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

Gene Heskett gene.heskett at verizon.net
Wed Dec 28 08:23:16 EST 2005

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.

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Cheers, Gene
People having trouble with vz bouncing email to me should add the word
'online' between the 'verizon', and the dot which bypasses vz's
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