Standard PCs have only two IRQ lines for serial ports, therefore
    COM3 must share COM1's IRQ line and COM4 must share COM2's 
    IRQ line.  Unfortunately, two devices using the same IRQ 
    line can't be active at the same time.  COM3 can't be used for 
    a printer or modem if a serial mouse is active on COM1, for 
    example.  Likewise, COM4 can't be used if a serial mouse is 
    active on COM2.  In other words, if you have four COM 
    ports and a mouse is attached to one of them, you do not 
    have three COM ports available for other devices, you only 
    have two.  The COM port that shares the IRQ line being 
    used by the mouse is not usable.  
    
	The following table shows what ports are available for other
    devices in a conventionally configured PC, XT, or AT when a 
    serial port mouse is present on a multiple-port system:

       Serial port mouse on     Ports available for other devices 
       _____________________    _________________________________
        COM1 or COM3 (IRQ4)          COM2 or COM4 (IRQ3) 
        COM2 or COM4 (IRQ3)          COM1 or COM3 (IRQ4) 

	Windows system.ini file holds all settings for the environment
devices, including the communication drivers and serial ports parameters. 
The table below shows the recommended general seetings for serial ports from
COM1 to COM4, as a starting reference,  minimizing the chances of conflicts. 

system.ini

[386Enh]

COM1Base=3f8h
COM2Base=2f8h
COM3Base=3e8h
COM4Base=2e8h

COM1Irq=4
COM2Irq=3
COM3Irq=4
COM4Irq=3


IMPORTANT:The IRQ's must be compatible with the Hardware setup (Refer to
the hardware installation manual). It means if you have a sound board or a
scanner installed, you have to check the actual IRQ's used and choose  the
appropriated setup for the system.ini.
At this point it should be emphasized that YOU have to find your own optimal
setup because there are no software and hardware standards  for 
communication devices.