I thought that I had sent a message to the list a few months ago showing the procedure to use the software DDCMP driver in RSTS V10.1. That's a standard feature, but undocumented and probably unsupported. It works well and interoperates with PyDECnet and with the DDCMP implementation in SIMH. Presumably it will talk to the software DDCMP in RSX, though I have not tried that. Attached are the instructions and the program mentioned. paul