(a)  
Three CDU's are used as input/output devices for various electronic systems. They can be used to manually tune the ILS receivers and to initiate system self-tests through the central maintenance computer (CMC).

 (b)  
Each CDU consists of an alphanumeric keypad, mode and function select keys, and a display screen with twelve line select keys to choose items from menus displayed on the screen.

 (c)  
The left and right CDU's are located on the forward electronic control panel (P9) and the center CDU is located on the aft electronic control panel (P8).

 (3)  
FMC select switch

 (a)  
The FMC select switch is used to select the source of FMC data for the ILS and other systems. In case of failure of one FMC, the switch is used to select the other FMC.

 (b)  
The FMC select switch is located on the EICAS select panel/FMC switch module on the pilots' center instrument panel (P2).

 G. Central Maintenance Computer System (CMCS)
 (1)  
The component of the CMCS that interfaces with the ILS is the central maintenance computer.

 (2)  
Central maintenance computer (CMC).

 (a)  Two central maintenance computers, mounted CDUs, an optional electrical equipment bay mounted CDU, the IDS, and an optional cockpit printer, provide maintenance functions for various airplane systems. The maintenance functions include: 1) Selection and display of maintenance messages corresponding
 to equipment faults for present and past flights. 2) Selection and de-selection of maintenance page displays for the IDS. 3) Flight deck initiation and display of the test results for the ILS ground test.
 11. Operation_________
 A. Block Diagram (Fig. 2)
 (1)  
The ILS consists of three subsystems designated as the left, right, and center ILS.

 (2)  
Each subsystem has an ILS receiver that accepts rf and digital data, processes the data and provides audio and digital ouput to various interfacing systems.

 (3)  
ILS receiver inputs

 (a)  
Localizer and glide slope rf signals of the tuned frequency are provided by the antennas selected by the VOR/LOC antenna switches and the G/S antenna coaxial relays.

 (b)  
Receiver frequency tuning input is normally provided automatically by the FMC when in autotune mode or manually by the CDU through the FMC. If neither FMC is operational, the CDU provides a source select discrete that allows its onside receiver to accept tuning input directly from that CDU.

 (c)  
An external self-test request is provided by the CMC when initiated using a CDU.

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747-400 MAINTENANCE MANUAL
 (d)  
A test/tuning inhibit discrete is provided by the FCC to prevent the initiation of a self-test or the retuning of a receiver during an approach.

 (e)  
An air discrete signal is provided by the air/ground relays which enables the ILS receivers to store fault messages in internal memory while airborne.

 (4)  ILS receiver outputs
 (a)  
ILS digital output from each receiver, along with discretes from the antenna relays and switches, are provided to the EIU's to generate video signals for ILS displays on the ND and PFD and message displays on the main and auxiliary EICAS displays.

 (b)  
ILS digital output from each receiver is provided to the DMU for recording purposes.

 (c)  
ILS audio output from each receiver containing ILS station identification in morse code, is provided to the audio management unit for controlled routing to cockpit speakers, flight crew headsets, and voice recorder.

 (d)  
ILS digital output from the left receiver is provided to the left FMC for position update. Digital output from the right ILS receiver is provided to the right FMC.

 (e)  
ILS digital output from the left receiver, along with discretes from the left VOR/LOC antenna switch and left G/S antenna coaxial relay, is provided to the left FCC for use during automatic landing. Right system digital and discrete outputs are provided to the right FCC, and center system digital and discrete outputs are provided to the center FCC.
　
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