(4) There are two important equipment used in navigation system, one is the integrated flight systems accessory unit (IFSAU), the other is the digital to analog adapter(DAA). This IFSAU is line replaceable unit (LRU) located

in the electronic equipment compartment. Is contains two printed circuit card(A1,A2). Each of the cards is connected to a different rear connector on the LRU. The A1 card is the IFSAU autopilot card, it has circuits that operate independently to monitor, engage, and set autopilot and navigation modes. The A2 card is the IFSAU flight instrument card, it has circuits that operate independently to monitor, set, and test flight instrument system. It also contains suppression diodes and load resistors for externally connected circuits.

(5)
Tow digital to analog adapters(DAA) are installed in the 737 airplane. The DAA is part of inertial reference system(IRS). It stores inflight failures of the IRS in memory and monitors the IRS for current status. THE DAA converts the ARINC 429 data bus output from the IRS into an analog format and sends that data to the radio distance magnetic indicator9RDMI). The DAA converts an IRS BITE request discrete from the flight management computer system into an analog signal and sends it to the IRS.

(6)
The DAA perform other tasks not related with the IRS. They convert VOR/ILS navigation system signals from analog format into ARINC 429 format and sent the data to the electronic flight instrument system (EFIS) for display to the flight crew. The DAA convert analog information from the fuel summation unit into data information and send the information on an ARINC 429 data bus to the FMCS. The DAA also converts the low speed N1 from ARINC 429 format into analog format and sends the N1 information to the engine indicator. The DAA alos converts DME distance from ARINC 568 to ARINC 429 and sends it to the flight control computer(FCC) and the electronic flight instrument system (EFIS).