Digi One IAP as a serial to IP bridge

I am using 2 x Digi One IAP’s. Basically they are connected back to back and are being used to convert IEC 60870-5-101 serial to IP and then back from IP to serial at the remote end. The problem that is being experienced is that the Digi One IAP’s seem to be inserting an additional FF into the message whenever there is an FF already there so that the message:
10 5B FF 70 16 is transmitted as
10 5B FF FF 70 16

I am unsure if I have incorrectly configured this, but this is causing some serious problems since the Master throws the message away with Checksum errors.

Any assistance would be appreciated, I have attached the configuration file for the digi one IAP