[Coco] MC6883 x 74LS785

Daniel Campos daniel.campus at gmail.com
Sat Sep 10 16:14:56 EDT 2016

The same 4164s that I'm using in the 2B now for testing with the 6883, 
were also tested on another CoCo2 with a 6883 and worked just fine. 
Tested and retested with the CoCoCheck utility.


Em 10/09/2016 16:12, Bruce W. Calkins escreveu:
> 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

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