(3)
DME distance is provided to both RDDMIs and a DAA (D/A Adapter) for use by the FMCS and to the EFIS symbol generator for use by the EHSI's. Each RDDMI displays the distances measured by both DME systems. The EHSI's display either DME-1 or DME-2 distance. The DAA converts the 6-wire ARINC 568 DME signals to a 2-wire ARINC 429 signals and distributes these digital signals to the FMC and FCC-A and symbol generator. DME audio is provided to the audio integrating system where it is mixed with VHF Nav audio to provide identification of the ground station.
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A 737-300/400/500MAINTENANCE MANUAL
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M164 DME-1 INTERROGATOR (E2-1) M1163 D/A ADAPTER-1 M924 FCC-A (E1-3) EFIS SYMBOL(E1-2) GENERATOR-LEFT
DME SYSTEM NO. 1 SHOWN - DME SYSTEM NO. 2 SIMILAR
DME System Schematic
Figure 3 (Sheet 2)
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A
737-300/400/500MAINTENANCE MANUAL
(4) The DME interrogators and ATC transponders are interconnected by a suppression cable. When one system is transmitting, a suppression pulse is provided to the other three systems inhibiting their operation to prevent mutual interference.
C. Functional Description, VHF NAV Control Panel
(1)
The control panel provides control of the VHF NAV and DME systems (Fig. 3). Switches are provided to select automatic or manual frequency control, select the frequency during manual operation, and initiate self-test of the VHF NAV and DME systems. The frequency range is 108.00 to 117.95 MHz.
(2)
Manual Tuning
(a) Manual tuning is selected by pressing the AUTO/MANUAL switch-indicator. Relay K1 is de-energized and tuning is performed manually. MAN is annuciated on the AUTO/MANUAL selector switch. The annunciator is illuminated by 28v dc from the appropriate VOR/LOC DC circuit breaker, the ground is provided from the Master Dim circuit through K1. K1 applies the frequency select common to the 2-out-of-5 encoder, enabling the switched output. The manual lines are connected to the VHF NAV, and to the DME. The manual frequency display is driven from the unswitched output of the encoder, and AUTO is blanked.
(3)
AUTO Tuning
(a) Auto tuning function is enabled by pressing the AUTO/MANUAL switch-indicator a second time. The auto tune arm 28v dc power is applied to the AUTO/MANUAL switch-indicator relay control. 1) AUTO will annunciate. 2) The frequency select common thru relay K1 is removed from
the encoder and sent to the DAA.
3) The frequency select common from the the VHF NAV receiver is applied to the AUTO decoder/display driver thru energized contacts of AUTO relay K4.
4) 28v dc display power from the VOR/LOC circuit breaker will be applied to the AUTO decoder/display drives to enable the AUTO frequency display.
5) 28v dc is applied through energized contacts of AUTO relay K4 to the manual bar solenoid. 6) The AUTO/MAN switch is enabled which allows the pilot to switch back and forth between manual and auto modes.
(b)
The Flight Management System provides tuning signals to the DME, the VHF NAV, and to the control panel AUTO frequency display through the DAA.
(c)
Also when in the AUTO mode the flight management system may desire to use agility tuning, which will cause the AUTO frequency display to show dashes. A ground dash enable signal is applied thru energized contacts of relays K2 and K4 to the decoder/display driver to cause the frequency display to display dashes.
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