(14)
The sync signal is used to indicate which clock cycles are to be counted and used. When the sync line goes from a zero to a one, the next clock cycle is called "clock cycle 1”. Distance information is then transmitted using this clock cycle as a reference. The sync line goes to a zero during the last half of clock cycle eight to signify that the label has been transmitted. This is done to allow equipment (PDC and AFCS) which uses more than one information input to "read" the label and select only the information it uses.
(15)
The monitor circuits (Fig. 3), provides output data monitor (continuous), automatic self-test monitor, and track monitor. Data monitoring is continuously accomplished and consists of checking the serial data output lines for presence of clock, word sync and DME label. Automatic self-test is accomplished upon changing channels or every 45 seconds during search or automatic standby. Track monitor is accomplished while the DME is tracking a ground station.
(16)
A secondary digital ranging circuit (within the monitor) continuously seeks synchronous targets and compares the position with the DME range gate position. In the event a synchronous target exists at a range less than the DME range gate position (indicating the DME is locked to an echo), the monitor immediately causes the DME to unlock and begin a new search cycle, allowing, the ranging circuits to acquire the closer (nonecho) reply.
(17)
Rotating the switch to the STBY position turns off the transmitter. In this position, dashed lines appear across the DME indicators. With the DME function select switch in STBY or ORIDE a ground is applied to the BCD frequency converter which in turn appropriately programs the distance circuits. The system also has an automatic standby feature which places it in the standby mode when a ground station is not available or when the ground station signal is too weak to be usable. When this occurs, the search operation stops, the transmitter is turned off, the dashed lines appear, and the system is in all respects on standby except for the position of the DME control switch. When a usable ground station signal reappears, the system automatically starts operating.
(18)
Operation of the system in the O'RIDE position is essentially identical to normal operation. However, the 200-mile range limitation of certain selected DME stations is overridden and all channels can be utilized for full-range DME operation (390 nautical miles).
(19)
When manual TEST is activated, the distance circuits develop a synthetic reply pulse and apply appropriate distance information to the control circuits. The control circuits then develop the serial BCD word which is applied to the DME indicator. When the test is activated, the DME indicator reads blank for 2 seconds, followed by dashed lines for 2 seconds, then followed by 000.0-nautical mile readout.
(20)
Since both the DME and ATC systems are pulse coded and operate in the same frequency range, the DME interrogator sends a suppression pulse to the ATC transponders, the suppression pulse acts to prevent system interference. Similarly, the ATC transponders, when transmitting, suppress the DME interrogators.
5B8
Oct 20/83 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-55-0 Page 9
5B8
34-55-0 Page 10 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Oct 20/83
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
DME 2 AC
B. Tune the system (using the VOR/DME frequency tuning controls) to a DME channel with the DME function select switch set to the NORM or ON position.
C. The DME indicators show dashed lines if the DME is out of range; or during search, until miles indication is displayed on the indicators.
5C8
Jun 20/85 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-55-0 Page 11
H25250
DME System Binary Coded Data 5C9
34-55-0 Figure 4 Jun 20/83
Page 12
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
DME SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
GJ ALL EXCEPT B-2509 and B-2510 RH ALL EXCEPT 4X-RAA
1. General
A. The tree-type format makes use of the self-test features of the interrogator unit and gives a quick procedure for isolating faults without the use of additional test equipment.
B. The procedure is written for one DME system. When two systems are installed, trouble shooting may be accomplished by interchanging components.
C. If the procedures do not rectify the malfunction, airplane wiring should be suspected and checked by reference to the wiring diagram.
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