Date   : Mon, 07 Feb 2005 17:57:06 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: BBC SWRAM module

> On Mon, 7 Feb 2005, Michael Foot wrote:
> > Quoting John Kortink <kortink@...>:
> > > It samples NR/W nand 2MHzE, which makes for a cleaner and
> > > safer write strobe, because it is then synced to the data
> > > transfer part of the bus cycle, as it should be (instead
> > > of relying on accidental 6502 behaviour of defaulting to
> > > 'read' on R/nW).
> >
> > I agree that it's a homemade job and that whoever did it knew what they
were
> > doing. I've not seen a schemetic for this type before but it looks overly
> > complex to me. The green wire going to S37 is interesting.
> > Unfortunately because it is an Econet only machine, I've not go around to
> > testing it out yet.
> 
> I notice the lack of (D)NFS chip in that machine - did it come like that
> or did you take it out?
 
It looks like the IC between the MOS and BASIC is the DNFS.
 
> I can't quite make out what's under the homemade thinggy, it looks like it
> could be 2 chips "stuck together" - maybe one of those is DNFS and the
> other is the RAM - so the extra logic is to use both in the same socket?
 
It looks like at least 32K of sideways RAM. One of the three wires
goes to the ROM select cluster just right of the absent disk
interface. If, say, ROMSEL bit 2 is fed to a SRAM A15, then you
will see 16K of SRAM in socket 15 and 16K of SRAM in socket 11.
 
I've done this often with 32K ROMs so I can plug DNFS and VIEW
into one ROM socket. With ROMs you only need the one flying lead,
of course.
 
Normally, you would only need a 32K SRAM and two flying leads -
R/nW and A15. It could be that the two smaller ICs are to generate
whatever signal goes to the keyboard LED.
 
Is there any possibility of you telling us what the small ICs are
and sketching the wiring arrangement, or uploading a very detailed
picture of just the ROM area, detailed enough to read the IC
legends? All I can make out is 74LS02 at the bottom and
74LS-something at the top.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
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