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SubjectRe: serial ports

> The problem with why COM1 and COM3 can't share FIFO is electrical, not a
> matter of software. The problem is that the UARTS for COM1 and COM3 are
> directly driven by the board, and so if both UARTS are active, they end
> up fighting for the line

I wish people would be more careful when making all encompassing
statements like the above. Especially when in that context they
are plain wrong.

Every 2, 4, and 8 multi-uart card I've ever used or seen has the
extra logic required for electrically correct ISA interrupt sharing.
In fact, one set of cards I have provides connectors for convenient
sharing of interrupts between similar cards in other ISA slots :-)
Every motherboard with built-in uarts that I know of does not support
interrupt sharing (and I can't imagine any that would).

Interrupt Sharing Summary:
You _CAN_ share interrupts between UARTS if the card(s) support it.

There is no rule for interrupt sharing support. The best way to tell
if the hardware supports sharing is to read the manual, or contact
the manufacturer or vendor. If they can't provide the info, take your
business elsewhere (run, do not walk...)!

Note: Ask _before_ forking over the money, and make sure you have it
in writing that they are responsible for any resulting damages.
(I've insisted on this even at small retail stores' parts counters.)
If they will not stand behind their product or claims, again start

For those with no way of finding out if their card supports
interrupt sharing, assume your card _DOESN'T_ support it.

- --
Andrew E. Mileski
Linux Plug-and-Play Project Leader
PGP Public Keys & Signature are at

Version: 2.6.2


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