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5C8 
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5C8 DME System Interface SchematicFeb 20/86 Figure 2 34-55-02Page 7/8 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

(12)
The distance data is transmitted during 32 clock cycles (Fig. 3). During the first clock cycle, a "one" is transmitted. During clock cycles two through seven, a "zero" is transmitted. During clock cycle eight, a "one" is transmitted. This makes up the "label" which identifies the information to follow as "DME Distance". During clock cycles 9 thru 12, zeroes are transmitted to create a "pad" to make the information fit a standard format.

(13)
During clock cycles 13 thru 16, a one-hundredth mile data is transmitted. Clock cycles 17 thru 20 define one tenth mile data, cycles 21 thru 24 define one mile data, cycles 25 thru 28 define ten mile data and cycles 29 and 30 define hundred mile data (Fig. 3). Clock cycles 31 and 32 define information used to determine the status of the DME. A "one" during clock cycle 31 indicates that the DME is undergoing an internal functional test, and a "one" during clock cycle 32 indicates that the DME has "no computed data" to transmit and the RDDMIs must display flags.

(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) which uses more than one information input to "read" the label and select only the information it uses.

(15)
The monitor circuits (Fig. 2), 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 (non-echo) reply.

(17)
The system 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 (less than 450 squitter pulses per second). 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 (more than 450 squitter pulses per second), 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 the DME TEST button is activated, the distance circuits develop a synthetic reply pulse spaced to represent the functional test distance of zero nautical miles. The distance circuits measure the time between the interrogation and self-test 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 RDDMI's. When the test is activated, the DME indicator reads blank for 1 second, followed by dashed lines for 1 second, then followed by 000.0-nautical mile readout.

5C8 
Feb 20/86  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-55-02 Page 9 

8.  Control
A. Power is applied to the DME system and flight instrument displays by closing the following
circuit breakers on the P18 and P6 panel:
DME 1 INST XFMR-1
DME 2 INST XFMR-2

B.  Tune the system to a local VOR/ILS station with the DME function select switch set to the NORM position.
C.  The RDDMIs display flags if the DME is out of range; or during search, until miles indication is displayed on the indicators.
　
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