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A
737-300/400/500MAINTENANCE MANUAL
 B.  Equipment cooling supply and exhaust OFF lights are mounted on the P5 overhead panel. Each light illuminates amber if the corresponding airflow detector detects a no-airflow condition. If the air/ground relay is closed (airplane on ground) a no-airflow condition also causes the ground crew call horn to sound.
 17. IRS Digital Interface (Fig. 16 and Fig. 17)_____________________
 A.  Simplified schematics of the IRS digital interface are shown on Fig. 16 and 17.
 B.  Figure 16 shows the input busses from interfacing components to the two IRU's. The data bus tables on the left list the parameters (from the DADC, FMC, ISDU and DAA) used by the IRU.
 C.  Initialization of the IRU's from the FMC is accomplished on the FMC 1 and 2 output buses. These buses also handle all the digital data from the FMC (AMM 34-62-01/001) to the DAA's, FCC's, and autothrottle.
 D.  Figure 17 shows the IRS output bus interface. The table on sheet 2 shows parameter information used by each interfacing component.
 E.  The three output buses from each IRU are all ARINC 429 high speed type. Identical data is output on each bus.
 F.  Digital data output bus No. 1 delivers inertial vertical speed to the VSI, and flight essential digital data and ground speed to the DAA and EFIS symbol generator. Heading data is received on this bus for digital-to-analog conversion in the DAA for analog output to the RDDMI.
 G.  Digital data output bus No. 2 is a dedicated bus which delivers the digital IRU data directly to the flight control computer (FCC).
 H.  Digital data output bus No. 3 is a utility bus. This bus provides digital data from the IRU to the VSI, DAA, flight control computer (FCC), flight management computer (FMC), CDU, ISDU, autothrottle computer, EFIS symbol generator, digital stall warning computer (DSWC), ground proximity warning computer (GPWC), the weather radar R/T unit and the flight data acquisition unit (FDAU).
 18. Operation_________
 A.  IRS Alignment
 (1)  
The IRS can be initialized by the FMCS-CDU or ISDU whenever the IRU is in the alignment (ALIGN) mode. When a mode select switch on the MSU is switched to ALIGN or NAV from OFF, the respective IRU enters the alignment mode.

 (2)  
In the alignment mode, the IRU reference axis is aligned to the local vertical. The IRU calculates latitude by estimating the horizontal earth rate components. While in the alignment mode, the IRU outputs a discrete signal to light the ALIGN annunciator on the MSU. When the mode select switch is switched to ALIGN from OFF, present latitude and longitude (lat/long) must be entered to complete the alignment process. Present latitude and longitude are not required when switching from NAV to ALIGN.

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INPUT DATA WORDS  SOURCE BUS
DADC  ISDU  FMC 
041 SET LAT  X  X 
042 SET LONG  X  X 
043 SET MAG HDG  X  X 
203 ALTITUDE  X 
210 TAS  X 
212 ALT RATE  X 

INPUT DATA WORDS  SOURCE BUS
FMC 
357 BITE WORD  X 

A 737-300/400/500MAINTENANCE MANUAL
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A 737-300/400/500MAINTENANCE MANUAL

IRU NO.2
SCHEMATIC
IRS Digital Output Interface Figure 17 (Sheet 1)

B

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