Thanks to both of you (as usual!). I was working on a ‘live’ machine and what was supposed to take 15 min. turned into 2 hours, so I had to wrap it up for today. I also came to the conclusion that it must be my cheapie 9-pin to USB adapter.
FWIW, my experience with USB adapters and groov products was always in the RS485 mode and I never had issues thanks to recommendations like this.
I will order this adapter since the above linked post lists it as tested and compatible with the Rio, as well as the Gearmo model listed in the Rio manual. Hoping that our EPIC will accept at least one of these.