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

Mike Pepe lamune at doki-doki.net
Wed Dec 28 09:09:06 EST 2005

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
>>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.

...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.


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