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COM Port Conflict:
Every COM port on a computer requires an IRQ line to signal the processor for attention. In most computers two COM ports share the same IRQ line. The IRQ4 line works for COM1 and COM3, and the IRQ3 line works for COM2 and COM4.
Most computers two COM ports share a common IRQ line. COM1 and 3 always use IRQ3 and COM2 and 4 uses IRQ4. IRQ line is used by the COM port to signal the processor for its attention.
In case more than one device is connected to the same IRQ line there could be a conflict. Therefore, the first step to take while installing a modem is to check the computer to determine how its interrupts and COM ports are configured.
Since the COM ports share IRQ ports when you connect more than 1 device there will be a conflict. Therefore it is best practice to know the how the interrupts and COM ports are configured before installing the device.
