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After 340 |40 seconds on battery power, the DC power disconnect relay in the flight instrument accessory unit opens to remove battery power from IRU-2. IRU-1 stays on battery power until the voltage drops below 18 V.

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 (f) (g)  When the mode select switch on the MSU is moved from OFF to ALIGN or NAV, a self-test BITE signal disables the 115 V ac input and inserts the 28 V dc battery as the power source when both sources are available and valid. This tests the battery and associated power switching circuits. The ON DC annunciator lights momentarily during this test.The ISDU or the FMC supply initialization data and the ADCs furnish required air data inputs to the system. The MSU also supplies mode selection discretes to the IRU. A remote test discrete directs IRS to output "canned" data for checks of the
 (h) (i) (j) (k) (l) (m)  system.Gyro electronics - Each laser sensor assembly contains a laser gyro fringe pattern detector, and control circuits. The main function of each laser gyro assembly is to produce pulses indicating CW/CCW rotation.The laser gyros detect pitch, roll, and yaw rates and provides this data to the IRU computer for computation and navigation. A temperature sensor provides a signal that is used for correction of thermal drifts. Each gyro has correction values stored in a PROM which details the gyro's unique bias, thermal, and alignment characteristics. The PROM data is routed to the CPU at turn-on. It is used to provide error correction of laser gyro outputs.The laser gyro is started by -950 V dc and +3500 V dc levels from the HV power supply. Once laser action starts, the +3500 V dc is turned off and operation is sustained on the -950 V dc supply. Control signals standardize the path length for each gyro such that gyro operation effeciency is maximized. A dither drive circuit controls the operation of the dither motor. The dither motor develops a vibration to keep the gyro out of the lock-in region during low rotation rates.Accelerometer electronics - These circuits consist of three accelerometers, temperature sensors and wave-shaping circuits. This portion of the IRU outputs analog and digitized acceleration. The output of each accelerometer is an analog current proportioned to acceleration. The current is integrated and digitized and the digital counts are stored. They are then used to produce a component of velocity and change in velocity (acceleration) for each axis.The accelerometer electronics also contains a 7.68 MHz precision system clock. The clock outputs are divided down to 3.84 MHz and 640 kHz. The clock signals control the timing of data transfer and computer program execution.Sensor electronics and A/D multiplexer - The A/D multiplexer converts analog input signals from the sensors into digital words. The input signals are accelerometer analog currents and sample and hold signals, pitch rate, temperature sensor signals, and BITE monitor signals.

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Acceleration and gyro data are received under timing control of a clock signal. The accelerometer pulses are accumulated as velocity and change in velocity. The gyro pulses are summed as changes in angular rate and direction. Coning circuits correct the gyro data for errors developed by the cone-shaped motion of the gyros. Sensor data is then routed to the common CPU bus.IRU sensor data is supplied by the laser gyros, accelerometers, and their temperature sensors. Signal outputs of these devices are formatted into data words for manipulation by the IRU computer. A common data bus links the following blocks of the IRU. 1) Input electronics. 2) Sensor electronics and A/D multiplexer. 3) CPU 4) Output electronics. 5) BITE electronics.Input Electronics - The IRU receives altitude, altitude rate, and true airspeed (TAS) from the air data computer (DADC). The flight management computer (FMC) and the ISDU, each send initial latitude/longitude and heading (in the ATT mode only) to the IRU. This data is placed in a buffer which is accessible to the CPU. Mode 1 and Mode 2 discretes, switched from the MSU, select the operating mode using open/ground logic as shown on the schematic.CPU - The IRU computer is controlled by a stored program. The computer uses the sensor data to perform calculations.A 16-bit processor is used as the central processing unit (CPU) for each IRU. Input control signals direct the CPU to perform specific tasks with the aid of software. The program is run at the system clock frequency of 3.84 MHz. The sensor inputs are compensated for inherent drifts, bias, and misalignments. The program uses stored coefficients to perform this function. The inputs are used to compute airplane attitude. This attitude is referenced to the local level navigational coordinates. The angular rate of the airplane over the surface of the earth is also included in the computation. This rate accounts for the airplanes velocity and the earth's rotation. This provides for rotation of the local vertical. Attitude data is also used to resolve acceleration data into the local level coordinate frame. Acceleration is used to compute the airplane's horizontal and vertical velocity. It is also used to compute the airplanes present position.A magnetic deviation look-up table is used to compute magnetic heading. The look-up table contains values of global deviation stored in memory. As the result, heading is computed as a function of present position latitude and longitude. This computed data is added to true heading and true track to produce magnetic referenced data. The IRS uses inputs from the ADC to compute windspeed and direction. It also uses these inputs to smooth out the vertical velocity and altitude computations.
　
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