757 AMM 飞机维护手册 仪表 CHAPTER 31 - INSTRUMENTS 1(29)

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 (1)  The clock GMT output is displayed on the FMC-CDU (AMM 34-61-00) POS INIT page. Since the FMCs have a cross talk connection, they both use the same clock output. Both CDUs display GMT from the captain's clock, unless it has failed. Both FMCs would then switch over automatically and display GMT from the F/O's clock. The FMC-CDU GMT hour display can be manually changed through the FMC, but the minutes and seconds will still be that which is received from the clocks.
 3.  Operation (Fig. 2)_________
 A. Functional Description
______________________
 (1)  Power
 (a)  
The 28 vdc hot battery bus operates the digital displays, the sweep second hand, and the time base electronics. The 28 vdc primary bus is monitored by the clock for failure.

 (b)  
When primary dc power is interrupted, or it drops below 18V, or is lost, the LED in the voltage detector (optoisolator) dims or turns off. This causes the photo transistor to switch off informing the power supply that the primary dc power has failed.

 (c)  
Loss of 28V dc primary power causes inactivation of all clock controls, blanks the display, and freezes the sweep second hand. When 28V dc primary power comes on again, all functions are restored to the correct times without loss of accuracy.

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 (d)  Loss of back-up battery power for more than 200 ms disables all clock controls, indications, and outputs. When power comes on again, all functions return to zero. They hold at zero until the GMT control is turned to slew for a minimum of one count on the GMT display. This restores all functions to normal.
 NOTE: Partial loss of battery power may cause the time base
____ microprocessor to "latch up" such that the unit displays are blank when normal power is reapplied. If this occurs, the microprocessor can be reinitialized by cycling the time base battery bus circuit breakers. Clock reset can then be accomplished per par. 3.C.(1) thru (5) of this section.
 (2)  Clock
 (a)  
A crystal controlled oscillator provides the time reference. The microprocessor, under software control, uses the oscillator signal to generate the time base. Computation of GMT, ET, and CHR time is done by software. Countdown functions produce hours, minutes, and seconds. Microprocessor output data is converted to serial form to drive the LCD's. With pin 16 floating, data is sent out in ARINC 429 digital format. Binary and BCD GMT data words are transmitted sequentially for a total of 10 words/sec. The chronograph pointer is driven by seconds data from the microprocessor using a stepping motor with a closed loop controller.
　
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