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and receive uplink messages using VHF 3. VHF 3 is used for data only, and not for
communications. Control of the ACARS functions is provided via the MENU pages on the
FMS CDU.
The ACARS consists of a Communications Management Unit (CMU) and an Aircraft
Personality Module (APM).
A. Communications Management Unit (CMU)
The CMU functions as the central processing and routing computer. The CMU gathers
aircraft data from the various systems such as: aircraft tail number, flight number,
departure station, etc. for use in the downlink messages and for event detection. The
system allows the aircraft to locate and acquire a ground station for communications.
The CMU interfaces with other avionics systems to provide ACARS function and
operation. These systems/components include:
S The FMS CDU, which provides the crew with a means for data entry into the
ACARS, ability to receive and display uplink messages and a visual indication that
the data communication link is operating properly.
S VHF 3 configured for data link. The CMU tunes VHF 3 to the ACARS data
frequency (Canada: 131.475 Mhz) (United States: 131.55 Mhz) (Europe: 131.725
Mhz). When the aircraft moves from one network coverage to another, the ACARS
will automatically search for the appropriate frequency. While searching for a usable
frequency, a NO COMM advisory is displayed on the CDU. The advisory will be
removed when the ACARS is communicating with a VHF ground station.
S The CMU, which uses ARINC buses for incoming data from the Data concentrator
units (DCU’s).
S The CMU also uses the Integrated Avionics Processor System ARINC buses for
incoming data.
S The transponder, which provides the aircrafts unique ICAO address (Mode S).
Transmitted information can include:
S Requests for air traffic control (ATC) data
S Flight plans and weather data
S Operational and maintenance data
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COMMUNICATIONS
Data Communication System
Flight Crew Operating Manual MASTER
CSP A--013
Received information can include:
S Text weather data
S ATC messages and clearances
S Operational messages
B. Aircraft Personality Module (APM)
The aircraft personality module (APM) is a memory unit which stores information unique
to the aircraft, such as tail number, ICAO address, aircraft type and configuration.
C. ACARS Printer
The printer is installed in the copilot’s side console. It is used for preflight and inflight
activities to print pilot selection of messages and weather data sent to it from the CMU.
The printer is also used by maintenance to download information.
D. ACARS Messages
(1) Downlink Messages:
Downlink messages originate onboard the aircraft and are transmitted to the
ground station automatically (appropriated system event detected) or by the crew.
The CMU encodes the downlink message and stores it in a downlink queue until
successfully transmitted.
(2) Queued Messages:
Queued messages are transmitted in sequence or by priority. Once a message is
transmitted, the CMU waits for the ground station to acknowledge (ACK) positive
receipt of the message. If ACK is not received within a specific time frame, the
message is transmitted again. Any downlink messages that are generated during
a NOCOMM state (link to ground station is not established) will be stored in the
queue buffer until the link to the ground station is established.
(3) Uplink Messages:
Uplink messages originate on the ground and are transmitted to the CMU. When
the message is received, the CMU validates the data content and compares the
aircraft ICAO address within the messages to the ICAO address programmed in
the APM. If the addresses match, the ACARS then transmits an ACK or negative
acknowledgement (NACK) of receipt to the ground station. The ground station
will re---transmit the message if an ACK is not received within a specific time
frame. When an uplink message has been processed, a MESSAGE prompt is
displayed on the CDU (if the ACARS application is running). The message can
be viewed by selecting the message prompt. An EICAS advisory message
SELCAL VHF 3 and a SELCAL SELCAL aural is triggered whenever there is
reception of an uplink message.
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COMMUNICATIONS
Data Communication System
Flight Crew Operating Manual
CSP A--013
MASTER
ARINC Communications Addressing and Reporting System (ACARS) Block Schematic <MST>
Figure 05---40---1
VHF 3
Cockpit Printer
APM
ACARS CMU
AUDIO IN
AUDIO OUT
TUNE
COMM LINK
ARINC
DATA KEY
ARINC
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