[Coco] MC6883 x 74LS785

Sat Sep 10 15:12:58 EDT 2016

This may or may not relate to the issues:  The 74LS785 has a different 
RAM refresh rate than the 74LS783 AKA 6883.  Also, not all RAM responds 
well to the different refresh rates.
Bruce W.

On 09/10/2016 12:17 PM, Daniel Campos wrote:
> Thanks to everyone that privately sent me the 74LS785 datasheet. I 
> have done some more tests and discovered that the CoCo2B board only 
> recognizes 32Kb of RAM when using a MC6883. If I replace it with a 
> 74LS785 it works just fine and recognizes the full 64K RAM.
>
> - Also tested replacing the 2x 4464 DRAMs for 8x 4164 DRAMs (the board 
> has the places for both), but the result is the same.
> - For those who asked, yes, I tested all the 3 MC6883 used in the 
> tests and they work just fine on another CoCo2 (not 2B) that I have here.
> - I have tried to replace the CB 1.3 + ECB 1.1 EPROM from another with 
> the CB 1.2 + ECB 1.1, but with no effect.
>
> Daniel
>
> Em 10/09/2016 04:40, Simon Jonassen escreveu:
>> Here you go
>>
>> http://roust-it.dk/coco/SN74LS785N.pdf
>>
>>
>> /Simon :-)
>>
>>
>>
>> -----Oprindelig meddelelse-----
>> Fra: Coco [mailto:coco-bounces at maltedmedia.com] På vegne af Daniel 
>> Campos
>> Sendt: 9. september 2016 21:50
>> Til: CoCoList for Color Computer Enthusiasts
>> Emne: [Coco] MC6883 x 74LS785
>>
>> I just received this week a CoCo2B, but instead of have a 74LS785 
>> SAM, it
>> have an old 6883. The result is an erratic memory behavior with NitrOS-9
>> crashing when loading, etc.
>> Anyone has any documentation or datasheet about this newer SAM ? I have
>> done a quick search on google but found nothing relevant.
>>
>> Daniel
>>
>
>
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