Transponder wiring

[Terry Seaver <terrys@cisco.com>]

Help,

while wiring my Garmin GTX320 transponder to an
ACK altitude encoder I came up missing one signal.
The GTX320 has inputs A1, A2, ..., C2, C4, and D4,
while the encoder has only A1, A2, ... C2, C4 outputs,
but no D4 signal.
Does anyone know about these signals?
Shall I just ground or leave open the D4 input
to the transponder?

Terry Seaver