The program only needs a simple “infinite gain” circuit to square off the input tones so that they can be read by the serial port. This is achieved using a 741 operational amplifier. The basic circuit of the unit is given below. The RS232 pins are given for a 9-pin serial port. A 25-pin port is given in brackets.circuit_modem

I have taken the liberty of modifying DL5YEC’s design a little by not using the two back-to-back diodes in the TX AF path. I found they created a square-wave into the microphone input of the rig. Without them you have a triangular-wave with the tops a bit rounded, more like a sine-wave!! I have also increased the value of the resistors in the AF filter to reduce the level a bit more. circuit_modem_kitEven so there is more than enough AF drive for most transmitters. Switch OFF any audio compression and adjust your rig AF-GAIN control for the correct output power. I run the rig at 50% power since, in RTTY mode, the rig is delivering 100% output which can cause transmitters to overheat.