(9)
A pushbutton CHR control switch is located on the clock bezel upper left corner. The CHR function can also be controlled by the corresponding CLOCK switch located on the lightshield. The pushbutton switches can be used alternately and function as follows:
(a)
Press CHR once, the chronograph starts running and the CHR display starts at zero.
(b)
Press CHR a second time, the CHR time accumulator and display stop and hold at the indicated value.
(c)
Press CHR a third time, the CHR time accumulator resets to zero, the sweep second hand returns to zero, and the accumulated minutes on the CHR display is blank.
3. Clock Switch (Fig. 1)
____________
A. The Captain's and F/O's CLOCK switches are located on either side of the lightshield panel (P72).
B. The remote CLOCK switch for the Captain's or the F/O's clock performs the same function as the CHR switch for that clock. Both switches cycle the chronograph through the start, hold and reset functions of the chronograph mode.
4. Operation (Fig. 2)
_________
A. Functional Description
(1) Power
(a) The 28 volt dc main hot battery bus supplies power for all clock functions. As long as power is available on the main hot battery bus and the ELEX CLOCK circuit breaker remains closed the clocks will continue to operate. 1) The main hot battery bus will remain energized as long as
the main battery is installed.
2) Loss of main hot battery bus 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.
(b) The 28 volt dc APU battery bus is used by each clock to generate a negative dc voltage which is applied to the clock display circuitry. If power is removed from the APU battery bus, or the CLOCK DISP circuit breaker is opened, the negative dc voltage will be removed from the display circuitry which will cause the displays to go blank. 1) The APU battery bus is normally energized under the
following conditions: a) Engine generated, APU generated or externally applied AC power is available.
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747-400
MAINTENANCE MANUAL
SWITCH
F/O CLOCK
Clock System Block Diagram
Figure 2
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
b) The airplane is on the ground and unpowered and the BATTERY switch on the Pilot's Overhead Panel is set to ON.
(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 LCDs. 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/second. The chronograph pointer is driven by seconds data from the microprocessor using a stepping motor with a closed loop controller.
(b)
Loss of the time base will blank GMT and disable all the indications.
(c)
The control switches provide electrical grounds for logic circuits in the microprocessor. The clock and CHR switches are latch (F/F) circuits to prevent interference from the switch contacts. Grounding the test pin (10) causes the seven segment display to show all 8s.
B. BITE
(1) All clock failures are indicated in the same manner. The clock displays go blank and the sweep second hand stops. In addition, transmission of GMT output data words is halted.
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