Make sure you check all of the tabs in Setup and Test to make sure that you don't have the same serial Com port assigned to two tasks.
Split Second needs a legal Com port between 1-9 for each task.
Set scoreboard to NONE if not using and scoreboard, etc.
Set the Timer Com port to None AFTER selecting Dual TIMY at 9600 baud if using the USB connection and a scoreboard.
Otherwise the serial port for timer might be on Com 1 and you are trying to use Com 1 to send data to a display.
ALWAYS exit the program and reboot after making changes like this.
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