HA-8-3 (video/game board) port address decoding

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Terry Smedley

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Sep 10, 2021, 1:40:21 AM (13 days ago) Sep 10
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Looking at the photographs of the HA-8-3 schematic that Glenn posted, you can see that all the port I/O addresses are decoded from the upper byte of the address bus (A8-A15).

As discussed in an earlier thread, this renders the board incompatible with a Z80 CPU card that does not contain circuitry to emulate the 8080 port addressing behavior.  That would include the v3.x and v4 CPU boards.  The fix for the HA-8-3 card is the same as what was described for the H8-7 breadboard card - rewire the I/O address buffer to use A0-A7 (on the HA-8-3, that's a 74LS240 at U6).

There may be an HA-8-3 clone coming in the pipeline, and that board will be fully compatible with both 8080 and Z80 port addressing.

Terry

norberto...@koyado.com

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Sep 10, 2021, 1:46:59 AM (13 days ago) Sep 10
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I will do the HA-8-3 clone if you help me with the testing and which IC’s to use on the schematics.

 

Norberto

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Terry Smedley

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Sep 10, 2021, 1:58:36 AM (13 days ago) Sep 10
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Norberto:

I have a stash of the TMS9918A, AY-3-8910, AD7501, and AD7574 chips just arrived from UTSource today.  The RC2014 TMS9918A board with static VRAM will be here tomorrow, and I will assemble and test it right away.  I'll send you a parts kit as soon as I have a handle on how the video circuitry is working.  

I was also thinking of experimenting with the XBEE RF modules to support wireless joystick/pushbutton controllers.  I used them to make a wireless joystick remote for the HERO-I robot, and the same approach should work with the HA-8-3.

Terry



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Terry Smedley

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Sep 10, 2021, 2:31:24 AM (13 days ago) Sep 10
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But.....looking at Glenn's "X-Ray" view photograph, it appears there was a running production change made to the HA-8-3.  The address lines used on Glenn's board are very clearly A0-A7, while the schematic shows A8-A15.  We can't tell whether this change was made prior to any boards being released, so there may be a mix of address decoding circuitry "in the wild".

We also saw similar running changes with the HA-8-2 board, with filter component values changed on the production board compared to the schematic.

Terry

norberto...@koyado.com

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Sep 10, 2021, 3:08:37 AM (13 days ago) Sep 10
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So initial boards did use 8080A behavior and then added Z80 behavior. Interesting!

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