[Coco] On John Linnville's Color Computer Game Master Cartridge Design

[RETRO Innovations]

   Hmm, it seemed like the datasheet shows that if either WE or CE are
   tied low, READY stays low.  I thought I saw the behavior as well, but
   perhaps it was a flawed test.

   As for the rest, I agree the approach looks good.  I was thinking of
   having the SCS/RW/E going low would clock the data into the register,
   and then that line AND the READY line would be tied to WE/CE.  THus,
   the line going low would bring WE/CE low and clock the data into the
   register, and then the SCS/RW/E would go high, but READY would be low,
   so the AND would still be low until READY went high again.  It avoids
   the open collector.  But, both designs assume READY comes up in high
   state, and I was not sure it could be guaranteed to do so.

   Jim

     On August 2, 2017 at 2:53 PM Darren A <mechacoco at gmail.com> wrote:

     On Wed, Aug 2, 2017 at 12:23 PM, RETRO Innovations wrote:

     >

     And, after 3 nights working (fighting) with this IC, I think people
     should
     give more credit to John for his design. He didn't just wire up this
     IC to
     the Coco expansion bus, he performed some magic to successfully map
     it into
     the Coco address range.

     The SN76489 takes 32 4MHz cycles to successfully store a value in a
     register, which is 8uS, though that might be worst case, as I see
     3uS
     typical. In any event, this need to hold the data valid for so long
     makes
     a simple connection to the Coco bus more complex (essentially,
     unless one
     wants to HALT the CPU while the write completes, it appears one
     needs to
     store the data in a faster "buffer register" and then hold the bus
     signals
     to the IC until the IC signals the write is complete (there is a
     "READY"
     pin)).

     I have been going through the same exercise recently. Before trying
     to
     write some HDL for a CPLD implementation, I rigged up the following
     circuit
     on a breadboard for testing:

     [1]https://drive.google.com/file/d/0B_o8KocZC3DSenp4em0xWFloOVk/view
     ?usp=sharing
     * Darren

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References

   1. https://drive.google.com/file/d/0B_o8KocZC3DSenp4em0xWFloOVk/view?usp=sharing
   2. https://pairlist5.pair.net/mailman/listinfo/coco