Tim Houlihan wrote:
> I have a Terra AT3000 altitude encoder connected to a Garmin GTX 320
> Transponder, during initial flight tests I found that only mode
> A with no altitude information was being received .
> ....
> Can anyone advise me of a simple way to ground test
> this setup off the aircraft?
With xponder on, the and encoder turned on for at least the length of any
warm-up period specified my encoder mfr, measure the voltage at each encoder
data line. These lines are "active low," meaning if all are high (5-10V)
that's an invalid code which ATC will ignore. Tends to indicate the
encoder is at fault, or not warmed up. If working and at any ground altitude,
some will be low - 0V. What the true code is generally not important if you
can inhale on the static line and change some lines every 100'. However, one
bad data line -- one supposed to be low but isn't -- can cause ATC to see an
invalid code and no apparent output. For that you need the grey code chart.
> I plan to produce a test lead with LED's on the data lines
> to see if the encoder output code changes as pressure is reduced.
I'd be leery of doing this myself. Narco's Mode C input lines are "buffered,"
so OK to fiddle with homebrew circuits, but on Kings, you're going straight in
to a humungously expensive, big IC chip. What Garmin does I'd want to know
before proceeding with my inserted circuitry. Note also on King and Narco,
the internal "pull-up" resistors on the data lines are 10K ohms, meaning they
won't light an LED!
RST's kit with only LED's I presume is designed to be safe for all xponders.
But when I made one of these 25 yrs ago, I cautiously buffered the LED's with
transistors, which also made the LEDs light up to read active low. But I
have not used it since, since it hasn't been necessary for diagnosis.
Good luck!
Regards,
Fred F.
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