Date : Wed, 29 Mar 2006 00:52:30 +0100 (BST)
From : Pete Turnbull <pete@...>
Subject: Re: Model B NMI PITA.
On Mar 28 2006, 21:01, Fragula wrote:
> Ahh.. I'm getting to the root of my various problems it seems. Well,
the
> Beeb related ones anyway.
:-)
> So I tried to transfer the DFDC from one model B to another, cut S9
as i
> need the NMI. quick test, and voila, another non-functional econet!
:'(
> Shorting S9 out with a screwdriver seems to "sort" the problem, but
that
> wont do the FDC'd model any good.
>
> The Good Old Model B seems to need Link S9 closed to work with
Econet,
> but open for the disk controller.
That's not right. I've got Beebs here that have both; it's always been
possible to have both. In fact, Level 1 and Level 2 file servers have
to have both, as they're floppy-based.
It's possible you have an early PCB that needs some fix to use the disk
interface properly, though offhand I can't remember what the fix would
be (something to do with that 7438 at IC27, and S9). It's also
possible that the interrupt on the STL interface works differently from
Acorn's, but that's less likely. It's also possible if you have Issue
4 or older boards that the Econet interfaces are not correctly fitted;
they sometimes "mostly" work if all the mods are not done.
The FDC interrupt is active-high and comes from pin 11 of the 8271, to
pin 9 of a 7438, which (pin 8) drives the /NMI line. Pin 10 of this
7438 is tied to +5V. Pin 9 is connected to S9, which shorts this to
ground, thereby preventing this gate from going low, when the FDC is
not fitted.
S2 does a similar job for the interrupt from the 68B54 ADLC, although
that is active-low, and passed from the ADLC pin 7 through part of a
74LS132 (IC91 pin 13) coming out on pin 11, then into pin 2 of the same
7438 used for the FDC interrupt, coming out onto the /NMI line on pin
3. However, pin 1 of that part of the 7438 is fed from a flip-flop
that can be used to turn the Econet interrupts on and off.
--
Pete Peter Turnbull
Network Manager
University of York