COM3 & COM4 under SCO Xenix ?

[at adec23.uucp]

md at sco.COM (Michael Davidson) writes:


>wcurtiss at x102c.harris-atd.com (Curtiss WC 67625) writes:

>>I am trying to install a serial card configured for COM3 and COM4 (INT 3 & 4,
>>I/O Addr 3E8 & 2E8) in a machine which already has COM1 and COM2 (INT 3 & 4,
>>I/O Addr 3F8 & 2F8).  SCO says it can't be done and I should get a multiport
>already active so you can't get an inactive -> active transition,
>there is no transition for the interrupt controller to see and
>you lose the interrupt.
Actually you loose ALL interrupts after this, you have to reset the two serial
ports before you can get an interupt again. I have successfully managed to
work both these ports by judicious polling and a hardware hack at 9600 Baud
on a plain 4.77Mhz XT.

The other problem is that the interupt is NOT wire-or. The output drivers are
three state devices, they drive the line continuously when they are enabled.
This means that if one device asserts an interrupt (HI) and the other doesn't
(LO) the one that doesn't takes precedence (due to LSTTL driving capability)
so NO ONE gets the interrupt. A simple RDL or gate looks after this, but who
will support a hack like this? No one in their right mind is the answer.

The multiport boards solve this second problem by providing an OR gate on the
board to combine the ports interrupt signals BEFORE they get on the buss.
Some provide superior interrupt control logic as well.

Ciao,
-- Mark Salyzyn  adec23!mark
-- 
Ciao -- Mark Salyzyn   adec23!mark