(5)  
Each item on a menu either calls up another menu or calls up a display to perform the indicated function.

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A
737-300/400/500MAINTENANCE MANUAL
 (6)  
Some screens have multiple pages. Entering all the data on a screen will automatically invoke any following screens.

 (7)  
The position of the cursor on the IDU display is indicated by the dashes under the data.

 (8)  
Backspace (BKSP) erases last character and shifts remaining
 characters right one space.

 (9)  
When sending data to the ground station, always allow the MU to link-up before proceeding to another data mode. Link-up may take as long as 5 minutes due to traffic on the ACARS RF channel.

 (10)
 If the IDU displays a NO COMM message then the MU has failed to link-up with the ground station sucessfully. When this message occurs, repeat the transmit sequence again.

 (11)
 The NO COMM will be annunciated any time a message has been transmitted, either automatically or manually, and was not received and acknowledged by the ground station. This does not constitute a system failure; the message is stored for later transmission. No communication may be experienced for numerous reasons, such as out of range, antenna shadow, or ground station down.

 (12)
 The VHF transceiver XMIT light, lights during ACARS transmission.

 (13)
 On the MU, the MU FAIL indicator lights when the built-in self-test diagnostic program detects a failure in the Management Unit.

 (14)
 On the MU, the CU FAIL indicator lights when the built-in self-test diagnostic program detects a failure in the IDU.

 B. Operational Modes (Fig. 2)
 (1)  
The airborne part of ACARS operates in conjunction with a ground-based data processor in either a polled or a demand mode. In the polled mode, the ground processor controls the transmission of all messages from the airborne system to the ground. In the demand mode, the airborne system can itself initiate transmissions. On initial power-up, the airborne system adopts the demand mode of system operation. It continues to function in this mode until it receives an instruction from the ground to transfer to the polled mode.

 (2)  
OFF Mode

 (a)  In the OFF mode, 115v ac power is removed from the management unit.
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A 737-300/400/500MAINTENANCE MANUAL

WHEN SWITCHED TO VOICE
ACARS Operational Modes Diagram Figure 2
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 (3)
(4)

(5)

A 737-300/400/500MAINTENANCE MANUAL
 (b)  
The GMT clock continues to run, provided the hot battery bus is powered.

 (c)  
Certain data, such as 000I times from previous operations, are retained in nonvolatile memory.

 (d)
Power to the control unit is off.Demand Mode

 (a) The demand mode can be entered from the following modes: 1) Off mode, upon application of power to the management unit. 2) VOX mode, upon operator command or upon command from the ground, if the system was in demand mode prior to going to VOX mode.
 3)  Polled mode, upon sensing an "on" event, or if there is no poll for more than 1.5 minutes, or if the uplink contains the demand mode general response label.
 (b)  
In the demand mode, the airplane transmits to the ground when interrogated, when the crew initiates transmission, or automatically after the occurence of a predefined event. This mode is used to allow freedom of transmission at times when it will not generate message traffic conflicts.
　
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