Help with Homemade ZX80-81- HIRES problem

Discussions about Sinclair ZX80 and ZX81 Hardware
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sirmorris
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Re: Help with Homemade ZX80-81- HIRES problem

Post by sirmorris » Fri Nov 24, 2017 3:27 pm

Great news! Congratulations :D
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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PokeMon
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Re: Help with Homemade ZX80-81- HIRES problem

Post by PokeMon » Fri Nov 24, 2017 3:49 pm

1024MAK wrote:
Fri Nov 24, 2017 2:53 pm
Funny that I had studied that part of the ZX80 schematic a couple of months ago, I wondered why Sinclair had wired up that multiplexer input like that (pin 5). But I got distracted and never investigated any further...
I guess this is simplified to reduce logic (as Sinclair always reduced anything not really necessary). But it has the problem, that HRG from RAM does not work. I think this is the reason why they corrected this behavior in ZX81 with more logic in the ULA. And spend a resistor in /ROMCS to override this signal from external hardware as well (like they did with /RAMCS in ZX80). During development they maybe had not the idea to use this computer for high resolution graphics.

Luciano
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Re: Help with Homemade ZX80-81- HIRES problem

Post by Luciano » Fri Nov 24, 2017 3:52 pm

Yes .. good news.. now is working fine.

Thank you all ..

Probably on zx80 Sinclair didn´t considered the future HIRES applications :D

On the zx81 ULA ,maybe, the "equivalent" internal circuit , pin 5 is conected to A14?

By the way.. do you have the ULA ZX81 schematics?

Luciano
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Re: Help with Homemade ZX80-81- HIRES problem

Post by Luciano » Fri Nov 24, 2017 4:05 pm

Usiing Mreq and A14 (pin5 connected to pin 6) the Hires works well but in low res mode the picture have vertical lines..
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Image_00005.jpg
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PokeMon
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Re: Help with Homemade ZX80-81- HIRES problem

Post by PokeMon » Fri Nov 24, 2017 4:18 pm

I have a guess - is this with no additional memory ?
1k RAM ?
When you enter Text/BASIC - is it shown correctly ?

Luciano
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Re: Help with Homemade ZX80-81- HIRES problem

Post by Luciano » Fri Nov 24, 2017 4:30 pm

16K Ram

Using MREQ and A14 the vertical lines are there all the time ,for text and low res games.. and disapear with HIRES

Using A14 only on pin 5 , low and hi-res works perfectly

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PokeMon
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Re: Help with Homemade ZX80-81- HIRES problem

Post by PokeMon » Fri Nov 24, 2017 10:25 pm

It has to do with character display and creating NOPs during HALT. I thought that the ZX80 same as ZX81 has a condensed display file when RAM <3k but in fact this routine isn't existing - display file is always condensed even with 16k RAM.

Is your clone 100% identical to ZX80 schematics or any custom changes ?
So what is displayed as "blank" character is the pattern with D6=1 (line) coming from the pullup resistor on D6 and the combination of forced NOPs. I think an original ZX80 wouldn't behave identical when looking at the schematics - that's the question if anything changed.

Using A14 is a good idea. It doesn't work with /MREQ together because /MREQ isn't active during refresh.

Luciano
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Re: Help with Homemade ZX80-81- HIRES problem

Post by Luciano » Fri Nov 24, 2017 11:27 pm

Hi Pokemon

Yes.. it is identical to z80 schematics plus NMI slow circuit... i mean.. i tried to soldered identical.
(now the difference is A14 on pin 5 and controll pins on 16k expansion)

I will double check the D6 and the pull up resistor.

If you have another suggestion, hardware or software, to test ,feel free to send..

Any modifications in this board are very easy to implement .

One of the objectives of this project is to understand better how zx80/81 works..

Thank you!

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PokeMon
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Re: Help with Homemade ZX80-81- HIRES problem

Post by PokeMon » Sat Nov 25, 2017 9:58 am

The question is, it looks like it is loading the binary 00000010 into the shift register here in my eyes (D6=1). What I don't understand is, why it is loading the characters (K cursor) correctly. I reread your postings and you are using ZX81 rom with 16k RAM = fully expanded display file. So my theory failed, that it is caused through the HALTs in display file. Even as soon as a HALT is executed, the shift register is blocked for further loads. That's why I asked if it is original to ZX80 (video logic). The NMI logic can not cause such a behavior, I think. Maybe just a curious timing thing.

Anyway using just A14 for pin 5 seems to be a good solution as this input is active during /RFSH only and would not harm io instructions, so /MREQ is not needed here. And it is blocked for high resolution coming from RAM (A14=1). So good work. Did you solder all manually ? ;)

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siggi
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Re: Help with Homemade ZX80-81- HIRES problem

Post by siggi » Sat Nov 25, 2017 12:51 pm

PokeMon wrote:
Fri Nov 24, 2017 10:25 pm
It doesn't work with /MREQ together because /MREQ isn't active during refresh.
That is wrong! /MREQ IS active during refresh, but it is much shorter active than during nomal READ/WRITE cycles:

Z80-Refresh.jpg
Z80-Refresh.jpg (124.3 KiB) Viewed 81 times

See also:

http://z80.info/zip/z80cpu_um.pdf

Siggi
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