I’ve been happily using a RigBlaster NoMic and a PK-88 for data modes for the past decade. However, a recent interest in 9600 baud packet meant neither of these devices were suitable. The PK-88 is a fixed 1200 baud hardware TNC and the RigBlaster NoMic has an inadequate frequency response due to the isolation transformers.
For this reason most seem to interface radios directly to soundcards without isolation, then create an additional PTT solution. I like everything in the one neat box, so this is my solution (click to zoom):-
Correction: The PC Out schematic has an error – the resistor to ground should be 100 ohm, not 10K.
I decided to use a small relay for the PTT although an opto-isolator would have been an alternative solution (but the click is a nice audible confirmation of TX). Here the resistors divide the voltage from the RTS pin to a more suitable level for the transistor (I think I used a 2N2222) and the first diode scrubs off any negative voltage. Real RS232 ports swing +/-, forgetting this, I omitted this diode at first then wondered why two transistors quickly failed. Connecting to a scope quickly reminded me of my error! The final diode should probably be directly across the relay contacts – if building this, I’d suggest you move it.
The Radio (data out) ports are switched then DC blocked via a small capacitor to remove any DC bias. I’m hoping the small capacitor won’t effect frequency response too dramatically.
The PC (soundcard out) is a basic voltage divider with again a small capacitor to scrub off any DC bias.
The adjacent photo shows the assembled device (click to zoom).
I added a power LED plus a TX LED connected to the secondary relay contacts. It seems to work well.
Update: Attenuation needed to be removed for 9600 baud. This doesn’t appear to effect 1200 baud (alsamixer provides enough control over audio levels).