777 AIRCRAFT MAINTENANCE MANUAL
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BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details.
ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER RECEIVE FUNCTION
General
The antennas send the interrogation signals to the transponder. The signals go through the diplexer circuits to two individual receivers. The 1030 MHz interrogation signal mixes with a 1090 MHz signal supplied by the 1090 MHz oscillator. This makes an intermediate frequency signal that the signal processor uses.
Suppression
When the transponder gets a suppression pulse from another system, the signal processor circuits do not permit the receiver circuits to operate.
Antenna Select
The antenna select circuit chooses the duplexer and antenna that supplies the strongest receiver signal. The transmission of the reply signal goes through the selected duplexer and antenna.
Signal Processor
The signal processor determines if the transponder receives a valid interrogation signal. It also determines the correct reply mode; mode A, mode C or mode S.
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ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER RECEIVE FUNCTION
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ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER TRANSMIT AND CMCF FUNCTION
Transmit
The central processor unit (CPU) controls the transponder to reply to a mode A, mode C or mode S interrogation. The mode format circuits use the identity code, airplane altitude and the unique 24-bit airplane address from the memory to format the data for the reply signals.
A mode A reply contains only the airplane identity code.
A mode C reply contains both the airplane identity code and the airplane altitude.
Mode S interrogations come from the TCAS equipment of other airplanes. The mode S replies contain a unique 24-bit airplane address.
After the signal processor chooses the correct mode for the reply data, the modulator makes a reply signal from that format. The modulated signal goes to the power amplifier. The signal mixes with a 1090 MHz frequency to make the transmission reply signal. The transmission reply signal goes to the diplexer and to the ATC antenna.
The antenna select circuit makes the transmission signal go to the antenna that receives the strongest interrogation signal.
Test
When you apply power or do a test of the ATC/Mode S transponder system, the built-in test equipment (BITE) does a check of the operational status of all the internal circuits and for the correct antenna impedance. The BITE circuits identify any failures and put them into the non-volatile memory.
The LEDs on the front panel of the transponder show:
*
Transponder status
*
Antenna status
*
Transponder control and altitude interfaces.
AIMS CMCF Function
The ATC/Mode S transponder transmits fault summary data on an ARINC 429 bus to the CMCF of both AIMS cabinets. To do a test of the ATC/Mode S transponder system, a test command comes from the maintenance access terminal (MAT) through the CMCF of the left AIMS cabinet.
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ATC/MODE S TRANSPONDER SYSTEM -TRANSPONDER TRANSMIT AND CMCF FUNCTION
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ATC/MODE S TRANSPONDER SYSTEM -ANTENNA COAX RELAY CONTROL
General
Use the transponder panel to select the active ATC system. When you do this, the transponder panel sends a discrete open or ground to the top and bottom ATC coax relays. This connects the right transponder or left transponder to the top and bottom ATC antennas.
Left System Select or Standby
When the airplane power is on and when the transponder panel has the left transponder or standby selected, the S1 switches open. Relays R1 and R2 do not energize. The top and bottom antennas connect to the left ATC/Mode S transponder.
Right System Select
With the right transponder selected, switch S1 closes to supply a ground to energize relays R1 and R2. The top and bottom antennas connect to the right ATC/Mode S transponder.
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ATC/MODE S TRANSPONDER SYSTEM -ANTENNA COAX RELAY CONTROL
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ATC/MODE S TRANSPONDER SYSTEM -SELF-TEST
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