Let me see if I can do this with words instead of a sketch. To cut tennons on the end of logs, first drill a hole in the end of the log, say a 1/2" hole, about in the middle and running as close to paralell to the log as you can accomplish by hand. Next, bolt a bracket to the top of your table saw that has a 1/2" headless bolt sticking out of the side of it. If you want your tennons to be zero radius between the tennon and the body of the log, make the bolt crosswise to the blade. If you want a radius between the tennon and log, make the bolt run paralell to the blade.
Mount a wobble dado blade on the table saw, set to full width. Adjust the height of the blade so that the distance from the edge of the teeth to the center of the bolt is the radius of the tennon you wish to make (you'll have to play with this). Slide the bolt into the hole in the log and start turning and feeding the wood into the blade. You can clean up when the bolt bottoms out in the hole.
I have only done this with the bolt paralell to the blade and the saw running in a direction that tried to push the wood away from the blade, NOT suck it in, so if you change anything you may run the risk of grabbing and drama. Also,arrange the bracket in such a way that the bolt enters the wood before the blade touches the wood(obvious, but it had to be said).
The other home grown way to do it is to mount the log between centers with a rounter running on two rails above the centers. Adjust the router height to get the size tennon you want and have someone rotate the log while you control the router. This way of doing it makes the two tennons always run in a straight line ( yeah, I know, more or less straight).
I have seen both these setups in log furniture shops that did it for a living, but of course there are more proffesional set-ups out there. Method #1 gives a nice look to the wood if you include the radius between the tennon and the log, but also leaves your table saw with a couple of threaded holes in the top, which drives some folks nuts. I guess there has to be a way to clamp that bracket on there without the holes. Method #2 is more disagreeable in it's execution (hate router noise) but does make a zero radius joint between the tennon and log and lines the tennons up more nearly straight....Joe