28-05-2025, 12:26 PM
Hi OzHolmes,
Sorry for the delayed response because someone has been on holidays.
Thanks for the wonderful and thorough explanation.
Wait states present - so sad,
but expected as someone was wondering how to handle the Z80's variable length memory access cycles.
Interestingly what you have described is similar if not identical to how the Microbee Alpha Plus model accesses the video memory.
If someone is repeating themselves, apologies.
The difference between the DGZ80's & microbee's tape interfaces is that the microbee's is bit bashed, reliant on software delay loops &
uses an alternating '*' then space activity indicator to minimise VDU access (along with a VDU wait state disable circuit for some models) rather than
the DG680's fantastic animated wiggly LORES dot which updates more rapidly.
This is because the microbee doesn't have a Z80 CTC to generate the 1200Hz & 2400Hz tape tones (to save costs).
Cheers
Someone
Sorry for the delayed response because someone has been on holidays.
Thanks for the wonderful and thorough explanation.
Wait states present - so sad,
but expected as someone was wondering how to handle the Z80's variable length memory access cycles.Interestingly what you have described is similar if not identical to how the Microbee Alpha Plus model accesses the video memory.
If someone is repeating themselves, apologies.
The difference between the DGZ80's & microbee's tape interfaces is that the microbee's is bit bashed, reliant on software delay loops &
uses an alternating '*' then space activity indicator to minimise VDU access (along with a VDU wait state disable circuit for some models) rather than
the DG680's fantastic animated wiggly LORES dot which updates more rapidly.
This is because the microbee doesn't have a Z80 CTC to generate the 1200Hz & 2400Hz tape tones (to save costs).
Cheers
Someone