no "+V"

The AXP340 must be connected to an approved altitude encoding source. The AXP340 can use either a parallel Gillham code altitude input, or serial RS232 altitude input. Both of these interfaces are on the 24 way connector.  If the altitude encoder you are using offers both, we recommend using the RS232 serial input.  Serial formats allow a higher resolution altitude representation that can be used by Mode S interrogations, whereas parallel Gillham (gray) code format can only represent altitude to the nearest 100 feet.  You must choose between serial or parallel Gillham formats – you should NOT connect both.  If a parallel Gillham encoder is connected the AXP340 will always use that as the altitude source even if a serial encoder is also connected.

The parallel Gillham encoder inputs are active when the voltage to ground is pulled below approximately 4 Volts.  The AXP340 includes internal isolation diodes which prevent the unit from pulling the encoder lines to ground when the transponder is switched off.  The AXP340 can therefore share the altitude inputs with other devices without needing external isolation.

Parallel Gillham output altitude encoders intended for operation below 30,000 feet may not have a signal connection for D4.  In an aircraft with a service ceiling below 30,000 feet input D4 will never be active, and can safely be left unconnected.

The serial encoder input uses RS232 input levels.  The communication should be 9600 bps, no parity.  The AXP340 will correctly recognise either “Icarus/Trimble/Garmin” format altitude data, or “RMS” format altitude data.  Refer to the encoder documentation to determine jumper settings as appropriate.

The AXP340 can also accept Shadin family Format G, Format S and Format Z air data protocols which supply both altitude and airspeed information.  The airspeed information can be used to provide an automatic air/ground determination for an ADS-B installation.

The AXP340 includes a serial altitude output which repeats the altitude received on the encoded input (either parallel Gillham or serial) for connection to a GPS or other equipment.  The serial output supplies RS232 output levels, and runs at 9600 bps, no parity.  The output format is always “Icarus/Trimble/Garmin” format.  If the altitude source is a parallel Gillham encoder, the serial output is reported every 0.5 seconds; if the source is a serial encoder, the output simply repeats the input reports, each report delayed by up to 10 milliseconds from the corresponding input report.

The AXP340 transponder can accept altitude from multiple sources, the AXP340 will prioritize the altitude inputs in the following order:

1. Parallel Gillham code input (Gray code input)
2. GPS RS232 Input with Airdata information (e.g. Avidyne R9 IFD)
3. ADC/Serial Input

All the altitude sources above must meet either TSO-C10( ) or TSO-C88( ) per 14 CFR 91.217.  

The ADS-B output does not alter any existing regulatory guidance regarding the barometric altitude accuracy or resolution. Aircraft operating in Reduced Vertical Separation Minimum (RVSM) airspace must have the same accuracy and resolution for the ADS-B airdata transmission.