Hello Paul
I am using a trafo between aux/mic (3,4) and the sound card. If your cat control is working ok, you have to switch on RTp Tx mode in Control unit. Because I want to reduce data on my 4G connection I do not switch RTp Tx mode on. From my computer I use DTR/rs232 and a relay for PTT (5,6 in aux/mic). It is working very nice both on my Win computer and on my Raspberry Pi.
73 de
Alf, la1zm