The FT-891 does do true FSK as well as AFSK RTTY, and of course PSK, FT8, SSTV, FAX or other data modes. Its just a matter of having the correct interface and the radio menu settings correct.
The difference between the FT891 and the FT-991/A. FTdx3K , FTdx1200 and the FTdx101D is that those models also include a built in USB sound card that the FT-891 doesn't have.
All these models with a USB socket in the back have at least a dual USB serial port chip inside. This provides two separate COM ports (usually consecutively numbered) and identified as Enhanced and Standard.
The Enhanced COM port is the radio CAT port using RXD/TXD with hardware flow control using RTS and DTR. Note X-ON/X-OFF software flow control is not supported.
The Standard COM port RTS and DTR lines are used for Keying and PTT. Keying can be either CW or FSK depending on the mode. The RXD and TXD lines on this port are not used or connected in the FT-891.
If you only want to do RTTY using FSK and not PSK or FT8 using MMTTY then you only need a single cable from the Data out pin of the DATA/PSK socket to the sound card input of your PC and a USB lead between the FT-891 and your PC for CAT, PTT and FSK TX Data like this diagram from the Advanced User Manual for the FT-891.
If you want to use the FT-891 for PSK or FT8 then you will need to include TX AFSK capability with either another cable to connect the PC Sound Card output to the Data In connection in the DATA/PSK socket or use an external sound card adaptor like the Signallink USB to provide the connections.
These are the menu settings that need to be set for RTTY FSK keying and PTT using MMTTY.
Mode to RTTY
Whether you use LSB or USB (R-L or R-U ) and High or Low tones will depend on your preferences, but note that you may need to reverse either decode or encode to match RTTY received. See below *
PC Keying and FSK using Menu 07-12 as below
But note that whether RTS or DTR controls FSK or PTT depends on how PC keying is set vis:
With MMTTY FSK keying you use the Virtual Comport driver (EXTFSK64 if 64bit or EXTFSK if 32Bit Windows) to link the FSK out of MMTTY to the FSK connection on the standard comport of the radio.
Here is a capture from my setup
You can see that the two Com ports in my case are COM3 and COM4. COM3 is the CAT port set at 4800Baud. The TX keying page shows that I am using the EXTFSK64.exe for FSK and PTT and the screen for that is at the bottom left showing that COM4 is being used for FSK and PTT with FSK on DTR and PTT on RTS to match the Menu 07-12 setting above.
Note also that the frequency of the FT-891 shows correctly on the MMTTY screen because I am using V1.70e or later, which fixed the CAT frequency error which occurred when the FT-991 was introduced that added the 100's of MHz to the CAT Frequency setup.
Hope this helps. it does work and the TX signal is much cleaner from using the internal FSK generator in the radio.
73 de Paul G3WYW
* Tones and Shift in RTTY: Convention is that the HIGHEST RF frequency is the MARK tone and also by convention LSB ( Lower Side Band ) is used for RTTY even above 7MHz. This means that the Lowest Audio frequency is the Mark one. See in the MMTTY screen capture above, my MARK frequency is 1275Hz. I am using LO tones ( I'm in Europe) and with 170Hz shift my SPACE tone is 1445Hz. Using Lower Side Band you can see that the MARK tone (1275Hz) will generate the highest RF frequency and the SPACE tone (1445Hz ) will generate a lower RF frequency. e.g. Carrier 14090.00KHz , MARK tone is 1275Hz so MARK RF frequency is 14090.00 - 1.275 = 14088.725KHz and the SPACE RF frequency is thus 14088.555KHz which is lower than the MARK frequency by the 170Hz Shift.
It is important to recognize that using USB with Lower or Higher Tones will mean that your signal could be inverted compared to someone using LSB resulting in decoding gibberish and/or transmitting it as well. A Google search will provide more information on this topic.