H.  The lights may be tested by pressing each light switch assembly, by activating the bright-dim-test circuit or by initiating an IRU test.
 4.  Inertial System Display Unit (ISDU) (Fig. 9)
___________________________________
 A.  The inertial system display unit (ISDU) provides pilot interface with the IRUs. The ISDU allows entry of initialization data for the IRU's. The display of track angle, ground speed, present position, wind direction and speed, heading and system status is available.
 CAUTION: FAILURE TO OBSERVE ELECTROSTATIC DISCHARGE PRECAUTIONARY PROCEDURES
_______ WHEN HANDLING THE ISDU MAY RESULT IN DEGRADATION OR FAILURE OF THE ISDU.
 B.  The ISDU is located on the aft overhead panel, P5. The unit weighs four pounds (1.8 kgms) and is interfaced with airplane wiring through two rear mounted connectors.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

ISDU Schematic
Figure 9

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 C.  The ISDU front panel contains two sets of, seven segmented displays. Mounted on the left side of the front panel is a two-position system display (SYS DSPL) switch and a five position display select (DSPL SEL) switch. On the right side of the panel are a set of twelve keys called the keyboard.
 (1)  
The SYS DSPL switch is a two position switch for selecting either the left or the right IRU. The ISDU can only initialize or receive data from the IRU which has been selected on the SYS DSPL switch.

 (2)  
The DSPL SEL switch selects the type of data to be displayed on the ISDU numeric displays. The IRU, as selected on the SYS DSPL switch, supplies the data. The five switch positions and the data displayed for each position is as follows:

 (a)  
TEST - The test position provides a remote test signal to the selected IRU. When held in the TEST position, fixed outputs from the IRU selected by the SYS DSPL switch are displayed on the ISDU and the associated pilot's instruments. The test switch is inhibited during the navigational mode when the airplane ground speed is greater than twenty knots and during the attitude mode. During the first 2 seconds, all display segments and lighted pushbuttons, except the lightplate lamps, illuminate simultaneously and the highlighter bars on the ENT and CLR keys illuminate. Then, for 8 seconds fault messages are generated which are followed by test value displays.

 (b)  
TK/GS (Track Angle/Ground Speed) - True track angle from 0 to

 359.9 degrees is displayed in the left display with a resolution of 0.1 degree. Ground speed from 0 to 2000 knots is displayed in the right display with a resolution of 1 knot.

 (c)  
PPOS (Present Position) - Latitude from 90°S to 90°N is displayed in the left display and longitude from 180°E to 180°W is displayed in the right display. Resolution is 0.1 minute. The display is used when inserting present position during initialization of the two IRUs or when monitoring present position from an IRU during flight.

 (d)  
WIND (Wind speed and Direction) - Wind speed from 0 to 256 knots is displayed in the right display with a resolution of 1 knot. Wind direction from 0 to 359 degrees is displayed in the left display with a resolution of 1 degree. When the airplane is on the ground, the wind speed display will read 100 knots.

 (e)  
HDG/STS (heading/Status) - True heading from 0 to 359.9 degrees is displayed in the left display with a resolution of 0.1 degree. The ALIGN STATUS appears in the left side of the right display and is a countdown of the last 7 minutes of the ALIGN cycle. Malfunction codes (M/C) appear in the right side of the right display.

 (3)  
BRT - A brightness control knob is concentric with the DSPL SEL switch and is a potentiometer to control brightness of the displays.
　
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