(b)
Dashes also appear in the display when the interrogator is being agility tuned in the auto mode.
E. Functional Description, RDDMI Distance Displays
(1)
The DME distance displays in Fig. 4 on the RDDMI provide the pilot with slant range distance measured by the DME interrogator. The indicator receives digital data and a valid signal from the DME interrogator. The digital data is received in a six-wire BCD format. Two wires carry the 32-bit data word, two wires carry the word sync, and two wires carry the clock signal. The data decoder and shift register extract the distance data from the data word and provide the distance information to the magnetic indicator drivers.
(2)
The drivers position the magnetic wheel indicators according to the distance information. When an absence of computed data exists the magnetic displays are rotated to their eleventh position and four white dashes are displayed. This occurs during auto tuning and when the decoder detects a NCD (no computed data) bit in the digital data word. The monitor circuit monitors the DME valid discrete input, operation of the data decoder and shift register, and the internal power supply. A loss of valid input data, or a faulty power supply, results in the red and white striped flag obscuring the display. When all functions are normal, the flag is retracted out of view.
F. Functional Description, EHSI Distance Displays
(1)
The DME distance displayed in Fig. 4 on the EHSI provides the pilot with slant range distance measured by the DME interrogator. The FMCS digital-to-analog adapters (DAA) receive the same digital data and DME valid signals the RDDMI indicators receive from the interrogator.
(2)
The DAA receives the digital data in a six-wire BCD format. Two wires carry the 32-bit data word, two wires carry the word sync, and two wires carry the clock signal. The DAA sends DME slant range distance on an ARINC 429 bus to the FMCS flight management computer (FMC), flight control computer (FCC) and the EFIS symbol generator.
(3)
With the EFIS source select switch in NORMAL position. DME-1 slant range is processed through the left symbol generator and displayed on the captains EHSI. DME-2 slant range similarily is processed through the right symbol generator and is displayed on the first officers EHSI.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
(4) The DME distance is displayed on the EHSI in the upper left corner. The three digit display shows miles above 100 nautical miles (nm) and tenths of miles below 100 nm.
(a)
Manual mode operation. 1) Normal operation - DME message and 3 digit distance displayed. 2) No computed data - DME message and three dashes are displayed.
3) Invalid data - DME message and distance blank when there is a DME failure.
(b)
Auto mode operation.
1) VOR and ILS operation - DME message and distance are blank. AUTO message appears in the lower right hand side of the EHSI's.
G. Functional Description, DME Monitor
(1) The digital flight control system (DFCS) as shown in Fig. 5 continuously monitors the DME and provides a DME fault indication on the BITE control display unit (CDU). For further information refer to 22-03-10.
H. Functional Description, DME Self-Test (Fig. 5)
(1)
A DME-test is started by a push of the DME TEST switch on the VHF NAV control panel. The VHF NAV control panel should be in the DME position. This applies a ground to the interrogator range circuits which send a synthetic reply pulse. The reply pulses are synchronized to agree with zero nautical miles. The range circuit finds the time between the interrogation and self-test reply pulses. The distance is found from this data. If the DME-test is started, a DME VALID is held low for two seconds, followed by two seconds of no-computer-data. The output shows 000.0-miles for as long as the test input is held LO, plus the memory time of 11.4 seconds.
(2)
The DME self-test can also be started through the flight control computer while a lateral assurance ground test is done.
8. Control_______
A. Power is applied to the DME system by closing the following circuit
breakers on the P6 and P18 panels:
DME 1 AC
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