Ports may not work due to resource conflict. For instance, if you are trying to simultaneously use the devices that are available on COM1 and COM3, or COM2 and COM4, while the default IRQ settings for COM3 or COM4 are not changed, there is a probability of conflict as the above pairs share an IRQ setting by default.
In order to overcome resource conflicts, it is always best to change the resource settings of the COM3 and COM4 ports from the default standards. Always check the settings for communications software and be 100% sure that there is no specific setting for resources such as IRQs. If it exists then make sure that these are set to the correct values for the setup.
