.: Why a interface.

Computer switch mode PSU. have a noise filter at the input form the mains  normal there are pleased a 4,7nF capacitor from the phase to earth & one 4,7nF from neutral to earth. But if the earth terminal on the computer not are actually connected to Earth ( rod in ground ) "Normally the mans outlet have only the Phase & neutral"  then you have the mains voltage / 2  on the Earth on the computer & the chassis. that voltage maybe can damage the data input of the radio when you connect or disconnect the connector to the radio. Or noise from the mains can be routed to ground of your radio

.: Below a schematic of a standard filter  

.: deskription

Now I decide to make a interface for one compute and to radio for easy selecting radio for tx and rx I decided to make the interface for to radio example 1 Hf radio & 1 VHF/UHF radio I decided for relays for TX'ing the radio be course problems with optocupler on some radio types  The TX controlled a tree poison switch 1: controlled buy COM port 2: off ( radio don't tx when reboot computer) 3: manual TX if you wont to send a sound from a program than don't have COM control e.g. a recorded WAW file. the mining whit the level control photometers are you can set the level on the to radio so you don't have to change the setting on the soundcard when you switch between the to radio   the 12V on the Schematic off the interface ARE from a SEPERAT 12V min 100mA supply