Power
The ACARS CMU receives 115v ac from Elex bus 1.
The ACARS CMU also receives 28v dc from bus 1 for the aural warning
output signal generation.
Call Interface
An uplink message tells the flight crew there is an ACARS message.

When a message comes, the ACARS CMU gives these indications: -Chime from the aural warning module. Further activation of the chime is disabled until the crew selects the new message for display on the MCDU. Once all messages have been displayed, the chime will be re-enabled for future activation. -The CALL light is illuminated on the MCDU when ACARS is not selected on one of the MCDU’s and an uplink messaged is received by ACARS.
-The MSG light is illuminated on the MCDU when ACARS is
selected on one of the MCDUs and an uplink message is
received

NOTE: The chime will not sound when the aircraft is below 10,000 ft. of
altitude, and on the ground when the engines are >70% N1.
The visual call light on the MCDU will alert the crew to an
incoming message.
Communication Interface
The VHF 3 transceiver is a Honeywell RTA-44D VHF Data Radio (VDR), and is the primary means of ACARS communication. The frequency range is 118 to 137MHz. ACARS is assigned to specific frequencies according to the geographic region. in the U.S. the primary frequency is 131.550 MHz.
The advantage of VHF is that it is relatively noise free and requires low transmitter power, about 25 watts. The disadvantage is that communication is limited to line of sight. An aircraft a 40,000 feet the max range is about 250 miles. The VHF transceiver interfaces through the ARINC ground network for downlink and uplinks. messages to and from the aircraft are routed to Southwest Airlines.
The #3 VDR receives data from the CMU. The VDR can transmit this data using one of two modes: VHF Digital Link (VDL) Mode A, or VDL Mode 2.
VDL Mode A transmits ACARS data at a 2.4 kbps rate. This transmission mode has been in use for many years, but its narrow digital bandwidth causes ACARS frequency congestion when large amounts of data is transmited from the aircraft.

23-27-AC-005
December 1, 2004
23-27
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SOUTHWEST AIRLINES PROPRIETARY(c) All rights reserved

SOUTHWEST AIRLINES.
SWACOM TRAINING GUIDE

ACARS - POWER, DISCRETES, CALL AND COMMUNICATION INTERFACE
VDL Mode 2 transmits data at a 31.5 kbps, which is over 10 times the ACARS VDL Mode A rate. VDL Mode 2 will allow SWA to take advantage of emerging flight-operations applications such as graphical weather descriptions, electronic charts, and engine/airframe health monitoring programs.
The ACARS CMU sends and receives this data from the #3 VDR:
-Frequency to tune the VHF transceiver
-Port select discrete. This discrete selects the ACARS CMU.
When the discrete is a ground, the VHF transceiver selects
　
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