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Re: FW: FW: Old modem list

On Mon, Mar 22, 1999 at 05:40:30PM -0800, Tom wrote:
> On Mon, 22 Mar 1999, Ken Cornetet wrote:
> > You will loose your external serial ports anyway. Even if you set your
> > external ports to COM1 and COM2 and set you internal modems to COM3 and
> > COM4, COM1 & COM3 will be trying to share IRQ4, and COM2 & COM4 will be
> > trying to share IRQ 3. You will not be able to use COM1 & COM3 at the same
> > time. Likewise COM2 & COM4. Although there is nothing in the design of the
> > IBM PC ISA bus preventing boards from sharing interrupts, the boards would
> > *have* to be designed with this in mind. Also, the OS and serial port
> > drivers would have to understand shared interrupts.
>   Uhh... IRQ sharing between devices on a ISA bus is impossible.  The
> cards cannot be designed to make this work.

I'm afraid that this is not quite correct.

> > I have never seen any ISA serial boards designed to share interrupts with

I have.

>   Because it isn't possible.


Older multiport boards, like certain models of Computone, and also some
voice cards, like Dialogic, can in fact share interrupts on the ISA
bus.  It depends primarily on the drivers on the board; I believe they
have to be open-collector, rather than whatever the alternative
arrangement is.

Of course, you _also_ have to have drivers that will poll _all_ the
installed cards to see who the interrupt came from, and _this_ was
always the rub in the days before Open Source.

>   Sharing an IRQ between multiple UARTs is messy.  The interupt routine
> must poll all UARTs to find out which one interupted.  Your operating
> system must have a serial driver that supports this.

And time enough to actually do it.

It's much easier on the PCI bus.

(And even easier on a terminal server on the other end of an Ethernet

-- jra
Jay R. Ashworth                                                jra@baylink.com
Member of the Technical Staff     Buy copies of The New Hackers Dictionary.
The Suncoast Freenet            Give them to all your friends.
Tampa Bay, Florida     http://www.ccil.org/jargon/             +1 813 790 7592

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