Equipment Installation Manual,
GDC31 Roll Steering Converter
GPS Mode Annunciator Control
The GDC31 contains a Normally Open relay contact configured as a SPST switch to control the external
GPS mode annunciator. This annunciator is further controlled by external switching and is active when
the pilot selects GPS mode in place of heading mode with the external switch. Refer to the typical
interconnect diagram later in this manual. If GDC31 output data is valid, the relay contacts are closed,
allowing the mode annunciator to illuminate. If the data is invalid, the GDC31 will cause the mode
annunciator to blink at a 1 Hz rate and command the steering output to zero degrees of bank (wings level).
The GPS mode annunciator blinks if either the super flag input is invalid or the serial data from the GPS
is missing or not the correct format (see Appendix A).
Super Flag Input
The GDC31 reads the Super Flag (14 or 28Vdc) from the GPS receiver. If the flag is greater than 5Vdc,
the flag shall be considered valid, if less than 2Vdc, the flag shall be considered as invalid. If the Super
Flag indicates invalid, the GDC31 shall produce a steering signal of zero degrees, blink the GPS
annunciator relay and invalidate the ARINC 429 label 121, bank command data, and label 312, ground
speed data.
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