Operation
The CPU uses tune inputs to tune the frequency synthesizer. The CPU commands the transmitter to send interrogation pulses. The transmit pulses go through a circulator then to the DME antenna.
The transmitter sends a signal to the suppression circuits. During a transmission, the suppression circuit in the left DME interrogator sends a suppression pulse to the:
*
Right DME interrogator

*
Left and right ATC transponder (XPNDR)

*
TCAS computer.

The suppression pulse inhibits receiver operation in the other LRUs to prevent receiver interference.
Receive
The circulator sends the received RF pulse pairs from the antenna to the receiver. The receiver sends the pulse pair to the CPU. The CPU calculates the slant range distance with the time it takes to transmit pulse pairs and get a reply from the ground station. When another L-band system transmits, a suppression pulse inhibits the receiver operation.
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DME SYSTEM -SIGNAL PROCESSING

Interrogator Output
After the CPU calculates slant range distance, the CPU sends it to the ARINC 429 transmitter. The ARINC 429 transmitter sends the slant range data to the AIMS cabinets for the flight deck displays. The CPU sends the pulse pairs to the pulse pair decoder. The decoder sends the DME audio to the AMU.
Test
The CPU does a test of the interrogator when it receives a test command from the left AIMS cabinet or when you push the test switch on the front panel of the interrogator.
The CPU turns on the status LEDs on the interrogator to show the test results.
The CPU also sends tests results to the AIMS cabinets.
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DME SYSTEM -SIGNAL PROCESSING

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DME SYSTEM -SELF-TEST

Self Test Many LRUs on the 777 are compatible with other model
airplanes that do not have a central maintenance computing Do this to start a DME self-test:
system (CMCS). For that reason these LRUs have a front panel test feature.
*
Push the TEST switch on the DME interrogator.

*
Hold the TEST switch until the test ends.

Front Panel Displays
The LED status indicators go through this sequence:
*
LRU status and control fail LEDs come on red for two
seconds.

*
LRU status LED changes to green.

*
All LEDs go off for two seconds.

*
Applicable LEDs come on to show the LRU status.

The red CONTROL FAIL LED shows when the tuning source is invalid.
The LRU STATUS LED shows interrogator test results. The LRU STATUS LED shows green for a good test or it shows red for a test failure.
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 DME interrogator. You must do all tests for the DME interrogator from the MAT or a PMAT.
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DME SYSTEM -GROUND TEST

General
You can do a test of the DME system from the MAT. These tests are available from the operational test selection on the MAT:
*
Left DME System

*
Right DME System.

Operational Test
The operational test for the left and right DME systems does a check for normal operation of the DME interrogators and the control inputs to both interrogators.
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DME SYSTEM -GROUND TEST

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BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details.

AUTOMATIC DIRECTION FINDER SYSTEM -INTRODUCTION

Purpose
The automatic direction finder (ADF) system is a navigation aid. The ADF receiver uses AM signals from ground stations to calculate the bearing to the station from the airplane longitudinal axis, and to receive standard AM radio broadcasts.
　
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