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

["Sam Currie" <Sam.Currie@xxxxxxxxxxxxx>]

Thanks very much.  This has cleared everything up.

I have started some research and it looks like a PCI card will fix the
issue.

Thanks again,

Cheers, Sam 

-----Original Message-----
From: Lee Howard [mailto:faxguy@xxxxxxxxxxxxxxxx] 
Sent: Friday, 21 November 2003 4:47 AM
To: Sam Currie
Cc: hylafax-users@xxxxxxxxxxx
Subject: Re: [hylafax-users] 2+ External serial modems?

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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