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

[Stephen Arnold <arnold.steve@xxxxxxxxx>]

On Thu, 2003-11-20 at 09:47, Lee Howard wrote:
> 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.

Also, if you use another ISA serial card, you should probably go into
the PCI config section of your motherboard BIOS and switch to Legacy/ISA
for the IRQs used by the ISA card.  As long as the serial card allows
you to specify dedicated IRQs for each port (ie, via jumpers) you should
be fine, however, I've had mixed luck with cheap little ISA cards
(serial, parallel, etc).

If you have a few bucks to spend on this extra card, you might want to
look at one of the Cyclades multi-port serial cards.  They are high
quality and well supported under Linux.  Another option might be to try
a USB modem, or one of the serial/IP solutions like a Digiport box.  I
have no idea what the cost would be, but if you have serious reliability
requirements, it would probably be worth the investment (if only to save
time on maintenance, troubleshooting, etc).

HTH, Steve


-- 
Stephen Arnold <arnold.steve@xxxxxxxxx>
ENSCO, Inc.


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