ZX21 Project ZX81 Replacement PCB

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

Paul wrote: Wed Nov 25, 2020 12:20 pm Will it be possible to jumper to 1K, 2K, 16K, 32K?
Would the 1K and 2K not be able to be done with a ROM change and maybe holding the 1 or 2 key down at power on?
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Paul
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Re: ZX21 Project ZX81 Replacement PCB

Post by Paul »

To limit RAMTOP isn't sufficient. You need the correct RAM-Shadows as well. Which means you have to disconnect the unused RAM addresslines and use pull-ups or pulldowns instead.
In theory, there is no difference between theory and practice. But, in practice, there is.
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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

Paul wrote: Thu Nov 26, 2020 12:20 pm To limit RAMTOP isn't sufficient. You need the correct RAM-Shadows as well. Which means you have to disconnect the unused RAM addresslines and use pull-ups or pulldowns instead.
The best idea I can come up with at the moment is an adapter plugged into the RAM socket to provide the 1K, 2K functions. It is early days however and I may come up with a better plan for a future PCB issue!
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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

zx21_placement.jpg
I think I am making some progress.

A looooong way to go.

Constructive feedback always welcome!
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PokeMon
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Re: ZX21 Project ZX81 Replacement PCB

Post by PokeMon »

fordp2002 wrote: Wed Nov 25, 2020 9:56 pm The AS 6C1008-55PCN is CMOS only so not great for a TTL Computer. You can use the 512K Byte big brother however and I do have some of those.

Is there much software that needs more than 32K?

You would, of course, need to add additional logic to get more than 16K working.
A6SC1008 works great with TTL, don't hestitate to use it. It also supports 3.3V as 5V supply depending on your hardware. This chip is used in several memory attachments, at least in my ZXmore or ZXblast. ;) And yes, little extra logic is necessary to get it to 56k RAM. 32k RAM is often used but run out of memory fast when using different modules/drivers, HRG software and so on.

628128 would be also fine but source more current, AS6C1008 is a direct replacement. Just in case.

.
Bildschirmfoto 2020-11-26 um 20.49.17.png
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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

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Yes, that datasheet for AS 6C1008 contradicts itself. It states it is compatible with "all" in one part, while only claiming CMOS levels in others.

My PCB will take that chip, however.

I have added the 1K, 16K switchable feature as an option I worked out how to do it. You need to add an additional IC for it.

There comes a time when you cannot support every request, however.

Let's see if I can finish routing it!
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superfo
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Re: ZX21 Project ZX81 Replacement PCB

Post by superfo »

No mounting holes as original PCB?
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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

I deleted the mounting holes by mistake after a reimport of the circuit diagram. Luckily I did not delete the construction lines on the user layer so it only took a few seconds to put them back in.
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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

zx21_placement2.jpg
Making progress!

I have put that version on Github!
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

The ZX81 is a hard board to route (for me anyway) but I am getting there with less than 100 connections to go and I think the hardest bits are done.
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zx21_placement3.jpg
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