>
> At 07:41 PM 2/26/98 -0000, you wrote:
> >I thought you could only have/use  2 Com ports at a time? but he insists
> >that it should be possible.
>
> COM1 and COM3 share the same interrupt (IRQ4) and cannot be used at the
> same time. COM2 and COM4 share an interrupt (IRQ3) and cannot be used at
> the same time. To use two COM ports at the same time, one must be an odd
> number port and the other an even number port.
>
> David Figueroa
> --
> Internet Communications of America, Inc.
>

        No, it hasn't why to do it this way: you can hardware assign irq's
other than 4&3 to COM3 and COM4, and use all four at the same time, as long
as the os or specific communication program is aware of it. For example,
COM1/IRQ4, COM2/IRQ3, COM3/IRQ5 and COM4/IRQ9. Think in Linux without lan,
with some pc's around accessing it through the com ports. For eight bit
boards you have IRQ5, IRQ7 and IRQ9 to choose.
        I have even done: on mobo COM1/IRQ4 and COM4/IRQ3, internal modem
COM3/IRQ5, and on an old com/lpt board COM2/IRQ9, soldering a wire to
B4 on the soldering side, which is IRQ9. If you have a PS/2 mouse (IRQ12)
you have all four com ports at your disposal. And if you want more than
four, you can use a non standard com port address, although it would be
better to use a multi com board (several com's using an interrupt, and
software to distinguish the port who caused the interrupt).
        It has also been suggested to share the irq "oring" it using
diodes, but you must provide support in the os (as with the multi com
above).

************************************
Javier Vizcaino. Ability Electronics. [log in to unmask]

Starting point:        (-1)^(-1) = -1
Applying logarithms: (-1)*ln(-1) = ln(-1)
Since ln(-1) <> 0, dividing:  -1 = 1
     (ln(-1) is complex, but exists)