CUFAIL
MUFAIL
D50B34 95% RPM
M280 APU CONTROLUNIT (E3-3)
D1458A40ENGINE NO. 1 LOW OILD1458B40 ENGINE NO. 2 LOW OIL

M528 ENGINE ACCESS UNIT
SWITCHING MODULE (E3-2)  M1109 MANAGEMENT UNIT (E2-1)
ACARS SchematicFigure 5
 G38260
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(8)

A
737-300/400/500MAINTENANCE MANUAL
Clock Driver
 (a)  
Clock driver outputs used in MU circuits are: modulator, CU

 interface, and GMT interface.IDU Interface

 (a)  
The IDU interface contains the logic modules which serve to control transfer of data between the MU and IDU.

 (b)  
Data sent to the IDU is done by the .P on the CRU OUT line via the interface and is clocked into a shift register by CRU CLK.

 (c)  
The .P reads the IDU data on the CRU IN line.

 (d)  
Each transmission occuring between the MU and IDU is examined by the .P.

 (e)  
Should a failure in the transmission exchange be detected, the .P will read the data output to the IDU interface which is being coupled back. If the data is correct, the processor will read the IDU gated clock output. This output s gnal is checked by the .P for 55 hexidecimal or AA hexidecimal. Should neither of these appear, the MU FAIL light will be activated by the .P. Should either the 55 hexidecimal or AA hexidecimal be detected, the IDU FAIL light will be activated by the .P.

VHF Modulator

 (a)  
The VHF modulator converts the digital data to be transmitted to the ground into a sine wave of 2400/1200 Hz for modulating the transceiver.

 (b)  
When a downlink message is to be transmitted, the micro-processor loads the data into a parallel-to-serial shift register and is clocked out at a 2400 Hz bit rate.

 (c)  
The digital signals are fed into a digital-to-analog converter

 where the actual sine wave is generated.VHF Tuning

 (a)  
The purpose of the VHF tuning circuit is to select the frequency of the transceiver. The VHF frequency for data mode is set at 131.55 MHz by the .P as part of its initialization program. The voice frequency desired by the flight crew is latched and outputted by the .P to the transceiver.

 (b)  
The flight crew can change the voice mode frequency by pressing MISC - 6 - 6 - ENT on the CU. The present frequency will be displayed in the five righthand alphanumeric displays and can be changed by clearing the display and entering the desired frequency.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 (c)  Twenty lines from the MU to the transceiver contain the 2 by 5 code to select the desired frequency. Frequency select common is connected between the MU and the transceiver, and controls the selection of voice or data mode.
 (9)  
VHF Demodulator

 (a)  
Provides phone hang-up signal detection.

 (b)  
Data received by the demodulator in an uplink message is buffered and conditioned. After the signal is squared, the cell width of the square wave is determined.

 (c)  
The output along with some control signals indicate an interrupt to the .P. When the .P receives the interrupt, the cell width is read on the CRU IN line. The .P reads 16 cell widths for each character received by the demodulator. The .P software converts this information into a character.

 (d)  
When the demodulator interrupt signal is fed into the programmable system interface. An INTREQ signal is issued to the .P. Addresses IC0-IC3 indicates to the .P that the interrupt request signal comes from the demodulator circuit and that the demodulator is to send data to it.

 (e)  
The .P software enters a demodulator subroutine which issues an address decode signal and the cell width of the incoming signal is read by the .P on CRU IN line.

 (f)  
After the .P has stored the incoming message into RAM, a check is performed to determine if the message is error free and that the registration mark is for the proper airplane before the message is accepted.

 (g)  
During a link test, which is performed when the MU is in the data mode, a wraparound test on the modulator and demodulator circuits is performed by the .P as part of its self-test.

 (h)  
　
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