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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
 (3)  
BRT - A brightness control knob is concentric with the DSPL SEL switch and is a potentiometer to control brightness of the displays.

 (4)  
There are two numeric displays on the ISDU. When the ISDU is receiving IRU data, the DSPL SEL switch determines the data on the display. When the ISDU keyboard is used to initialize an IRU, the data punched in at the keyboard is shown on the two displays. For invalid data from an IRU, both displays are blanked. A brightness control for the displays is located concentric within the DSPL SEL switch.

 (5)  
The keyboard consists of twelve lighted keys. To change the numeric display from the IRU receive mode to a keyboard display mode, one of the following keys must first be pressed, N(2), W(4), H(5), E(6), or S(8). Any other initial key closure is ignored.

 (a)  
N(2), W(4), E(6), or S(8) when pressed the first time cause a N or W to appear in the left display or an E or S to appear in the right display. They represent north, west, east, and south and are used to initialize the IRU when inserting longitude and latitude. Numbered data can now be inserted which will be appropriately displayed on the numeric display (in ALIGN Mode only).

 (b)  
The H(5) key when initially pressed first, also changes the ISDU display to the keyboard display mode. This key is used in the ATT mode to enter the magnetic heading.

 (c)  
Keys with letters on them as well as numbers, represent the letter value when they are the initially pressed key in a program sequence.

 (d)  
When the N, S, E, W or H key is pressed, the ENT highlighter bar illuminates and stays lit while digits are keyed in. When the ENT key is pressed, the ENT highlighter bar extinguishes and a reasonableness check is done on the data. When reasonable, the data is transmitted to the IRU and the display shows the selected parameters.

 (e)  
If the data is unreasonable, the CLR highlighter bar illuminates and the display retains the unreasonable entry. Pressing the CLR key causes the CLR highlighter bar to extinguish and the ISDU to display the selected parameters.

 D.  The ISDU is powered from both IRUs with 28 volt dc. The two power sources are connected in parallel to the filter which supplies dc for the operation of the unit. A single 28 volt dc source is sufficient to power the unit.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
 E.  Both IRUs supply ARINC 429 format into the ISDU. The system select switch selects one of the busses and supplies it to the processor which continually updates the stored information from the selected bus. In addition, the processor senses pilot inputs during alignment or during the attitude mode. When supplied with information through the keys from the pilot, the processor functions as a transmitter, supplying ARINC 429 information to the IRUs. When the ENT or CLR keys are pressed, the processor functions as a receiver.
 F.  The processor consists of a clock, synchronized by the selected IRU, a 32 bit register, which converts the ARINC 429 32 bit word to parallel 8 bit information, the data processor which controls unit operation and a bit sensor. The data processor senses discrete inputs from the display selector and controls the display accordingly.
 G.  The display circuit consists of a display logic, drivers, decoders and 13 display units. The logic circuit senses display selector position, decoder input and dimming control and accordingly controls the light segmented display.
 5.  Inertial Reference Unit (IRU) (Fig. 10)_____________________________
 A.  The inertial reference unit (IRU) provides attitude, acceleration, angular rates, velocity, true and magnetic headings, positional data, absolute altitude and wind data signals. The signals are developed from a set of three laser gyros and three accelerometers mounted to airplane reference. The signals are provided to other systems including the flight management computer system, the digital flight control system, the autothrottle, the VHF navigation system and the pilots' instruments.
 CAUTION: THE UNIT CONTAINS STATIC SENSITIVE DEVICES, AND PRECAUTIONS MUST
　
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