The new plasmas come from the factory in SR mode on the serial ports. You have to switch it to RS-232 mode for it to talk normal strings.

With the set off, press the Menu key, then power within 3 seconds. The set should come up with "UART Select <-->" on the screen at the bottom. Use the left right arrows to select your baud rate, then hit menu and power down the set.

Once you've done this, you should be able to talk to the unit via RS-232.

Hope this helps.

Brad