[Coco] Re: [Color Computer] Sam/Ram question.

jdaggett at gate.net jdaggett at gate.net
Wed Dec 28 14:21:50 EST 2005

The ferrite beads will have little effect on the problem. They generally have 
very low impedance, <10 Ohms, at say 100 Hz and at 100MHz they w ill 
have and impedance of about 300 to 900 Ohms. 


On 28 Dec 2005 at 9:09, Mike Pepe wrote:

To:             	ColorComputer at yahoogroups.com
From:           	Mike Pepe <lamune at doki-doki.net>
Date sent:      	Wed, 28 Dec 2005 09:09:06 -0500
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> Gene Heskett wrote:
> > On Wednesday 28 December 2005 06:47, Phill Harvey-Smith wrote:
> > 
> >>Hi there,
> >>
> >>I mentioned a while back that I was working on getting the SAM in the
> >>CoCo 1/2/Dragon to talk to static RAM, I think I have this mostly
> >>working, currently implemented with a pair of LS latches and an Atmel
> >>16V8 PLD. I still think this requires a little work as it does have a
> >>couple of problems, however I can boot into basic and everything
> >>seems to work ok. Asuming I can get this working correctly, the next
> >>stage would be to graft on some sort of bank mapping to allow >64K
> >>memory.
> >>
> >>However the most serious problem I am currently having is that it
> >>seems to stop working after about 10-15 minutes, I suspect something
> >>is getting too hot, though nothing appears to be getting hotter than
> >>with normal DRAM connected.
> >>
> >>One thing I did wonder about is that the original CoCo/Dragon circuit
> >>sheets include a bank of 33 ohm resistors between the SAM and the RAM
> >>address lines, I have not included them, in my circuit could this be
> >>the cause of the problems, I could never work out just what they
> >>where there for anyway :)
> > 
> > 
> > They were there for the express purpose of src terminating the lines to 
> > the memory, helping to absorb the ringing enough that there was only 
> > one crossing of the level going each way, whereas without it, the 
> > memory may sometimes see 3 transitions per actual intended logic level 
> > change.  ISTR there were also some very small pf capacitors used there 
> > also, but these may have been needed to quiet the circuitry for 
> > FCC/DOC approvals.
> > 
> > 
> >>Cheers.
> >>
> >>Phill.
> >>
> ...what Gene said is true, though some CoCo models don't have resistors 
> there, they instead have small ferrite beads. I don't suspect that would 
> cause the problem you're seeing (though it might)
> Have you modified the CoCo ROM to put the SAM into SRAM mode? Maybe all 
> the superfluous refreshing is causing some glitching or timing problems.
> -Mike
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