FT991A_9600

Configuring the FT-991A for 9600 bps FM packet

This is a replication of https://github.com/rfrht/FT-991A/wiki/Configuring-the-FT-991A-for-9600-bps-FM-packet
By: Rodrigo Freire, PY2RAF.

The FT-991A is capable of 9600 bps FM packet radio - with the catch that is not possible to transmit using the USB interface, this have to be done via the RTTY/DATA jack. In this document I'll share my personal findings after investigating the feasibility of this operation mode.

Why not the USB port!

This is due to the FT-991A design. The FT-991A USB audio enters a low-pass filter (LPF) right after the digital-analog converter inside the radio (blue box). After this LPF, the audio enters the 'regular' audio transmit chain (red line).
PIC2
Conversely, the audio coming from the RTTY/DATA Jack is applied to Q1136, where in the presence of the 9K6 signal, diverts the signal to a different signal processing chain (see green line).

How do I set up my FT-991A for 9600 bps FM packet radio?

Reinforcing- you will need to transmit your audio through the RTTY/DATA port. For this test I did a very simple cable, with a 4.7 K resistor using the computer phone jack to feed the RTTY/DATA port, using pins 1 & 2:
PIC3
If you will be using your computer to process the receiving audio, there's no need to fully wire the RTTY/DATA port - use a cable just to send the computer audio to the radio, and use the regular USB audio for the reception.

If you have an external TNC and will not be using the computer to process the incoming audio, wire it as the below diagram (from manual page 124):

FT-991A Packet Radio - DATA/RTTY External Audio wiring
PIC4

Setup

1. Connect your computer TX audio (or TNC) to DATA/RTTY port, as per the above diagram
2. Set the MODE to DATA-FM
3. Set the menu clause 076 FM PKT PTT SELECT to DAKY. That will be good for either direct PTT in Pin 3 or CAT PTT command.
4. For 9600 bps mode, the menu 077 is irrelevant
5. Set the menu clause 079 FM PKT MODE to 9600

Testing, level, etc.

It is of great help to have a nearby receiver to test the transmitted signal for proper gain. Define a suitable transmit gain and enjoy!

What's the difference?

The transmitted 9600 audio is way more detailed, with wider bandwidth.