Date   : Tue, 28 Mar 2006 22:25:16 +0100
From   : Phil Blundell <philb@...>
Subject: Re: Model B NMI PITA.

On Tue, 2006-03-28 at 21:01 +0100, Fragula wrote:
> The Good Old Model B seems to need Link S9 closed to work with Econet,
> but open for the disk controller.
> 
> Anyone got any easy clue for me? I can't read the circuit diagram until
> Thursday, when my new glasses arrive.
> 
> "Its not possible" is not the correct answer, I'm sure, but it doesn't
> look easy at the mo.

Hi M.,

S2 needs to be open for Econet; S9 needs to be open for the disk
controller.  In theory at least, you just need to cut both links and you
should be in business.  It's a bit weird that shorting S9 would make a
difference to the Econet: all it does is prevent the FDC from driving
NMI though one gate of IC27; S2 does likewise for the ADLC using another
gate in IC27.  Closing either of those links with the appropriate
hardware in place will short an output from IC70 or IC91 to ground, so
not a good idea.

What issue is your PCB?  The early ones, up to (and maybe including)
issue 4, needed some extra modifications in that kind of area to make
them work.

I think the service manual (which is on Mark Usher's site somewhere) has
some words about how to perform the disk and network upgrades, including
the link settings.  Might be worth checking that out and making sure
you've been through all the right motions.

I have a B with both 1770 and net lurking upstairs somewhere.  I'll go
dig it out.

p.