Sharing a Serial Device Interrupt
Legacy COM ports on the same multiport board share a single interrupt. COM ports on a multiport board are identified by an index or a mask that associates a port with its corresponding device object.
Stand-alone serial devices can also share interrupts. However, a shared interrupt can only be used by one device at a time. When a device is opened, Serial allocates the interrupt to the device. When a device is closed, the driver releases the interrupt for use by another device.