Aha!
Many thanks for the pic Patrick, I have a feeling I've seen that somewhere recently, but passed it over as not what I was looking for.
Now I know, great!
My excuse is I've recently moved house so the shack is being rebuilt from the chaos of moving.
Also thanks to others who responded.
Interesting the comments about introducing noise into the K3 via USB, I'll certainly look out for that.
If I find that it does I'll go back to the standard serial port which has been absolutely fine.