Thank you again for your help with this. I want to mention to you that I have been using the same cables
connected to the NCE Power pro command station for years but with an older Mac and older version of DP.
This leads me to suspect the driver for the Prolific cable. Prolific had a lot of problems with their chips being second sourced by chip foundries, and then selling them as original Prolific chips (rather like the problem FTDI has). I wonder if Apple has a driver in its newer OS that can distinguish the fake chips - the Prolific driver for Windows has been able to do this for some years, and is the reason ESU changed to FTDI based USB-Serial cables for their Lokprogrammer.
If this is the case the only solution is to replace the USB-Serial cable with one where the chips have known providence.
