1.2.5 Altitude Decoder BoardThe Altitude Decoder Board contains the 26-pin rear connector (J2), discrete/parallel inputs andGillham/Greycode decoder circuitry. Encoded pressure altitude data from a parallel altimeter device isreceived 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 viathe 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 theInterface Board.6
