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Re: Help with Homemade ZX80-81- HIRES problem

Posted: Mon Nov 20, 2017 7:49 pm
by Luciano
Hi.

Yes.. you all right.. the image is correct in the white stripe region!

Do you have a suggestion or idea about how to perform this adress decoding test?

It seems that it only happen using the HIRES routine , because "normal" programs runs ok.. even pseudo hires.

All sugestions are welcome..i will try to test all.. (i am trying to fix this issue for weeks.. without sucess .. :roll: )

Thank you!!

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Mon Nov 20, 2017 8:22 pm
by sirmorris
Using a pencil and a piece of paper try to work out what address range the correct portion of the image occupies. This will give some clues I think.

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Tue Nov 21, 2017 9:18 am
by Andy Rea
Oh my bad, I'll go and have a rethink

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Tue Nov 21, 2017 12:44 pm
by Luciano
Hi Andy.

Maybe is something related to the EPROM..
in this setup i am using 8k ROM 27C64.. and just to test, i placed one 28C64, the image changed completely (not fixed yet.. but much better result)

I will post the pictures soon.

Maybe the enable / disable timings? (CE OE)

Thank you!!

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Tue Nov 21, 2017 5:09 pm
by Luciano
Attached, the picture using 28c64 instead of 27c64 EPROM

Now the white stripe has no dada :roll: weird

I will try to change the 74ls165 parallel to serial shift(video) testing with 74LS and 74HC

It only happens with Hires.. very strange.. and difficult to solve

Thank you again

Regards

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Tue Nov 21, 2017 11:14 pm
by PokeMon
Looks like a timing problem with the RAM during refresh.
How did you enable RAM during refresh ?
Maybe just wrong resistor, diodes.
So the picture is shown correctly but missing pixels.
Normally the pixels come from the ROM in text mode but from RAM in HRG mode.

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Wed Nov 22, 2017 12:50 am
by Luciano
Hi Pokemon.

initially i tested using the original schematic of 16k ram expansion.. (attached file name: 5 original)
it worls very well for all software loaded , like 3d maze, manic(pseudo hires) and onder 16k games.
But after loading the some HIRES programs, i notice that the hardware was not capable to display it
I started to implement some modifications using /RFSH signal and disable the /RD ( fine name:4)
So the new problem was the "garbage on screen".. (file 1)
Yesterday i tested the eprom change, using 28c64 eeprom.. it improve the image.. but still not good(file 28c64)
Today i change the 74LS165 , to 74HC165 and place back the 27c64 EPROM.. now same result as with 28c64 and 74LS
The hardware does not have ULA.. is identical to ZX80 schematic , with 16k RAM Expansion and NMI generator (Grant web site)

I tested using 2 different SRAM chips brands, no change.

If you have some suggestion , i can implement here.. maybe trying another NMI circuit configuration?? (fig zx80slow2)

Do you have some program to test the HIRES mode that uses less memory than 16k? because using WRX1K works normally..

If you need additional information, like waveforms , pictures, please let me know, and i sent to you.

I really need some help.. i was trying to solve this for weeks without success.. :roll:

Thank you very much

Luciano

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Wed Nov 22, 2017 1:51 am
by Luciano
Another info.. may be useful.. this file attached , pacman.p is a HIRES game.. and works normally..no problems

it seems than only happens with steady pictures?? :roll:

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Wed Nov 22, 2017 3:48 pm
by siggi
Hi Luciano

you should use this logic as given above:
sirmorris wrote: Mon Nov 20, 2017 5:10 pm This is from the ZXpand RAM select logic.

RAM = true when RAM is required.

assign IFRAMOE = !nRD | !nREFSH;
assign IFRAMCE = (!nMREQ | !nREFSH) & RAM
Could you show us a picture of your ZX81? Maybe there are other problems? Not enough deblocking capacitors next to the ram? Thin GND or +5V lines?
This things could result in a lot of misterious problems ....

Siggi

Re: Help with Homemade ZX80-81- HIRES problem

Posted: Wed Nov 22, 2017 6:37 pm
by Luciano
Hi Siggi.

I will try to implement the logic suggested.

Before start soldering ,Could you please confirm if i did the logic convetions correcttly? (attached)

Yes, i can send the pictures of my setup.

Thanks a lot!! ;)