COM3 & COM4 under SCO Xenix ?

[Michael Davidson]

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
>card.  This is not an option due to project contraints.  Has anyone written a
>serial driver that will handle the above situation?

The problem (an the reason why we don't support this) is that you have
two similar devices sharing the same hardware interrupt line. Now you
*can* get away with sharing an interrupt line like this under certain
circumstances, but this isn't one of them. Interrupts on AT bus machines
are "edge triggered", meaning that the 8259 interrupt controller
has to see a false -> true transition on the line to latch the
interrupt (sorry, can't remember whether these lines are active low
or active high).

Now, consider the situation where COM1 has just interrupted on line
3 causing the line to change from the inactive to the active state.
The interrupt controller sees the transition and latches the interrupt.
Suppose that COM3 then attempts to interrupt on the same line
*before* the line has returned to it's inactive state - the line is
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.