[Coco] What are the important differences between the SN74LS783 and the 74LS785 SAM chips?

[Rocky Hill]

The link in the previous message has some non printable characters that generate a page not find when clicking on it.Here it is a little cleaner.https://github.com/qbancoffee/coco2_64k_sram


    On Thursday, October 6, 2022 at 10:49:03 PM EDT, Rocky Hill via Coco <coco at maltedmedia.com> wrote:  
 
 Hello All, 
Recently I made two versions of a static RAM board that plug into  the memory expansion port of one of my CoCo 2s. The static RAM boards work but the CoCo behaves differently for  each model SAM chip.  

When using the 74LS785 SAM, 64K of RAM can be accessed as one would expect but when using the 74LS783 SAM, only 32K of RAM can be accessed.  

Interestingly when the same 74LS783 is used in a CoCo 2 that has 8 DRAM chips, 64K can be accessed. 

I know the two SAM chips have different refresh rates and I think the 74LS783 was designed with refreshing 8 different DRAM chips in mind but I thought this wouldn't be a  consideration when using modern static RAM. 

Has anyone seen this before? Has anyone been able to access 64K of static RAM with an SN74LS783 before? 

The SRAM circuit is made so that the SAM thinks it's still interfacing to DRAM but the datasheet for the SAM says that it can be placed in static RAM mode. I'm wondering if some sort of poke could make it magically access 64K...... 

Has anyone used this mode before? If so, how does one go about putting it in SRAM mode? Does it still strobe the row and column parts  of the address with RAS and CAS?


Here is the repo if you want to look at the design. The SMD and THT versions are in the same repo. 

https://github.com/qbancoffee/coco2_64k_sram 


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GitHub - qbancoffee/coco2_64k_sram: Static RAM board for the Tandy Color...

Static RAM board for the Tandy Color Computer 2 model 26-3134 - GitHub - qbancoffee/coco2_64k_sram: Static RAM b...
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Hopefully it's something as simple as overlooking an address line or some logical error in the design. 
Thanks, 
Pedro 




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