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Front Panel Lights and Switches

The ACP has these front panel components:

Microphone select switches with mic select lights

Filter switches

Receiver select switches and volume controls.


The mic select lights get 28v dc or 16v dc from the MD&T circuits. A mic select light turns on when it gets a ground from the electrical interlock programmed logic array(,) (PLA) circuit.

When you push the microphone select switch, it sends 5v dc to the PLA. The PLA is an electrical interlock circuit. It makes a 4 bit binary code for the selected switch.

System selection is latched() into the PLA. The PLA sends the applicable code to the switching multiplexer. The PLA also sends a signal to turn on a receiver volume control when you push the microphone select switch.

Push the receiver volume control to hear audio from the related system. The receiver volume control switch sends a dc voltage from a potentiometer (0 to 12v dc) to the multiplexer. The multiplexer sends this voltage to the REU to set the radio output

volume level. When the audio volume control switch is off, 12v dc goes to the REU and you cannot hear the system audio.


Switching Multiplexer

The switching multiplexer gets these signals:

Microphone select binary code from the PLA circuit

DC voltages from filter switches

DC voltages from the service interphone, PA, and speaker volume controls.

The switching multiplexer sequentially selects the inputs. The data output goes to the REU as a control system data word. This multiplexer operation stops when the volume control multiplexer operates.


Volume Control Multiplexer

The volume control multiplexer gets DC voltages from the remaining receiver volume controls.

The volume control multiplexer sequentially selects the inputs.

The data output goes to the REU as a control system data word.

This multiplexer operation stops when the switching multiplexer operates.

Select Circuits

A multiplexer selects each input in sequence for a given time. A clock in the REU increases the value of a counter(). This counter provides the select address for the two analog multiplexers.

Two multiplexers combine the data from all the switches and the controls. The scan inhibit circuits prevent the operation of one multiplexer output while the other scans.


Emergency Circuits

The REU gives power for the ACP. You lose ACP power when for these conditions:


REU has no power

ACP has the ALT selection.

These conditions occur when you lose power in the ACP:

The ALT/NORM relay has no power

The data line to the REU identifies MASK/BOOM selection

28 volts from MD&T goes through the ALT/NORM relay contacts to the VHF 1 mic selector switch lamp for the captain and observer, and VHF 2 mic selector switch lamp for the first officer.


These components send a push-to-talk (PTT) signal to the remote electronics unit (REU):


Audio control panel (ACP) radio-intercom PTT switches

Control wheel PTT switches

Remote mic switches

Hand microphone PTT switches.


Set the radio-intercom PTT switch on the ACP to R/T to send a PTT signal to a communications system. Set the switch to I/C to talk on the flight interphone system.

Push the remote mic switch on the glareshield to send a PTT signal to a communication system.


Set the control wheel PTT switch to MIC to send a PTT signal to a communications system. Set the switch to INT to talk on the flight interphone system.

The observer can only send a PTT from the hand microphone or ACP.


The remote electronics unit (REU) controls the communication between these:


Three flight deck stations

Service interphone

Flight interphone

All related electronics equipment.


The REU contains three identical station cards. The cards are for these crew members:



First officer



The REU also has an audio accessory unit (AAU) card. The card contains circuitry for flight and service interphone, alert tone generation and various audio accessory functions.

REU cards process these signals:

REU inputs

REU outputs

AAU signals


REU Inputs

Each station card in the REU receives audio from these sources:

Flight compartment microphones

Communication transceivers

Navigation receivers

Passenger address amplifier

AAU card aural alerts.

The station cards get control inputs from the audio control panels (ACPs) and microphone push-to-talk (PTT) switches.

The mux receiver, in the REU, sends a data clock to the ACP.

The ACP replies back with crew selected receiver audio, navigation signals, volume and particular transmitter.

The mux receiver responds to the crew inputs and sends control signals to the following circuits:

Receiver circuits

Mic/PTT select logic

Transmitter circuits

Speaker mute/volume logic.

The receiver circuits receive audio inputs from the communication and navigation systems, passenger address amplifier, service interphone and flight interphone. The circuits process the crew selected audio. The audio goes to summing amplifiers. The summing amplifiers send the audio to the speaker mute/volume logic circuits, CVR amplifier and headphone amplifier.


The mux receiver sends boom/mask and interphone PTT signals to the mic/PTT select logic. The mic/PTT select logic processes input microphone audio and PTT signals. It routes() the selection to the transmitter circuits. The mic/PTT select logic circuits process speaker mute signals and hot mic audio.

The transmitter circuits route the microphone audio and/or PTT to the selected communication system.

The speaker mute/volume logic circuits process flight compartment speaker muting and set the speaker volume to the level as commanded from the mux receiver.


REU Outputs

Each station card sends audio to these:



Voice recorder

Flight compartment speakers from CAPT and F/O station cards

Communication transceivers

Passenger address amplifier.

The station cards also send PTT signals to the communication transceivers and the passenger address amplifier.

AAU Signals

The AAU uses these audio signals:

Altitude alert

Ground proximity alert

Flight interphone

Service interphone.


The flight control computers (FCCs) send an altitude alert discrete to the AAU card. This input starts a C-chord generator.

The ground proximity computer and the weather radar receiver transmitter send alert audio to the AAU card. The AAU card combines this audio with the output from the altitude alert tone generator and sends it to each station card summing amplifier.

The summing amplifier combines the AAU alert audio with TCAS audio. Station cards amplify these signals and send them to these components:

Voice recorder





The flight interphone amplifier amplifies microphone audio from station cards. Microphone audio goes to the flight interphone jack.

The AAU card amplifies audio from the external flight interphone jack and sends it to station cards.


The service interphone amplifier amplifies these inputs:

Audio from attendant handsets

Audio from service interphone jacks

Microphone audio when service interphone is set on the


This audio goes to service interphone jacks and station cards.

Emergency Signals

Emergency operation bypasses all active circuitry in the system to maintain communications between the aircraft and ground stations.

Microphone audio and PTT signals from the CAPT and OBS stations go directly to the VHF 1 transceiver. Microphone audio and PTT signals from the F/O station goes to the VHF 2 transceiver.

Received audio from the VHF 1 transceiver goes to the CAPT and OBS station headsets. Received audio from the VHF 2 transceiver goes to the F/O station headset.

Date: 2016-03-03; view: 819

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