The GTX330 transponder is the recent addition. The 330 works but the
auto ground/air mode switching (based on GPS ground speed) doesn't
work.

According to the GTX330 Pilot's Guide, the only thing that GND mode does is
turn off transponder replies.

It's my understanding that it is no longer neccessary to turn off or put the
transponder into standby after landing and before departure because with
modern ATC equipment, ground clutter at airports is automatically removed
from radar displays.

Can ATC folks confirm this? I believe I've seen this in various
rec.aviation groups for years now. A quick search of Google Groups confirms
that. Personally, I leave my transponder in ALT mode all the time and have
never had a controller ask me to confirm that it was on.

If the only functionality provided affected by the GND/ALT mode switch is
whether or not the transponder replies to interrogations, then I'd say
forget about trying to get it to read the RS-232 info.

The KMD550 will be much more useful with the enhanced data that the KLN94
sends out. I don't think turning off the enhanced data stream for just the
GND/ALT mode switch is worth it.