The following was posted by the author of fldigi to the linuxham list and I thought it might be of interest to some.

73, Bob W9RAN

at http://www.w1hkj.com/alpha/fldigi/

Includes a feature that has been on the wish list for the past 18 years:

    FSK keying for 45.45 Bauot ONLY
    
      * add ability to key xcvr in FSK using either a DTR or
        RTS serial signal line (default RTS).
        . serial port can be shared with rigio or a separate port
        . RESET button to restart the interface after config changes
      * SPACE always sent as LETTERS character
      * 100% functional on Linux including Pi3/4
      * 100% functional on on macOS 11.6 and 10.13.6
      * 100% functional on Win-10

view FSKconfig.png and rigcat-config.png for configuration setup on authors IC7300.  stats.png demonstrates timing stability of FSK generator..

Not available for OS-X earlier than 10.13.6 as the Apple compiler does not support the required system calls.  An alternate is being sought by the developer.

73, David, W1HKJ