(c)  
The system downlinks messages in response to flight crew inputs, an uplink message requiring response, or an event that requires automatic reporting.

 (d)  
The system exits this mode to one of the following modes: 1) OFF mode, when ac power is removed from the MU. 2) VOX mode, when selected by the operator or by uplink

 command. 3) Polled mode, when commanded by the ground station.Polled Mode

 (a)  
The polled mode can be entered from the following modes: 1) Demand mode, upon uplink command. 2) VOX mode, upon operator command or uplink command, when

 system was in the polled mode before going to VOX mode.

 (b)  
In the polled mode, the ground station periodically polls the airplane for any available downlink message and checks for a reply. If the airplane has information for the ground network, the appropriate message is transmitted automatically. If the airplane has no information for the ground, a general poll/response is transmitted automatically.

 (c)  
The system exits this mode to one of the following modes: 1) VOX mode, upon operator selection or by uplink command. 2) Demand mode, upon ground station command, if poll is not

 received for 1.5 minutes, or when there is an "on" event.VOX Mode

 (a)  
This mode is entered, from the demand mode, upon aircrew or ground station command.

 (b)  
In the VOX mode, the system provides an ARINC voice link for telephone calls by automatically tuning the ACARS voice frequency and turning over control to the flight interphone system.

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737-300/400/500MAINTENANCE MANUAL
 (c)  
On the ground, the phone number is dialed automatically in response to a downlink request. If the call is completed, the system automatically switches to the VOX mode, and the chime sounds.

 (d)  
When voice communications with the airplane is desired, an uplink command is transmitted from the ground station. This causes the management unit to alert the aircrew, auto-tune VHF-3 transceiver, and switch to VOX mode. The crew then converses as in regular VHF communications.

 (e)  
The system exits the VOX mode to the last previous data mode.

 (6)  Failed Mode
 (a)  The system will enter the "failed" mode any time a failure is detected by internal monitoring or link test. This can occur from any active mode.
 C. ACARS Block Diagram (Fig. 3)
 (1)  
The block diagram shows the various components that interface with ACARS and the interface between the IDU and MU components.

 (2)  
The IDU provides the interface for entering flight data for transmission to the ground station, recalling previously memory-stored data, and display of transmission information to and from the ground station. The IDU can also be used to tune the VHF-3 transceiver for voice operation.

 (3)  
The MU collects airplane digital data, allows for voice operation, and receives ground requests for data.

 (4)  
Also, the MU provides the automatic data processing, memory, VHF-3 transmission control, and auxiliary equipment control.

 (5)  
The ground sensing (squat) switch provides the on/off (ground) signal to the MU.

 (6)  
The equipment, cargo, and entry doors, along with the APU and parking brakes provide the in/out (gate) signals to the MU.

 (7)  
All 000I events are automatically recorded in terms of GMT
 (Greenwich Mean Time).

 D. Power Distribution and Control Diagram (Fig. 4)
 (1)  Power Distribution
 (a)  The MU power supply receives 115v ac, 400 Hz, single phase and 28v dc battery power from the P18 load control center and provides ten regulated dc output voltages. Two output logic signals are provided and one logic input signal is accepted. The regulated dc outputs are divided into three groups: standby, main, and Interactive Display (IDU).
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