When I wanted to try out PSK31, I hacked together a cable for my 857D out of spare parts. The 6 Pin Mini DIN connector came from a discarded keyboard and the audio jacks came from a spare auxiliary patch cable.
Before you build one for yourself, note that it is widely recommended that the radio and computer are isolated as not to cause a ground loop. This cable does not provide any isolation. So proceed at your own risk. There are plenty of people that use cables like this without issue–I’m not one of them. I had some issues with the cable, they weren’t the end of the world, but I’ve since upgraded to using a Signalink USB.
The pinout for the data jack is provided in the FT-857D Operator’s Manual (PDF). It is also shown here. One need only connect the data out (1200 bps) from the radio to the computer’s sound input and the data in from the radio to the computer’s sound output. Both of these connections also need to be connected to the ground. Remember, that the diagram here is of the data jack on the back of the radio, make sure the pins you solder to in the connector will mate with the right holes in the jack.
As I said, this cable has worked well for me, but it does have a few drawbacks compared to other methods of connecting a radio and computer. However, this is a very quick* and inexpensive way to try digital modes. If you like it, you may wish to enhance your audio connection.
* This method is quick if you have the parts. If you don’t you’ll need to find them somewhere. If you can’t get your hands on an old PS/2 keyboard or mouse, you can order a 6 pin Mini DIN connector (e.g. Digikey Part #CP-2060-ND).