H17 Controller with Z80 V4 board running at max speed.

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Jun 23, 2021, 12:55:10 AMJun 23
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Gery shared with me a modification feature in remark-issue38-1983. This is an article from Pat Swayne, where he runs the Heath Z80 CPU at 3 MHZ along with the H17 controller. The H17 is receiving a constant 2MHz signal from the Heath Z80 board. The new Z80 V4.0 supports this feature along with the new H8 Front Panel. However the H17 controller lack such support as the article came to me late. On the H17, I will need to cut a trace and add a wire. A very simple change.


Can you make the changes to HDOS so that when it switches the speed, it loads the correct constants for the H17 to operate, when CPU frequency can be any speed value? I wonder if this is feasible or not.


Attached is the article for your review.




H8 3 MHz modification 1.png

Terry Smedley

Jun 23, 2021, 11:48:22 AMJun 23
Back in 1982, Trionyx commissioned Tom Jorgenson (Software Wizardry) to figure out how to patch HDOS and CP/M to run the H17 reliably at 4MHz.   I've attached Tom's full 20-page study.  Probably worth noting that the introduction of I/O or memory wait states can make the re-calculation of the H17 timing constants more complicated.

In my opinion only, the "gear shift" solution provided by Ken and others - shifting the CPU to 2MHz before timing-sensitive H17 operations - works quite reliably and requires no hardware modifications.  That methodology allows both HDOS and CP/M 2.2.x to run quite well with the H17 at 2/4/8/10/16 MHz.  


Norberto Collado

Jun 23, 2021, 12:23:24 PMJun 23
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OK, let’s put this idea to rest. 😊



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