The test switch starts a self-test.
Training Information Point
The maintenance manual and the fault isolation manual do not have a task for the line maintenance technician to do a test from the front panel of the ATC/mode S transponder. You must do all tests for the ATC/mode S transponder from the MAT or a PMAT.
Many LRUs on the 777 are compatible with other model airplanes that do not have a central maintenance computing system (CMCS). For that reason these LRUs have a front panel test feature.
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ATC/MODE S TRANSPONDER SYSTEM -ATC/MODE S TRANSPONDER

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777 AIRCRAFT MAINTENANCE MANUAL

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ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER PANEL

General
The dual transponder panel controls the ATC/Mode S transponder and the TCAS computer.
The ATC controls and displays on the transponder panel are:
*
IDENT switch

*
Four digit identification code

*
Active system display

*
Transponder mode selector

*
Transponder code selectors

*
Transponder selector.

Transponder Selector and Active System Display
The transponder selector has a left and a right transponder (XPNDR) position. With the left position selected, the control panel sends a ground standby/on discrete to the right transponder only. Then the right transponder is in standby. The left transponder is active.
With the right position selected, the control panel sends an open standby/on discrete to the right transponder to make it active. A ground standby/on discrete goes to the left transponder and an antenna transfer discrete goes to the ATC coax relays. This puts the left transponder in standby, and makes the relays connect the ATC antennas to the right transponder.
EFFECTIVITY
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The selected ATC system shows on the LCD active system display.
Identification Code Display and Selection
The flight crew uses the code selectors to set the four digits of the identification code. The four digits show on the liquid crystal display (LCD). Codes are from 0000 to 7777, with 4096 different selections.
CAUTION: DO NOT SELECT CODES 7500, 7600 OR 7700.
THESE CODES ARE USED ONLY FOR
EMERGENCIES.
Transponder Mode Selector
The transponder mode selector has several positions. These are the functions that the ATC system uses:
*
STBY (standby) -a ground discrete goes to both transponders. This ground discrete prevents operation of the transponder, but does not prevent built-in-test-equipment (BITE) functions.

*
ALT OFF (altitude reporting off) -the active transponder responds to ATC interrogations with mode A replies only.

*
ALT ON -the active transponder responds to ATC interrogations with mode A and mode C (altitude reporting) replies.

These are the functions that the traffic alert and collision avoidance system (TCAS) uses:
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ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER PANEL

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TA ONLY -all intruder traffic advisories (TAs), except resolution advisories (RAs), show for the alert and collision avoidance system (TCAS).

*
TA/RA -all intruder traffic shows. This is the normal mode of operation for TCAS.

IDENT Switch
When the ATC controller requests the airplane identifier, the pilot pushes the momentary IDENT switch. The transponder adds a special position identification (SPI) pulse to the interrogation reply for the next 18 seconds.
Above-Normal-Below Switch
The above-normal-below switch changes the altitude envelope that shows the TCAS intruders on the NDs.
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ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER PANEL

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ATC/MODE S TRANSPONDER SYSTEM -ATC ANTENNA

Purpose
The ATC L-band blade antenna receives 1030 MHZ interrogations from ATC ground stations and other airplanes that have TCAS. The ATC/Mode S transponder transmits the reply signals through the L-band antenna.
Physical Description
Connect the coaxial cable connector to the antenna. The antenna has an O-ring moisture seal and attaches to the airplane by four screws. The ATC and DME antennas are the same and are interchangable.
CAUTION:  EXTRA FORCE ON THE ANTENNA BASE MAY BE REQUIRED TO BREAK WEATHERPROOFING SEAL. TO PREVENT DAMAGE TO AIRPLANE SKIN OR ELECTRICAL CABLE AT THE ANTENNA BASE, CAREFULLY PRY AROUND THE ANTENNA WITH A SEALANT REMOVAL TOOL.
　
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