Date   : Tue, 27 Apr 2004 21:27:24 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: 65c02

On Apr 27, 15:31, gARetH baBB wrote:
> Has anyone come across a reference before about using a 65c02 in an
Issue
> 7 BBC ?
>
> I've used them in Issue 4s no problem, but this Issue 7 just starts
up
> claiming to have only 16K and Language?

There shouldn't be any difference in that regard bettween Issue 4 and
Issue 7.  However different makes of 65C02 may behave differently (as
well as having different changes to the opcode set), and they all have
slightly different timing and drive capability to an NMOS 6502.  There
are two types of 65C02, not counting 65C102 etc variants.  Since some
of the Beeb bus timing is "close to the edge", using a slightly
different processor might cause a problem.  First suspect in my book
would be the 74LS245 data bus buffer at IC14 -- that's the one you
sometimes have to change to fix second-processor problems.  If it's an
LS245, try an ALS245 or F245.  Replacing the LS245 with an ALS245 was
at one time an official Acorn fix.  If it's an ALS or F type, try an LS
:-)

Check that the six main power connections are effective.  It's not
unusual for some of the connectors to be slightly loose, especialy if
an enthusiast has had the board disconnected a few times.  Imperfect
contact can cause slight voltage drops which the TTL doesn't like (it
adversely affects rise and fall times).  If they seem at all loose,
pull off the Faston connectors, tighten slightly with a pair of pliers,
and push them back onto the board.

Make sure the video ULA has an effective heatsink if it's a Ferranti
ULA rather than a VTI video processor (which needs no heatsink); this
dramaticaly affects the timing.  Also check all the RAM chips are the
same type.

Next thing to check is that the 74LS74 ICs used in the timing circuit
are driving to full levels and generating a clean waveform (you need an
oscilloscope for this).  Some brands are, well, not quite so good as
others.  I've seen a second processor completely stop because of this!

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York