COM port hangs after time

​ I am running software that controls an RS232 device on a Precision 650 workstation with Windows XP Pro. After a half hour with nobody at the computer, the screen goes blank and a new user would need to log in. The software continues to collect data over the COM port for 3-6 hours before it stops. As soon as someone sits at the computer (i.e. touches the mouse or keypad) the software and COM port resume functioning. No errors are reported. ​

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​ The software will try to communicate with the device based on the Windows clock. It waits until the proper time has elapsed and then tries to call the device. I get no message of a failed communication from the software. ​

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​ I assume this is some sort of energy saving / security setting in Windows but I can't find it. ​