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(1) Signal Source and Fault Detection provides independent selection and monitoring of sensor input data in each FCC. Signal selection is provided to maintain servo tracking during multi-channel operation and when dual flight directors are selected. Monitoring sensor data allows detection of a failed sensor and permits exclusion of the invalid data from autopilot servo and flight director command computations. The SSFD also provides failure flags for Caution and mode fail annunciations. There are three types of SSFD algorithms utilized for triplex, discrete, and dual/single source sensors.
(a)
Median Select SSFD Process computes the mid-value of triplex
(3) sensor inputs.
(b)
Discrete SSFD Process perform a majority vote of three discretes.
(c)
Force Select SSFD process selects one data source for use by all FCCs. ADC and IRU data is force selected.
E. Autopilot/Flight Director System CMC Monitor and Test (Fig. 10)
(1) The FCC provides the following system interface tests:
(a)
ARINC 429 inputs from system units are checked in two ways. First, the FCC checks the sign status matrix to verify the validity of the incoming data. The buffer memory locations on I/O data path assembly are also periodically checked to determine if the data words have been updated. If the words have not been updated, then a fault is assumed and the data words at the tested memory locations are not used.
(b)
Discrete input and output interfaces are monitored by software which detects shorted wires. These monitors are used for the caution/warning engage and arm solenoid power, trim outputs, engage and arm commands and inputs. The interface monitor circuits are tested, as appropriate, at power up or prior to engagement of autopilot/autoland.
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MAINTENANCE MANUAL
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AUTOPILOT/FLIGHT DIRECTOR SYSTEM INTERCHANNEL DATA -
____________________________________________________ DESCRIPTION AND OPERATION
_________________________
1. General_______
A. The three Flight Control Computers (FCCs) communicate with each other via interchannel digital data buses and analog discrete signal lines. There are two input and two output interchannel high speed ARINC 429 buses connected to each FCC. Also, four input and two output analog discrete signal lines are connected to each FCC. Sensor signals, engage, and autoland status are shared to allow signal voting, fault detection, and determination of proper annunciations based on the status of all FCCs.
2. Operation_________
A. Functional Description
(1) Autopilot/Flight Director Interchannel Data (Fig. 1)
(a)
General
1) All three FCCs communicate with each other via the four ARINC 429 high speed interchannel data buses and the six analog discrete signal lines. The interchannel digital data buses operate at 100 Kilobits Per Sec (100KBPS). The FCCs share sensor signals on these data buses. This allows signal voting and fault detection through comparison. The FCCs also share servo engage and detent monitor trip information using the analog discrete signal lines. This allows each FCC to determine proper engage/disengage criteria and annunciations based on the status of all FCCs.
(b)
Digital Buses
1) All data is transmitted as non-standard words. The ARINC word is reformated so that data in the most significant bit is right shifted one position. This makes bit 29 a SSM bit. The right shifting makes bit 28 the most significant bit. This causes the full scale input range to double since the maximum input range is .5. Therefore, test equipment for a standard 429 bus cannot be used without manual conversion and interpretation of the data.
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