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I have multiple devices using serial ports that are functional but they only work one at a time, not simultaneously
Explanation: You have a resource conflict between multiple serial devices in your system. They work when used one at a time, but there are problems (lockups, slowdowns, errors) when you try to use more than one at once. A common problem is a mouse that doesn't work when the modem is operating and vice-versa.
Diagnosis: This situation is almost always caused by having two devices set up to share an IRQ line; it is a resource conflict. By default, COM1 and COM3 share an IRQ line (IRQ 4) and COM2 and COM4 share one (IRQ 3). This is described in more detail here. If you try to use both devices at once a conflict ensues. Note that some add-in peripherals come with their own virtual "COM port". For example, an internal modem has the equivalent of a COM port built into it, which it uses to communicate with the system.
Recommendation: You should change one of the devices that is conflicting so that it uses a different IRQ setting. Try the following: