Did the OP ever get this figured out? I have a Dynon D10A comunicating properly with an iFly 740. It also worked with my previous iFly 720. I ONLY use the transmit pin on the USB to serial converter cable. A couple of years ago it stopped working and I corrected the situation by changing out the USB to serial converter cable. I use extended with the default 9600 baud data rate. This works when connected directly to the Dynon D10A directly and I recently added an HS34 and it works connected through the HS34 also.
I use the same line to send data to my recently added UAvionic AV-30 PFD which gives me ground speed and the ability to use the iFly data to drive the HSI screen. The same line also sends GPS position data to my 406 mhz ELT. Yes, my aircraft is an experimental/homebuilt which lets me use the iFly output as I please. It does a great job driving the autopilot servos through the D10A.
Henry