Path: utzoo!censor!comspec!lethe!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!helios!bcm!dimacs.rutgers.edu!rutgers!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Rev 4.4 motherboard viable?
Keywords: motherboard rev 4.4
Message-ID: <18987@cbmvax.commodore.com>
Date: 14 Feb 91 22:46:51 GMT
References: <1219@xroads.UUCP>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 55

In article <1219@xroads.UUCP> gmz@xroads.UUCP (Gerry Zeitlin) writes:

>I've been advised by a local retailer to upgrade my rev 4.4 motherboard
>to the latest from Commodore (via a swap-out offer, approx $350 +
>installation).

Sounds to me this guy's either misinformed, or just about for a buck.

>The reasons for doing this would include new memory timing (which would
>explain why the Aries memory board I just purchased there seems to
>be causing system crashes); fat Agnes; etc.

Well, I don't know why your new memory board is crashing, but there's no 
change in timing between old and new A2000s, other than the slight effect
the ECS Agnus has on bus timing (all A2000b Agnus chips are Fat, the new
ones are 1MB ECS Agnus).  You can add an ECS Agnus to your R4.4 board and
expect it to work just dandy.  Two PCB jumpers must be changed to map the
second 512K in as Chip memory.

There is one known problem on some Rev 4.4 boards.  In conjunction with a
DMA driven hard disk, a certain brand of 74ALS245 chip used in the U605
position can cause an overly long rise time on the AS* signal out on the
bus during bus master changeover.  This is ignored by the basic A2000 anyway,
but if the condition exists, it'll be a problem with accelerator cards that
have their own DMAable memory, such as the A26x0 cards (since they respond
to bus activity faster than Chip memory).  This has the potential to cause
problems with other memory devices, but none have so far been identified
(unless maybe that's the problem with this Aries board; I know nothing about
it).  In any case, the fix is to add a 1K pullup resistor between pins 11
and 20 of the U605 chip, which is between the CPU and first Zorro II slot
on your A2000.  This fix is perfectly acceptible whether you have a problem
or not, and it's the only difference between R4.4 and R4.5 A2000s.  We pay
less than 1 cent for such a resistor; I doubt Radio Shack could charge more
than $1.

As for R6 A2000s, no system timing was changed or anything like that.  The
main reason for the change was to replace the old 256K x 1 DRAMs used on the
A2000 R4 and earlier with 256K x 4 parts, which became cheaper per bit.  Some
FCC changes were apparently made as well, I wasn't involved in that stuff.

>Is this good advice?  Might I be able to find another memory expansion
>system that would work ok with my rev 4.4 board?

Unless you have the aforementioned U605 problem, I wouldn't expect any trouble
getting a correctly designed memory board to work fine in your system.  And
that U605 problem will only happen in conjunction with hard disk or other DMA.
There is certainly no reason to pay $350 for a new motherboard!

>G E R R Y  Z E I T L I N			(602) 996-1661


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	"What works for me might work for you"	-Jimmy Buffett