Thanks to suggestions from N8MH and WA8SME, Data jack cabling now working. Hope to be on Falconsat-3 soon although optimal passes here are 25-29 degrees elevation.
For those interested, the IC-910H only uses the Data jacks for I/O when PTT is asserted at the jack itself. Input level needs to be ~ 0.4 V p-p or less. Otherwise, internal limiter shuts down modulation.
Best to all,
Bob N6RFM Fn41iq