COM3: and COM4: devices

[Scott Reynolds]

pmartin at psmsd.UUCP (Paul Martin) writes:

>I know this has been asked many times before, but I never really paid any
>attention because it never applied to me, but does anyone out there know
>of a way to use COM3: and COM4: ports on a standard multi i/o board with
>xenix (ie a tty device)?

>COM3: shares the irq with COM1:
>COM4: shares the irq with COM2:

I asked myself this question some time ago, and I dug into the docs with
my system (an old, old XENIX, much like SCO XENIX/286 2.2).  I used tty
drivers from my own manual plus a couple I picked up elsewhere and came
to the conclusion very quickly that sharing interrupts just won't work
on an ISA bus because the IRQ lines can apparently support only 1 device
at a time properly.

What I did was hack on the drivers a lot to add a few features I needed
and support 4 ports by using IRQ 4, 3, 5, and 2 (in that order for DOS
addresses COM[1-4]:).  The last port shares IRQ2 with the 80287 and the
HD controller through a slave interrupt controller, eliminating the type
of conflict you have with sharing any of the other IRQ lines.  [In case
anyone is interested, on my IBM PC/AT and this old XENIX I ended up
having to use interrupt 0x19 for the port set on IRQ2.]

I'll mail copies of the 8250 tty driver to anyone who would like to see
it.
-- 
Scott Reynolds			=	scott at clmqt.marquette.Mi.US
Enterprise Information System	=	..rutgers!mailrus!sharkey!clmqt!scott
	"Where were you when I laid the earth's foundation?  Tell me,
		if you understand." -- YHWH  (Job 38:4)