Call Interface

An uplink message tells the flight crew there is an ACARS message. When a message comes, the ACARS MU gives these indications:

• Chime from the aural warning module

• VHF 3 light on the SELCAL control panel.

Communication Interface

The ACARS MU sends and receives this data from the VHF transceiver:

• Frequency to tune the VHF transceiver

• Port select discrete. This discrete selects the ACARS MU or the radio communications panel (RCP) for tuning. When the discrete is a ground, the VHF transceiver selects PORT A.

PORT A comes from the ACARS MU.

• Data keyline to key the transceiver

• Downlink audio to the VHF transceiver

• Voice/data select discrete. This discrete selects the VHF transmitter to send audio from the remote electronics unit (REU) or data from the ACARS MU

• Uplinked audio from the VHF transceiver.

Digital Inputs

The ACARS MU gets these digital inputs:

• Data entries and menu selections from the CDUs

• Status data from the printer

• Software loads from the data loader control panel.

Digital Outputs

The ACARS MU supplies this digital data:

• Data and menus to the CDUs and alert messages for the

scratch pad

• Report data to the printer

• Status to the data loader control panel.

Purpose

The ACARS management unit (MU) receives the uplink data and controls the transmission of downlink data to and from the VHF transceiver.

General

The MU does these tasks:

• Monitors input signals

• Formats downlink messages

• Monitors downlink data to make sure it is correct

• Monitors uplink messages to make sure they are correct

• Decodes uplink messages

• Controls the mode of operation

• Controls data transmission

• Gives acknowledged/not acknowledged response

• Tunes and controls a VHF radio

• Sends data to printer when commanded

• Monitors system operation.

The mu processes only uplink messages that come with the

airplane registration code. This same code also goes on all

downlink messages to identify the airplane.

Front Panel Control and BITE Indications

BITE operates continuously. The TEST switch causes the MU to do a BITE test.

The SYSTEM FAIL LED comes on if the ACARS system fails. You push a small rod through the RESET hole to push the reset switch. This causes the MU to do a power up test. General

The ACARS management unit (MU) receives the uplink audio and controls the transmission of downlink audio to and from the VHF transceiver.

The MU has these components:

• Power Supply

• Discrete input/output interface

• ARINC 429 input/output interface

• Data input/output interface

• Microprocessor

• BITE logic

• BITE displays.

Power

The power supply gets 115v ac from transfer bus 1 and 28v dc from the hot battery bus.

The power supply changes the 115v ac into several dc voltages for use in the MU.

The MU uses the 28v dc for these functions:

• To keep data in memory

• To keep time

• Annunciation voltage

• Chime output voltage.

Discrete Input/Output Interface

The discrete input/output (I/O) interface receives and sends discrete data to system components and to other systems.

This interface changes the discrete inputs to digital information for the microprocessor. It receives these discretes:

• Airplane identification and registration

• Out, off, on, in (OOOI) status

• Programming options

• Call reset.

The discrete I/O interface changes digital data from the microprocessor to discrete analog signals for the SELCAL, aural warning, and VHF communication systems. These are the discrete analog output signals:

• Data keyline

• Voice/data monitor

• Frequency port select

• Call annunciation (optional)

• Chime.