Ok mate, first thing is slow down the baudrate to say 19200 or even 9600 as I don't know how well your cable is shielded etc so you may get noise through the line that can change what is being sent.
Check all the com settings on the pc/updates software and make them match the rfm
Baud rate you currently have is 115k so the rfm rate has to bethe same
Parity setting can be either even, odd or none e,o,n again set both devices the same recommend Even
Databits can be 7 or 8 set both devices to 7
Stopbits 1 or 2 set both to 2
9600,E,7,2 is a safe comm setting to test. The transfer will be much longer in duration but data corruption is less likely. Slowing the baudrate improves data integrity
I will need to look up my notes on null modem wiring for 9 pin to 9 pin. I assume that's the cabling you are using and I haven't been in IT or cnc for some years no so this stuff is no longer burnt to memory
Try getting the settings synced first and see how you go
