(3)
The digital air data computer (DADC) performs the computations on the three sensed inputs. They are static pressure (Ps) and total pressure (Pt) derived from the pitot static system and temperature from the temperature probe.
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737-300/400/500MAINTENANCE MANUAL
Pressure Transducer and Temperature Sensing
(1)
Each pressure transducer converts pneumatic pressure to digital data. The sensor element is a piezoresistive unit and functions as a strain gage. The resistance varies with pressure applied to the transducer assembly. The temperature probe is a resistance that varies as a function of temperature. The resultant resistance is converted to a voltage. All three analog signals are multiplexed and converted to digital in the analog to digital (A/D) converter and applied to the microcomputer.
Altitude Synchro Simulation
(1)
The microcomputer sends the digital representation of altitude to a digital to analog (D/A) converter. The resultant analog signal is the dc equivalent of altitude which is demultiplexed and sent to a sample and hold circuit. The representative dc voltages are routed to a sine and cosine generator, modulated by V ref ac and developed by a Scott T transformer into X and Y synchro-type signals. The output is a simulated synchro signal equivalent to the airplanes course or fine altitude.
Airspeed and Mach Synchro Simulation
(1)
The airspeed and mach outputs are produced by methods similar to the
altitude outputs.Baro Correction Altitude
(1)
Sine and cosine voltages from the altitude synchro simulator and baro correction 1 and 2 are sent to the baro-set synchro module and demodulated using the baro altitude ref 1 and ac as reference voltages. The output of the demodulator is buffered and sent to the multiplexer and to the A/D converter. The microcomputer processes the signal and sends it to the Arinc 429 transmitter for transmission on the four Arinc buses (Ref Fig. 5).
Altitude Rate
(1)
The computer determines the airplanes altitude rate and sends it to the D/A converter and demultiplex circuit. The demulitplexed output is developed by the sample and hold circuit and the buffered output is sent from the air data computer as altitude rate.
Airspeed (Q) Pot
(1)
The Q pot receives digitized inputs from the microcomputer. The speed switch (SPD SW) changes state at 190 knots during increasing airspeed and at 170 knots during decreasing airspeed. This results in a step gain change in the output dc reference voltage. The gain output is 1/Q or reciprocal of airspeed which is sent to the yaw damper system.
Digital Data Bus (ARINC 429)
(1)
The digital bus module receives the digital output from the microcomputer and translates it into Arinc 429 format. The output on all four buses is the same and consists of the following signals: pressure altitude, baro corrected altitude, mach, CAS, TAS, TAT, SAT, ALT RATE, static air pressure (Ps), total air pressure Pt and a source destination identification (SDI).
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737-300/400/500MAINTENANCE MANUAL
(2) The table of Fig. 5 identifies all digitized bus output and the units receiving this information. For ADC-1 the buses are labled DADC-L1, L2, L3 and L4. For ADC-2 the buses are labled DADC-R1, R2, R3, and R4. A receiver in the units receiving the signals selects only the signals required for that units operation.
I. Failure Warning
(1)
The computer continually monitors itself. Program timing is monitored by a watch dog timer (WDT) which compares program loop time to the time produced by a reference time clock (Real Time) in the WDT. If a discrepancy appears, the WDT first attempts a restart. If unable to accomplish a restart after 3 consecutive real time intervals a computer shutdown occurs.
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