Victor wrote:
: I have installed a KLN-89B installed but the altitude comming from the
: xponder is not working. The xponder is an old Narco AT50. I wonder if
: other pilots have another model of xponder installed that actually
: interfaces with the GPS.

This assumes that the KLN-89B is reading the parallel (gray-code)
transponder output that the transponder is connected to.

You need to use isolation diodes on all lines from the Narco transponder
to the encoder, and from the GPS to the encoder. If you don't do this, the
GPS will read the encoder incorrectly. You'll also need pull-up resistors
on the outputs of the encoder.

ASCII schematic:
+V
|
(kln89B) -------|--- 10K
| |
--------------- (Encoder)
|
(AT50) -------|---

^^
Diode

--
Aaron Coolidge (N9376J)