Using internal modem cards with SCO Xenix

[Ross Oliver]

In article <1112 at ispi.UUCP> jbayer at ispi.UUCP (Jonathan Bayer) writes
about how to use more than two serial devices under XENIX:
>OK.  Since each device must have it's own interrupt, why not make two
>serial ports both use the same interrupt, and modify sioconf.c to show
>that the second serial port is nothing more that a second port on the
>first serial card?  For these serial cards that go to com 10 (or
>whatever), why not simply set up all the ports on the card to use the
>same interrupt, and set up sioconf properly (specificaly the port count,
>the interrupt vector, the base address of the board, the interrupt poll
>address, and the spacing of the io ports on the board).  The only
>potential problem is the interrupt poll address (as I see it)


This will work, provided one other condition is met: each of the
cards that share an interrupt must allow the IRQ line to "float"
when the card is not asserting it.  If one of the cards clamps
the IRQ line high (I believe the interrupt lines are active low),
another card might not be able to force it low to signal its interrupt.
The result at best would be missed or spurious interrupts, and at
worst, damage to the hardware.  I'm not trying to scare anyone out
of trying this; there is a lot of flexibility in sioconf and the
master file, so if you think you can make use of it, by all means
do it.  However, you should also be aware of the potential pitfalls
when using hardware designed primarily for MS-DOS.

Ross Oliver
Technical Support
The Santa Cruz Operation, Inc.