5
HLDCLATCH
DEMOD

6
LATCH CPU D/A

DEMOD

7
LATCH
DEMOD

8
LATCH
DEMOD

9
LATCH

20
21
22
RESET CLEAR
WJ123 4
FAILURE
MONITOR

POWER

12
SUPPLY LLDC MONITOR
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Fuel Summation Unit Schematic
Figure 8

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A
737-300/400/500MAINTENANCE MANUAL
 E.  The summation unit provides an analog output proportional to total fuel quantity. The CPU controls the analog output amplitude with a 12 bit Digital-to-Analog converter (D/A). A 10v ac winding on the power transformer provides the reference voltage to the D/A. The output of the D/A is scaled between 0 and 10v ac depending on the binary combination latched into the converter by the CPU.
 F.  A failure monitor circuit checks the CPU for normal operation. Any abnormal operation causes a reset which zeros the output of the CPU and clears the D/A. The power supply monitor will also cause a reset through the failure monitor if the LLDC is too low.
 G.  Four jumpers (for internal calibration) are within the fuel summation unit. The jumpers permit the unit to be pre-calibrated for full scale analog signal output.
 7.  Flight Mode Annunciator (N1 Limit Portion) (Fig. 9)__________________________________________
 A.  The N1 LIMIT portion of the Flight Mode Annunciator (FMA) provides two separate functions: N1 limit mode and FMC MSG/FAIL annunciations.
 B.  The N1 Limit annunciation function consists of four annunciation modules. Each module consists of two solenoids and a rotating prism. All four modules are identical except for the legend on the face of the prisms. The FMC 28 volt dc input provides the operating voltage for each solenoid with the appropriate ground supplied through the digital analog adapter (DAA).
 C.  If TO is the FMC mode then J1-22 is grounded through the DAA. This completes the cirucit for the A face solenoid in module D4. This causes the prism to rotate and display face A. DAA monitors ensure that only one face is activated on an annunciator module at a time.
 D.  The amber FMC annunciator on the FMA illuminates if the FMC, MSG or the FMC FAIL discrete is set by the FMC. The MSG discrete is normally open and the FAIL discrete is normally a ground. This makes gate 2 logic high which holds FF-1 reset. S1 is open so the FMC annunciator is off. If the MSG discrete goes low or the FAIL discrete goes high, the reset is removed and FF-1 is set by gate 1. S1 closes causing the amber FMC annunciator to illuminate if 28 volts is available from the airplane (A/P) 28 volt dc input.
 E.  Pushing the annunciator resets the warning by applying a reset to the flip flop provided A/T or A/P 28 volt dc is present. The reset pulse is also applied to the other FMA since the MSG and FAIL discretes are applied to both FMAs. Since FF-1 was set by a single shot the annunciation will not reappear unless the discrete that caused the annunciation is reset by the FMC or the FMC sets the other discrete.
 F.  The FMA has a built in test function which allows testing of all annunciation modules and the FMC light. When the test switch is held in position 1, switch S2 is closed. S2 provides a ground for each A solenoid which causes all annunciation modules to display face A. S2 also provides a ground to set FF-1 and turn on the FMC annunciator.
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A 737-300/400/500MAINTENANCE MANUAL
A/P 28V
FMC MSG
J1 J2
FMC FAIL
FNG RESET J1 TO
GA
REDUCED
CON

CRZ
SPARE

CLB
SPARE

A/P 28V
FMC 28V
A/T 28V
J2
A/P 28V

Flight Mode Annunciator (N1 Limit Portion) Figure 9
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