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Re: FW: FW: Old modem list

On Mon, 22 Mar 1999, Ken Cornetet wrote:

> I have my modems connected to an HP E35 unix system with 8 built-in (sorta)
> serial ports.
> 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.

> Some serial port boards let you choose IRQs other than 3 and 4, but not
> many. If you have one of these, you can set your modems to COM1 and COM2
> (IRQ 4 & 3) and set your serial port board to COM3 and COM4 *but* with IRQs,
> say, 5 & 9. Or perhaps you could find internal modems that allow IRQs other
> than 3 or 4.

  If you are using internal modems, support for IRQs other than 3 or 4 is
nearly universal.  All multi-IO cards that I can find that have serial
ports on them, support IRQs other than 3 or 4 too.

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

  Because it isn't possible.

> another board. Likewise motherboards with built in serial ports. Likewise
> internal modems. Keep in mind that I am talking about sharing interrupts
> between ISA boards. There are several serial port boards with multiple UARTS
> that share an IRQ - but only on the one board.

  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.


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