SELCAL -GENERAL DESCRIPTION
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SELCAL -COMPONENT LOCATION
Main Equipment Center
The SELCAL decoder is in the main equipment center on the E1-2 rack. The SELCAL coding switch is on the rack behind the decoder.
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777 AIRCRAFT MAINTENANCE MANUAL
SELCAL -COMPONENT LOCATION
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SELCAL -INTERFACES
Power
The left main DC bus sends 28v dc power to the SELCAL decoder power supply.
Audio
The SELCAL decoder gets audio from the communication transceivers.
Discretes
The SELCAL decoder sends call set discretes to the audio management unit. There is a different discrete for each transceiver.
The audio management unit sends call reset discretes to the SELCAL decoder. There is a different discrete for each transceiver.
SELCAL Coding Switch
The SELCAL coding switch on the back of the decoder has 16 switches in groups of four. There are four letters in the SELCAL code. The positions of the switches in each group identify one of the four SELCAL code letters.
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SELCAL -INTERFACES
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SELCAL -SELCAL DECODER
Purpose
The SELCAL decoder has these functions:
*
Monitors audio from the communication transceivers
*
Identifies the SELCAL signal that is the same as its code
*
Causes flight deck indications for a call.
There are no lights or test switches on the SELCAL decoder front panel.
SELCAL Coding Switch
Switches inside of the SELCAL coding switch set the airplane’s SELCAL code.
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777 AIRCRAFT MAINTENANCE MANUAL
SELCAL -SELCAL DECODER
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SELCAL -ACP CALL LIGHTS AND RESET SWITCHES
Control and Indications
On the audio control panel, a transmitter select switch CALL light comes on when the SELCAL decoder gets a SELCAL signal.
To make the CALL light go off, push the transmitter select switch. If the MIC light for that transmitter select switch is on, push any mic switch or that transmitter select switch to reset the call light.
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SELCAL -ACP CALL LIGHTS AND RESET SWITCHES
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SELCAL -ALERT MESSAGE AND AURAL
Indications
An alert message shows when the SELCAL decoder gets a SELCAL signal. The alert message, SELCAL, is a comm medium message. With the comm medium message there is a high/low chime from the aural warning speakers.
Training Information Point
The same comm medium message, SELCAL, also shows when the SATCOM system receives a call.
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777 AIRCRAFT MAINTENANCE MANUAL
SELCAL -ALERT MESSAGE AND AURAL
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SELCAL -FUNCTIONAL DESCRIPTION
General
The SELCAL decoder has these components:
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Audio compressors and filter units
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An analog-to-digital converter
*
A microprocessor
*
An output driver.
Power Up
At power-up, the SELCAL coding switch gives the microprocessor its SELCAL code. The microprocessor controls the SELCAL decoder.
Call Set
Each communication transceiver sends audio signals to the related audio compressor. The audio has a SELCAL signal. The SELCAL signal is a group of four tones. There is a letter for each tone. The audio compressor amplifies or limits the audio signal input to get a constant level. The audio compressor then sends the signal to the related filter unit.
In the filter unit, the signal goes to 16 different filters. Each filter passes an audio signal that identifies one SELCAL tone or letter.
The filter outputs go to the analog-to-digital converter. The converter changes the analog audio signal to a digital signal and sends it to the microprocessor.
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