The TS-850 has an internal keyer, which can be disabled and changed for direct keying via a switch on the back Unfortunately, it's not well set up to use the internal keyer while also being wired up to a computer.
To modify the TS-850SAT so it can be keyed with a computer or straight key while keeping the internal keyer active, add a jumper at the bottom of the IF board. I wired mine to the accessory jack, pin 1, which is unused.
To get the IF board, open the radio with the back towards you. The IF board is on the left:
You'll need to remove 6 screws, the three coax cables (black is near the radio front, orange in the middle, red to the rear in case you forget to mark it down), 4 ribbon cables near the front and 2 small 2 conductor cables near the rear.
Then carefully extract the board and turn it over. I ran a small wire jumper from pin 1 of the accessory jack to the middle, upper pad for the CW keying switch. Check your work carefully, and put everything back together.
Fine Print Disclaimer - this worked well for me, but may not for your particular radio. If you wire things wrong, you may damage your radio and/or yourself. The author makes no guarantees about anything.