23-12-03
ALL ú ú 03 Page 405 ú Sep 28/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757 MAINTENANCE MANUAL
SELCAL SYSTEM - DESCRIPTION AND OPERATION
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1. General_______
A. The SELCAL system provides notification to the flight crew when a ground station desires contact on one of the communication transceivers. SELCAL reduces pilot diversions and workload by eliminating the need for continuous monitoring of communication frequencies.
B. A ground station calls a particular airplane by transmitting a set of four audio tones over a communications transceiver. The SELCAL decoder receives the audio from the particular transceiver and compares the tones to an assigned code. When the tones and the code are the same, a chime sounds and a light on the pilots' call panel comes on, corresponding to the communications transceiver which received the signal. The decoder and indicator are manually reset after each call.
C. The SELCAL system consists of a SELCAL decoder, a shorting receptacle, and alert lamps on the pilots' call panel. System electrical power comes from the left main DC bus, through a circuit breaker on overhead circuit breaker panel P11.
2. Component Details (Fig. 1)_________________
A. SELCAL Decoder
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(1)
The decoder monitors audio from the VHF and HF communication transceivers, recognizes receipt of the SELCAL code assigned to the airplane, and provides flight compartment call alert signals.
(2)
The decoder has no front panel features, and is located on shelf 3 of the main equipment center rack E4 (E4-3).
B. Shorting Receptacle
___________________
(1)
The SELCAL shorting receptacle provides a 4-bit binary code to the decoder for each of the four tones that make up the SELCAL code.
(2)
The shorting receptacle is located on the E4-3 rack disconnect bracket assembly near the rear of the SELCAL decoder.
C. SELCAL Alert Indicators
_______________________
(1) There is one blue alert indicator on the pilots' call panel for each transceiver connected to the SELCAL decoder. Each indicator alerts the flight crewmembers of a ground-to-air call received by that transceiver. Pushing the indicator resets the light and decoder.
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23-21-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
DECODER SELCAL
E4-3
ACCESS DOOR, 119BL MAIN EQUIPMENT CENTER ASEE FWD
119BL
P11 PANEL
PILOTS' MAIN EQUIPMENT CENTER
CALL PANEL P5 PANEL A
SEE B
FLIGHT COMPARTMENT
SELCAL L-VHF-R L-HF-R
bb bb
CABIN CALL GND FWD MID AFT ALERT CALL
ALERT bbbbb
PILOTS' CALL PANEL
B
SELCAL System Component Location Figure 1
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(2) The SELCAL alert indicators are located on the pilots' call panel, on the pilots' overhead panel P5.
3. Operation (Fig. 2)
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A. Functional Description
______________________
(1)
Power and audio inputs are wired directly to the decoder. Thus, the only operational control is for the reset function. The system is reset by pushing the associated indicator or by keying the PTT line for the associated transceiver.
(2)
The decoder monitors audio from the VHF and HF transceivers. When the received code matches the code programmed by the shorting receptacle, the decoder recognizes it as a call to that airplane. The decoder turns on the indicator associated with the transceiver which received the call, and also sounds a chime in the flight compartment. The indicator and SELCAL decoder are then manually reset to wait for the next call.
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