I have just checked with myRIO web site and I had found that myRIO uses TTL (5V) logic on its UART port and this would match with the protocol for the DATA line of typical ROBOTIS Dynamixels:
If this is correct for your system, perhaps you could try this person’s approach (you’ll need to turn ON English Translation for your web browser as it is in Korean):
Basically, this person wired TX and RX lines together for his UART Port and this person claimed that this setup worked OK for his/her setup. So try at your own risk. Personally I prefer the extra hardware protection circuit provided by the U2D2.