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 When the software finds a failure, it:
 -isolates the source of the failure
 -does a failure-filtering process, to make sure that it is not something that has failed momentarily
 -keeps the failure data in the NOVOL RAM of the LGCIU
 -sends a failure signal to the CFDIU.
 (2) Software Monitor
 The LGCIU hardware uses a watchdog-timer to monitor the operation of the software in the unit. The watchdog-timer is a switching circuit which connects the software to the microprocessor. The software uses an 'audit trail check' to examine its internal circuits every 100 milliseconds. This makes sure that each software step goes in and goes out in the correct sequence. When the software finds that it is serviceable, it sets the watch-dog timer again. This must be done in less than 100 milliseconds from the time that it last occurred.
 When the software finds a program error during the 'audit trail check', it does not set the watchdog-timer again. If the watchdog-timer is not set in less than 120 milliseconds, the hardware gets data that a software failure has occured (and sets the microprocessor). This set of the microprocessor lets the software (if it is serviceable) set the watchdog-timer. The failure of the software to set the watchdog-timer (the hardware sets the microprocessor a maximum of 4 times) causes:
 -
the hardware to stop the microprocessor

 -
the software to send a failure signal to the CFDIU and keep the failure data in the NOVOL RAM.

 When the microprocessor stops, the software changes the condition of the LGCIU to 'ON Standby'. This causes the other LGCIU to control the L/G system (Ref. 32-31-00).
 1EFF : ALL 1 32-69-00Page 2 1 1 Feb 01/02 1 1 1CES 1 B. BITE Test

 The BITE test is software controlled and to start it you must make a test selection on the MCDU. The test is a process that operates most of the LGCIU hardware circuits. To prevent movement of the L/G and the L/G doors, it does not operate the solenoid driver of the:
 -selector valves
 -baulk solenoid of the L/G control lever.
 Approximately 7 seconds is necessary to complete the test.
 The failure conditions that do not show during the normal operation of the LGCIU can be found during the BITE test. This is because this function operates the hardware circuits. The Failure Condition Tables give these failure conditions.
 During a BITE test:
 -
the software failure-monitor function continues to operate

 -
the hardware failure-monitor function does not operate

 -
the LGCIU transmits incorrect L/G position information (ARINC 429 and discrete signals)

 -
the Sign/Status Matrix field of each ARINC 429 word is set to
 Functional Test

 -
the NOVOL RAM keeps all the failure data.

 At the end of the BITE test the BITE:
 -sends a CFDS message, or the signal TEST OK, to the CFDIU
 -sets the Sign/Status Matrix field of each ARINC 429 word to Failed or Normal
 -sends the failure data to the NOVOL RAM.
 A hardware interlock prevents the operation of the BITE test during flight. This interlock is operated by:
 -
a signal from the Brake and Steering Control Unit (BSCU) (when the aircraft wheel speed is 70 kts or more)

 -
a 'not compressed' signal from the L/G.

 C. Failure Data
 The failure monitoring software makes sure that the failure found is correct. This is done when it has completed its isolation and failure-filtering process. The failure data is then kept in a specified area of the NOVOL RAM in the LGCIU.
 1EFF : ALL 1 32-69-00Page 3 1 1 Feb 01/98 1 1 1CES 1 If a failure occurs when the aircraft is in flight, the failure data goes into the flight memory. The flight memory has the capacity to keep the data of 30 failures. Each failure data entry can be kept in the memory for a maximum of 64 flights. The data kept in the memory for 64 flights automatically erases at the start of the subsequent flight. If the memory becomes full in less than 64 flights, each new entry causes the initial entry to erase.

 The data for a specified failure only goes into the memory once during each flight. If there is an intermittent failure during a flight, the software:
 -puts the data into the memory the first time that the failure occurs
 -counts the number of times the failure occurs again (the software can make a maximum of 4 counts)
 -puts the count value into the memory with the applicable failure data.
 If a failure occurs when the aircraft is on the ground, (this includes those failures found during a BITE test), this data goes into the ground memory. The ground memory has the capacity to keep the data of 3 failures. When it is full, each new entry causes the initial entry to erase. All the failure data in the ground memory automatically erases at the start of the subsequent flight.
　
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