I just got some idea that might help with your Mac problem and DGT.
Though I had a Mac at my disposal for some hours I could not solve it.
Anyway, a collegue played arrount with a Novag Citrine that didn't work
either. His problem was that this board, as the DGT, does not use any
flow control, so it does not send CTS/RTS signals the PC was suspecting.
The DGT has most likely the same problem here, not using this part either.
The collegue solved the problem by soldering pin 7 and pin 8 to the
socket of the DB9 connector. Well only applicable for a serial port, but
I do not know wether you use the serial dgt as I do + some USB adaptor
or a native serial one. (But I'd suspect the native USB board would not
have a problem with flow control as this is AFAIK specified via USB
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