JE3HHT Mori-san kindly offers a very good example code for EXTFSK called COMFSK. comfsk105.zip  ( is available in the file downloads)  With  the COMFSK plug-in, you can do PTT/FSK through legacy serial port, USB-serial adapter, or legacy parallel port (USB-parallel adapter is not supported).

If you are using Windows 2000 or XP, you need the DLPORTIO driver. If you are using Windows 98 or ME, you do not need the driver. Read the document included in the COMFSK package for more information.

Mori-san has declared that all the files including source codes in the COMFSK package can be copied, modified, used for commercial and non-commercial use. What a bonus!

Parallel port interface

This is a sample circuit diagram of the parallel port FSK interface.

Serial port or USB?

Many recent laptop PCs do not have a serial port. That is bad news for radio amateurs, because many ham software use the serial port for PTT, FSK, and rig control. Instead of the serial port, modern PCs invariably have USB ports. OK, how about a USB serial adapter? Yes, you can use one for PTT and rig control. However, most USB serial adapters do not work for FSK,. It's because they do not support 45 baud rate, which is too slow. It's also because they do not support the 5-bit data mode.

If you want to use a USB serial adapter, go with COMFSK plug-in. I use COMFSK to do PTT and FSK through the parallel port. It is working pretty good.

73 de JA7UDE Oba