Added an "Op menu byte (=213)" before each serial transmission, to reduce interference from user apps that spam serial ports.
This especially helps with Ubuntu 14, which spams the port by default with an AT modem prompt (one byte in the prompt is a Pulse Pal op code! Other OSs don't do this - XP, Win7, OSX, Fedora...)
Now, any serial message that does not contain "213" followed by a valid op code byte, will be ignored.