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Combined Airspeed/Altimeter Indicator  553 
34-12-01  Figure 5  May 20/82 
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6.  Digital Air Data Computer
A. General
(1)  
The digital air data computer (DADC) is a solid-state device that uses pitot static pressures and air temperature inputs to compute air data information required by airplane displays and systems. In addition to the above inputs, 115-volt ac and 26-volt ac are provided to a multipin connectors at the rear of the computer.

(2)  
The computer is of modular construction and consists of two pressure transducers, 12 plug-in printed circuit cards, a power supply and a chassis assembly. The front panel (Fig. 6) provides two switches for self-test actuation, a self-test indicator light, a fault isolation test connector for off-aircraft trouble shooting and pitot static pressure input connectors. Three electrical connectors mounted on the back of the chassis provide interconnection with the airplane wiring.

(3)  
The TEST SELECT switch manually selects built-in test to be performed as follows: FUNCTION - transmits fixed values on all the computer signal output lines. SLEW - sets altitude rate to 600 feet per minute. FAIL - activates all failure warning flags.

The PUSH TO TEST pushbutton switch initiates the built-in test and the TEST VALID WHEN LIT indicator light comes on within 2 seconds after manually activated test is successfully completed and remains on if there is no failure.
B.  Functional Description
(1)  
The computer uses primary inputs of static pressure (Ps), total pressure (Pt), and air temperature (Ti) to compute air data information. Other inputs required by the computer are power, reference voltages, enabling and command signals, and programmed inputs to inform the computer what airplane it is installed in. The computer contains a microcomputer that operates on the input information to form output parameters of analog and digital voltages representing altitude, airspeed, air temperature, and derivations thereof.

(2)  
Two plug-in circuit card assemblies each contain a microprocessor that controls an arithmetic processing unit for making air data calculations. Both circuit card assemblies have large scale integration (LSI) integrated circuits that contain read only memory (ROM) for program storage, random access memory (RAM) for scratch pad memory, and peripheral integrated circuits (IC's) having I/O ports. All of the LSI integrated circuits are tied together by common buses. Calculations of the computer are performed on a time-shared basis.

(3)  
Two pressure inputs and one temperature input are converted into 16-bit data words. Various reference, enabling, and command signals are received from the airplane and are used by the computing circuitry (using the pressure and temperature data) to provide outputs that drive airplane systems and indicators. The computer uses solid-state electronics to perform functions that were formerly carried out by mechanical devices. Synchros and potentiometers are synthesized using solid-state electronics. Also developed are hold signals, rate signals, and failure warning signals.

(4)  
The computer contains a digital information transfer system (DITS) that generates a serial, bipolar output. An air traffic control (ATC) code is also generated. The computer also contains an extensive, self-test monitoring system.

(5)  
A static source error correction (SSEC) program is stored in the instructional program. The SSEC program is used by the computer to compensate for differences in airplane type. The program to be used is selected by airplane program jumpers that inform the microcomputer which type of airplane the computer is in.

(6)  
Outputs to the air data instruments from the air data computers and air data computer interface to other systems are shown in Fig. 7.

5C8 
May 01/98  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-12-01 Page 7 

Digital Air Data Computer  5C8 
34-12-01  Figure 6  Jun 20/85 
Page 8 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

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