Re: Question: Multiple Super Serial Card ][s in //e?

[dempson@actrix.gen.nz (David Empson)]

Paul Schlyter <pausch@saaf.se> wrote:

> Finally: isn't it possible to turn on an interrupt signal on the
> Super Serial Card?  Now try to do that on more than one of those
> seven SSC's, and you'll immediately run into a severe problem: the
> Apple II only has one IRQ line, not 8 or 16 as the Wintel machines
> have....

No problem, as long as the software is set up correctly.  The IRQ line
on the 6502 is designed to be shared between multiple peripherals.

For starters, there is a hardware daisy chain which prioritizes the IRQ
signal for cards in adjacent slots, as long as the cards actually
implement this AND all interrupting cards occupy adjacent slots.

This mechanism is obsolete with ProDOS's interrupt support anyway.

If multiple cards generate simultaneous IRQs, the interrupt handlers are
each responsible for polling their device to confirm that it is the one
generating the interrupt.  Depending on the architecture of the main
interrupt handler, multiple interrupts will either be handled in the
same interrupt cycle, or the first one will return and a second
interrupt will occur immediately.