Ron Natalie wrote:
Scott Rodriguez wrote:

Looking at buying an altitude encoder for the transponder in my
yet-to-be-born homebuilt.

I see some altitude encoders have an RS-232 output for sending altitude
information to a GPS. What kind of functionality does this provide?
Is it
"real-world" useful?

It's required for IFR approach certification I believe.

Negative. The encoder can be used to provide altitude alerting functions
through the gps, but it's not precise enough for providing vertical
guidance in an approach. The typical serial encoder reports in 100 ft
increments.

Baro-vnav (which is not required to utilize an otherwise approach
certified GPS) requires a much more precise input signal.

My KLB 89B allows the connection of an encoder to it. It's sole
usefulness is in serving as an altitude alert, and in calculating
density altitude/true airspeed (not having to enter pressure altitude
manually)

Dave