<david.corbett5@btinternet.com>
>
> Does anyone have experience of intermittent Mode C signal
between Garmin
> GNS 430 and Garmin GTX 320 xpdr? Having spent some time
under the
> impression that it was an installation problem, I am now
almost certain
> that the problem is internal. Any ideas?
>
It could be internal to either box, or the cable. So first
eliminate the cable, and then guess which box.
I believe the GTX-320 has large integrated circuits (LSIs),
which means the trivial job of differentiating what ATC has
requested (Mode A or C), and then appropriate bits to send
in reply (squawk or alt) is all done within one big LSI
chip. Those chips basically just don't fail.
It think it's required that if the supplier of the Mode C
data (your 430) believes it to be invalid (even like not
just yet warmed up), it brings all An, Bn, Cn, Dn Mode C
data lines high. They are "active low" inputs to the
xpondr, meaning code "0000" when all high -- which BTW is
not Mode C altitude of zero; so it's garbage for altitude.
That tells your xpondr to not reply to ATC's Mode C
interrogation. All those Mode C lines will read some
positive voltage above ground in this condition.
It's possible I suppose that the 320's LSI chip can also
perform a sanity check on the incoming Mode C data lines,
and reply with nothing. That could be just one of the wires
in the interconnecting cable is bad because the "Gray code"
they use affects altitude data sent in more random, and not
successive ways like our computer's simple binary code. So
it appears as an intermittent, but it's really not. You've
just changed altitude by +/- 100 feet. That means the first
thing to check is the integrity of the interconnect cable,
even under magnification.
So hope you botched the fabrication of that interconnect
cable! Else the prime suspect seems the 430.
Reg,
Fred F.
|