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1.2.5 Altitude Decoder Board
The Altitude Decoder Board contains the 26-pin rear connector (J2), discrete/parallel inputs and
Gillham/Greycode decoder circuitry. Encoded pressure altitude data from a parallel altimeter device is
received as a 10-bit data word at the 26-pin rear connector. Diode isolation on these input lines pre-
vents damage to the GPS 155. The data is then decoded and output to the Main Board data bus via
the Interface Board.
The Altitude Decoder Board also provides a discrete input for the GPS SEQ switch. When grounded,
this line activates the GPS 155’s HOLD mode. This input is sent to the Main Board data bus via the
Interface Board.
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