(3)  
Each input parameter is processed by the microprocessor system in the same manner as the Primary Display. The microprocessor computes, formats and outputs the parameters to the display drivers.

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737-300/400/500MAINTENANCE MANUAL
 (4)  
Total Air Temperature Parameter

 (a)  
Total Air Temperature (TAT) data is via an ARINC 429 data bus ("left" channel only) from the Digital Air Data Computer (DADC). The left channel signal processing module checks the input data for parity, sign, status matrix, label, etc, and is assembled as parallel data in a decoder. The data field is processed, formatted and then displayed on a two digit plus sign, 7 segment LED display under the interrupt control of the microprocessor.

 (b)  
Out of range values of total air temperature are signalled by slewing the display to its lower limit, holding the setting for 2 seconds, then displaying "dashes". When normal signals are received the display automatically indicates the new value.

 (c)  
If status matrix signals "no computed data" the display is "dashed", if the status matrix signal "Failure Warning" or there is loss of input signal, the display is blanked. In all cases, removal of fault conditions causes the display to indicate the new value.

 (5)  
Engine Oil Pressure

 (a)  
The engine oil pressure signal is derived from a three wire variable reluctance transducer. The signal conditioning consists of half wave rectification of the reference and signal voltages. VA and VB are the signal and reference dc outputs respectively. The dc values are smoothed and routed to an analog to digital converter, via a multiplexer. The digitized values are processed to compute the oil pressure, formatted and displayed on an LED pointer under the interrupt control of the microprocessor.

 (b)  
The oil pressure is monitored and if equal to or greater than 13 psi, manual BITE is inhibited. Open circuit or short circuit input signal conditions, or out of range values of oil pressure are signalled by slewing the pointer to zero, holding the setting for 2 seconds, then "blanking" the display. If oil pressure returns to within range, the display automatically indicates the new value.

 (c)  
An analog output of engine oil pressure is obtained by applying the digitized value to a digital to analog converter. The analog output is used by the digital flight data acquisition unit (DFDAU).

 (6)  
Engine Oil Temperature

 (a)  The engine oil temperature signal is derived from a resistive temperature bulb. The signal conditioning consists of applying a constant current, from an internal reference supply, to the resistance probe and applying the voltage drop across the resistance probe to a differential amplifier. The amplifier output is applied to an analog to digital converter, via a multiplexer. The digitized values are processed to compute the oil temperature, formatted and displayed on an LED pointer under the interrupt control of the microprocessor.
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A 737-300/400/500MAINTENANCE MANUAL
 (b)  Open or short circuit input signal conditions, or out of range values of oil temperature are signalled by slewing the pointer to zero, holding the setting for 2 seconds, then "blanking" the display. If the oil temperature returns to within range, the display automatically indicates the new value.
 (7)  
Engine Oil Quantity

 (a)  
The engine oil quantity signal is derived from a three wire capacitance probe which can be considered as two separate capacitors. One capacitor is a variable quantity dependent upon tank quantity; the other can be considered as a zero oil quantity reference capacitor. The signal conditioning consists of applying an 8 kHz reference voltage to the zero reference capacitor and a variable 8 kHz voltage to the other capacitor. A multiplying digital to analog converter (DAC) is used to generate the variable 8 kHz voltage. The microprocessor successively adjusts the digital input value to the DAC until a detector circuit indicates equality of currents in each capacitor. The digitized value represents the ratio of the two capacitors and is used to compute the oil quantity value. The oil quantity value is formatted and displayed on a 2-1/2 digit 7 segment LED display under the interrupt control of the microprocessor.
　
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