Using a single serial port to allow CW and RTTY in N1MM without any cable switching
On the K3 CONFIG PTT-Key setting, it needs to be "Off-Off."
It has been reported by some that (under some not fully defined conditions) it is possible to use a USB-Serial converter to key FSK RTTY with MMTTY so you don't need a serial port at all. I tried the recommended method and could not make it work.
Adobe Acrobat Document