[Coco] Bad memory reported by Disto Coco III RAM Test by Tony Distefano

[Gene Heskett]

On Wednesday 16 November 2016 11:36:39 Michael R. Furman wrote:

> Does anyone have a manual for the Disto Coco III RAM Test by Tony
> Distefano?
>
> http://www.colorcomputerarchive.com/coco/Disks/Utilities/Disto%20Coco%
>203%20RAM%20Test%20%28Tony%20Distefano%29%20%28Coco%203%29.zip
>
>  I ran it on one of my Coco 3’s and it is returning an error, says
> that block 60 is bad, which I interpret to be 0x78000 to 0x79FFF
>
> A second observation is that when loading certain programs in to BASIC
> from the CocoSDC those programs get corrupted at the same place every
> time.
>
> I was looking at the schematics to try to figure this out and I am a
> bit confused.  What I see on the schematic is each of the 256k x 1
> chips on the 512 ram expansion board is attached to one data line and
> there are two sets of chips selected by WE0 or WE1.  The part I am
> confused about is what it means that a particular range of memory is
> bad.  If one data bit is bad across the entire 256k range I understand
> that one, that’s a bad chip. But it only seems to be bad in a
> particular 8k range of addresses, so maybe this is some issue with RAS
> or CAS or a flakey address line to only the WE1 set of chips?
>
> What would happen if C65 or C66 still in place?  Why do they have
> those RC filters on RAS and CAS anyway?
>
Those were the result of the FCC's rules about radiated noise for home 
electronics. That was cheaper than a coat of metalized paint for 
shielding on the inside of the case. I don't think the advances in such 
paints have been enough to tip the balance toward the paint yet.  30 
years later its still expensive.

Before someone suggests aluminum paint, as a shielding paint, its a 
miserable failure because its not at all a good conductor because of the 
insulating coat of aluminum oxide that forms in a millisecond on 
exposure to normal atmosphere, which contains oxygen.  Its so reactive 
that if it was cut into 2 thousandths inch powder in a pure nitrogen 
filled enclosure, then the door was opened to retrieve the powder, it 
stands a very good chance of starting a fire when the air hits it.

When I am making a part out of aluminum by carving it with one of my 
machines, 95% of the heat generated in doing the carving is from 
this "rusting" of the alu, about a sixteenth of an inch behind the 
passage of the edge of the cutting tool.  We use water or an emulsion 
thats 95% water, directed at the tool to carry away that heat else it 
can begin to melt and stick to a cutting tool that may cost north of 
$20.  This oxide film is the 2nd hardest substance known to man, and 
wears the tool out quicker by far than any other material. So that 
coolant flow doesn't impede the formation of that coat of oxide, nor 
does it make the tool stay sharp any longer. Only a very fine mist of 
high flash point oil, directed into the rear of the cutting edge, 
safflower is good, keeps the oxygen away making the tool last longer. It 
only takes, if the mister is properly set, maybe 2oz of oil an hour, and 
it still fog coats everything in the building including my lungs and 
glasses.  TANSTAAFL.

> —
>  Michael R. Furman
> Email: n6il at ocs.net
> Phone: +1 (408) 480-5865


Cheers, Gene Heskett
-- 
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