Ham radio operators interested digital modes will often use these computer interfaces to automate CW contacts (send morse code from the PC), use the PC soundcard for RTTY and control push-to-talk functions among other things.
Many of the PTT interface drawings found on the net suffer the same problem. Well it is not really a problem.
Let's have a look at a typical interface :
You have the RTS and DTR line tied together with diode, then comes a transistor that operates as a switch. And finally the ptt line from you transceiver. So when the RTS or DTR signal goes high, you are on the air. So what is the catch ? Ever booted your computer or used other software besides your regular hamradio soft ? Of course you did 🙂 and what happened ? My transceiver went in to TX on the wrong antenna / receive converter / mistuned antenna. Luckely the radio was set in SSB and no harm done without modulation. But it could be worse.
So what have I done ?
A simple modification that costs almost nothing : add a simple switch in the PTT line. See the red switch in the interface drawing.
That's it. When the computer has booted and ready to use, I toggle the switch and can operate the digital modes.