Marcus,
I think if you are trying to just send and receive, you need nothing special. If you want to DNC (dripfeed), then you need to put the machine in the FE mode and use the protocol in the software (TNC) to support this.
Your on the right track with sending back to the software 1st and then loading the same valid program back to the CNC. Make sure the program, name or # is different when loading back in.