OptiPlex 3010 PC with Add-on Serial Card Stops Working After a Period of Inactivity

We have purchased three Dell 3010 PCs in the past 45 days. All three Dell 3010 PCs exhibit the following problem: a period of inactivity of at least 45 minutes or even shorter causes the serial port to go dormant and stop communicating with a keyreader device attached to it. All three PCs are Windows 7 Pro 64-bit. The serial card is an add-on card -- a Startech 2 Port Native PCI Express RS232 Serial Adapter Card with 16550 UART (part no. PEX2S553). The PCs have been updated to the newest BIOS A11 and chipset drivers and Intel Management Engine and have the newest serial card drivers (V 2.0.3.0) too. The power options in Control Panel have been changed to performance setting -- the PCs do not sleep or hibernate. The keyreader device uses serial comm port 2 since the Dell PC reserves comm port 1 although there is no serial comm port connector for it. We can tell the serial comm port goes dormant because when we perform a keyreader test, the RS232 interface fails. Immediately after a restart, the serial comm port starts working again (confirmed by the test) until there is inactivity of at least 45 minutes. Is there a setting either in the operating system or the device manager that may help the add-on serial card stay online and interfacing with the keyreader device?