I don't have any information on the Digi M8 interface. But on the FT747GX, all the connections are available for use with a typical digital interface. I will assume for a moment that the M* works like most of the others out there.
PTT should be controlled by a virtual serial port through the USB connection to the M8. The PTT output from the M8 should be connected to the PTT jack on the rear panel of the 747 with an RCA plug. The computer software you use will be looking for a serial port number to use for PTT. Assuming you are using Windows, you will have to go into the Control Panel - Hardware and Sound - Devices and Printers - Device Manager. The listing will show a category for 'Ports'. If the M8 is connected and everything is working, you should see a 'port' number associated with it. You may need to unplug and replug the M8 to identify which port it is. Once you've set this port number in the digital software, it should be able to key the radio when you go into transmit.
There is also an AF Output RCA jack on the back of the radio. This should go to the audio input on the M8 - it provides a constant level audio output so the AF gain control doesn't affect it. This way, you can turn the audio on the radio down or off, and still be able to do digi modes - a very nice feature.
The 747 does not have a separate input for the transmit audio, so you will have to obtain a mike connector and wire it to feed the audio from the M8/computer into the mic input. I think the M8 has a separate level control for the transmit audio, so you can set it for proper operation with the mic gain set at it's normal level for SSB.
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