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MAINTENANCE MANUAL
(4) GUN 001-050, 052-205; The functions of the DFDR are as follows:
(a)
The DFDR receives 115V ac power from the FLIGHT RECORDER AC circuit breaker through either the manual or automatic switching network. The DFDR contains a regulated power supply that generates the necessary operating voltages.
(b)
The DFDR receives flight data from the DFDAU in a 768 bit per second harvard bi-phase format. The data is converted to NRZ format and one second data blocks are alternately stored in two 768 bit blocks of RAM. Data from one block of RAM is converted back to harvard bi-phase format, written on the tape and verified for proper recording. At the same time, the other block of RAM is storing information.
(c)
The data is sequentially recorded on either tape tracks to achieve a 25 hour history of airplane operation. Two four track erase heads and two four track read/write heads are used for recording and playback. The proper read/write and erase channels are selected under CPU control based upon the track being recorded and/or operational mode. In the recording mode, the CPU monitors EOT and BOT sensors to initiate track changes at the completion of recording on each track. When the EOT/BOT sensor is detected, the direction of tape travel is changed and the next track is selected for recording. Data that identifies the track being recorded is stored in NVM so that in the event of power interruption, recording can continue on the same track after power is restored.
(d)
The tape transport is driven by a CPU controlled stepper motor. When operating in the recording mode, the tape is sequentially step driven two increments forward and one increment backward. During the first forward increment, the previously recorded data block is read and verifed. During the second forward increment, the 768 bit data is written on the tape. The tape is next driven backward one increment in preparation for the next cycle.
(e)
The playback data is routed to the DFDR front panel connector and to the DFDAU for output to a flight deck test connector. This enables the transfer of the recorded data to a portable copy recorder for ground processing. All eight channels are transferred simultaneously.
GUN 001-050, 052-999 ú 31-31-00
ú CONFIG ú 01 Page 14 ú Sep 28/01
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757
MAINTENANCE MANUAL
(f)
A primary function of the BITE is to monitor the recording verification that is accomplished during each step. The BITE also monitors tape motion, EOT and BOT sensors, track select, presence of input data, and CPU lock-up. The occurrence of any BITE failure, except for loss of input data, causes the BITE LED on the DFDR's front panel to be lit. A fault signal is also sent to the DFDAU, flight recorder control panel and to EICAS.
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