With the system-wide PTT, I suggest to make the feature with which we can use foot switch. We can utilize the PTT for many different purposes. USB port can be used for that purpose but still serial port(COM port) is easier to modify when we make the circuit.
Maybe we can copy UPS alarm circuit and its program:
- Interface (pin no. on DE-9 connector)
1. connect DSR(pin6) and DTR(pin4) directly
2. connect DTR(pin4) and DCD(pin1) with 10k resistor
3. PTT or foot switch can be put in between DCD(pin1) and SG(pin5)
1. assert "high" on DTR when CQ100 starts to run
2. monitor DCD and key up when DCD turns to "low"
Hope I don't give you too much headache.
Please just take this idea to your consideration if you may happen to think about external radio PTT control with RS-232.