*
A ground on the port select discrete.
When the right RTP is off, the ARINC 429 port relay de-energizes. This causes:
*
An open on the port select discrete
*
In the right transceiver, the port select switch relaxes
*
The right transceiver then gets tuning data through port B.
This switching allows any RTP to provide tuning data to any transceiver if one or more RTPs are off.
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VHF SYSTEM -TUNING FUNCTIONAL DESCRIPTION
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VHF SYSTEM -RECEIVE MODE
General
The VHF antenna receives RF signals and sends them to the VHF communication transceiver through the coaxial cable. The transceiver processes the RF signal and sends the audio to the audio management unit (AMU), the airplane information management system (AIMS), and SELCAL decoder.
The three VHF communication systems operate in a similar way. The right VHF system is an example.
Receive Operation
The microprocessor sends the receive frequency to the frequency synthesizer. The frequency synthesizer sets the frequency of the AM receiver.
The microprocessor also sends a logic 1 to the transfer switch when the transceiver is in the receive mode. This closes the transfer switch and sends the received RF signal from the antenna to the AM receiver.
The AM receiver demodulates the RF input and detects the audio signal.
The audio output from the AM receiver goes to:
*
Switch S1
*
The squelch comparator
*
The data output circuits.
The audio output circuits send the audio signal to the AMU and to the headphone jack. Push the TEST switch to close switch S1 and send the received audio to the audio output circuits.
The squelch comparator circuit compares the detected audio with a threshold value. If the level of the detected audio is greater than the threshold, the squelch circuits sends a ground to switch S1. Switch S1 closes and sends the audio to the audio output circuits.
The microprocessor changes the audio off signal to a logic 1 when the transceiver is in the data mode. This causes switch S1 to open and disable the audio output and the squelch comparator.
The data output circuits send unsquelched audio to the SELCAL decoder and to the right airplane information management system (AIMS) cabinets on an ARINC 618 bus.
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777 AIRCRAFT MAINTENANCE MANUAL
VHF SYSTEM -RECEIVE MODE
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VHF SYSTEM -TRANSMIT MODE
General
Audio signals from the audio management unit (AMU) and data (audio) from the airplane information management system (AIMS) go to the selected VHF transceiver. The transceiver processes the signals and sends them to the selected antenna for transmission.
The three VHF communication systems operate in a similar way. The right VHF system is an example.
Transmit Operation
During transmission, the microprocessor receives a push to talk (PTT) signal from the audio management unit (AMU) or a data key signal from AIMS. This causes the microprocessor to send a logic 0 to the transfer switch. The transfer switch connects the output of the transmit circuits to the VHF antenna.
The voice/data select discrete from AIMS determines the operation mode of the transceiver. A ground input sets the transceiver to the data mode. An open input sets the transceiver to the voice mode.
In the data mode, the microprocessor sets the audio off signal to a logic 1 to disable the sidetone.
Mic audio from the AMU or the data audio from AIMS go to the transmit circuits in the transceiver. The AIMS cabinet uses an ARINC 618 bus to send the data to the transceiver. The transmit circuits modulate the transmit frequency with the mic audio or the data audio. This makes an amplitude modulated RF signal. The signal goes to the directional coupler and transfer switch. The RF signal goes through the closed transfer switch and to the antenna. The antenna transmits the RF signal.
The RF output from the directional coupler also goes to the power monitor. The power monitor sends a logic 1 when the output power is greater than 15 watts.
The sidetone switch closes when the output power is greater than 15 watts and the transceiver is in the voice mode.
The sidetone switch closes and the mic audio goes to the AMU for the flight crew.
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本文鏈接地址：777 AMM 飛機維護手冊 通訊 Communications 1(14)
