Did you ever figure this out??? I have almost the exact same setup (XBR75X850E with Extron MLC 62 RS D controller via RS232). I had it working for a bit and it suddenly stopped. I went and reprogrammed the Extron controller and got it back up and running. Now, it has gone out again and I can't seem to get it back. I called Extron support to have a tech tell me they've had a lot of trouble with these TV's. He said that the serial ports will sometimes accept commands and sometimes will not. He had me unplug the TV for a bit and plug it back in which did nothing. Any help would be greatly appreciated as I am pulling my hair out trying to fix this!!!