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RE: [N8VEM-S100:5375] Z80 CPU board - D2/D3 port timing problem

David let me know if I need to update the site about this mod.



From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Philippe Elie
Sent: Sunday, October 12, 2014 4:08 AM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:5375] Z80 CPU board - D2/D3 port timing problem


hi folks,

I finally identified the issued that made some CPM (non banked version) hang the system : the RAM bank switching system had a timing issue. More specifically, when the monitor initializes the D2/D3 ports (with 00 and 04 respectively) the second IO access was actually trigerring also a 04 write into the D2 port.

I solved the issue by replacing U25 74LS04 by a 74F04 but a little glitch on D2 port enable still appears that makes me a bit concern about the reliability :


I was wondering whether the problem would come from sOUT : for U39 I used a 74LS08 instead of a 74S08 thinking speeds would be the same and considering the CPU operates at 4Mhz  ? Do you that would fix the problem ?


thanks in advance for your feedback


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