BUD JAMISON wrote in a message to MIKE ROSS:
-> You could then use either COM2 or COM4 as you please though obviously
-> not both at the same time. Using both at the same time would be only
-> slightly trickier. Simply select an unused IRQ for COM4 i.e. IRQ2.
 BJ> THAT is certainly not a good idea.  IRQ 2 is used to cascade to
 BJ> 9-16 on a 386 or better, and will cause MAJOR problems if used for
 BJ> anything.
You might also want to notice that IRQ 2 is not accessible on the PC AT bus.  
The ISA bus pin that is connected to IRQ 2 on PC XT and similar machines is 
connected to IRQ 9 on PC AT machines with the conversion being hidden from 
the user.
Regards,
       David
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