[Coco] It's a small win, but a win nonetheless

[RETRO Innovations]

Finally, after fighting with the cassette port for a week:

55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 3C 00 0F 4A 20 20 20 20 20 20 20 00 00 00 00 
00 00 00 39 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 
55 55 55 55 55 55 55 55 55 55 55 55 55 3C 01 02 00 00 03 55 55 3C FF 00 FF

Notes (for posterity):

This site was helpful:

http://www.cs.unc.edu/~yakowenk/coco/text/tapeformat.html

But, it is incorrect that bytes are stored "least" significant bit 
first.  They are transmitted "most" significant bit first.

I have a few lingering problems I need to fix, but I think I am over the 
worst of it:

  * The design runs on 5V currently, but will run on 3V, so the voltage
    divider on the comparator will have to change.  No big deal
  * The current design only works if my scope is connected to the
    cassette data out pin :-)  Evidently, the scope probe is loading the
    line, dragging it into the comparator center.  I'll have to thus try
    some way to figure out the center of the signal without my probe.

The above looks like I have a few straggler bytes to address, but the 
bulk looks like it is there.

Jim





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