Date   : Wed, 29 Mar 2006 22:42:01 +0100
From   : Andrew Benham <adsb@...>
Subject: Re: Model B NMI PITA.

Fragula wrote:

> Indeed. But *my* Good Old Beebs don't want to play.
> I have noticed one thing though.. ICs 79 and 80 are 7438 on the map at
> the back of the AUG, but my board appears to have 7437s, which are totem
>  pole outputs, not open collector. However these are buffers on the disk
> interface, as far as i can see, and that's working just fine.

What's IC27 ?  If that's a 7437 too, then you're in trouble, because the
Econet and FDC interrupts to the NMI line rely on 2 open-collector
outputs being wire-or'ed.  You can't wire-or totem pole outputs (or
rather you can, but you run the risk of shorting the Vcc supply).

-- 
Andrew Benham         adsb@...       
Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill"