Between Service Pack 4 and SP5, Microsoft changed the default Time Out value of serial ports such as those created by Digi drivers. For your printers to function properly, you will need to issue a mode command to each of the printer ports to turn off the Time Out value. For this command to remain in effect, it must be executed following each reboot.
For example, for a printer on COM4:
mode COM4 to=off
For ease of use, we suggest creating a batch file and placing a shortcut to the batch file in your Startup folder.