[GTALUG] Actual ttyS0 MIA

[Lennart Sorensen]

On Mon, May 11, 2020 at 09:41:27AM -0400, James Knott via talk wrote:
> I used to be a computer tech, on the big systems.  I also designed and built
> an 8 serial port card for my IMSAI 8080.  In all my experience, I had only
> seen level triggered interupts or IRQs.  This meant when something needed
> service, it would pull the signal line low.  This allowed multiple devices
> to share 1 interrupt line and the OS would sort out which needed to be
> serviced.  IBM, instead of using that common practice, when with rising edge
> triggers, which cannot be easily shared.  The Intel interrupt controller
> chip that was used supported either mode, so there was no valid hardware
> reason, that I could see, for IBM to choose that method.  If they had gone
> with level trigger, then we wouldn't have had that IRQ sharing mess we had
> to deal with on the PC/AT bus.

I wonder if edge triggered allowed for dumber devices since the device
doesn't need to be able to latch the interrupt until serviced.  I could
imagine anything that could make the IBM PC cheaper was considered
worth doing (like the 8 vs 16 bit data bus by using the 8088 rather than
the 8086).  But yes edge triggered interrupts are awful.

-- 
Len Sorensen