Many thanks for the replies and yes Duncan the replies were very fast.
I now think I have a good idea what to look for
Regards
Tim H
G-BZTH
----- Original Message -----
From: <n3eu@comcast.net>
Subject: Re: Europa-List: Mode "c" info required
>
> 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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