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