Re: [hylafax-users] 2+ External serial modems?

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

On 2003.11.19 23:29 Sam Currie wrote:

> I have been testing the system using two external serial modems.  They
> are old Netcomm E11F's, very reliable and stable.  But I would like to
> add a third modem.  Now my plan was to simply add an ISA 2 serial port
> card to the PC to give it a total of 4.  Then simply add another E11F
> the same way I did for the first two.
>
> But I was just reading the How-To and I noticed under the 'External
> Serial Modems' section it would seem this does not work.  Though I
> will
> admit I didn't understand why.  Can someone please expand on this?

The comments in the HOWTO stem from an assumption that the user is 
limited to the four "standard" on-board PC serial ports: I/O 0x3F8, IRQ 
4; I/O 0x2F8, IRQ 3; I/O 0x3E8, IRQ 4; and I/O 0x2E8, IRQ 3.  In Linux 
those are known as ttyS0, ttyS1, ttyS2, and ttyS3, respectively.  In 
MS-DOS they're COM1 through COM4.

The reason that you can only use two of those at the same time is 
because ttyS0 and ttyS2 share an interrupt as well as ttyS1 and ttyS3 
share an interrupt.  Because faxgetty is always using the modem you 
can't have it monitoring two ports on the same interrupt without 
experiencing conflicts.  If you don't use faxgetty (send-only) then you 
could do it, but you'd need to make sure that you don't simultaneously 
send out of two modems that share an interrupt.

Now, if you're going to install an expansion card that breaks the 
assumptions I mention above - i.e., it uses different I/Os and 
different IRQs - then you could very well use them.  Traditionally ISA 
type devices don't share IRQs at all.  You probably will fare better 
trying to install a PCI serial expansion card.

Lee.

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