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Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 3:39 pm
by Luciano
Hello Everyone!
I built the ZX80 from Grant´s web site, including NMI ,slow generator and 16k SRAM upgrade
All programs loaded works well, like 3dmaze,Manic M (pseudo hires) and others.
But after loading the "FullHDzeddy¨ demo i noticed that Hires is not working using the original configuration
I did some tests and changes , adding /RFSH signal and removing /RD from /OE and it "started" working but not OK yet.
Attached some pictures of the problem.
Do you have some idea or suggestion about how to fix this issue?
There is a small blank period on the screen.. Only in this portion the picture is displayed correctly..
It is weird.. On the ZX81 , theoretically, /RFSH is not necessary..
Even the 1K WRX1K demo only work after this RFSH modification.
Maybe using a different slow circuit?
Any ideas are very welcome!!
Thank a lot again!
Regards
Luciano.
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 4:02 pm
by sirmorris
IIRC you need both. Wire RD and REFS with an AND gate.
Code: Select all
+5v
|
|| 10K
|
!rd-----|<-+
!refsh--|<-+---!rdrefsh
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 4:15 pm
by mrtinb
(Where can "FullHDzeddy" be downloaded?)
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 4:35 pm
by Luciano
Hi Sirmorris.
I did a test now using your suggestion.. no image at all.. (using /RD , /RFSH and diodes.. )
Maybe another configuration?
Hi Mrtinb.. attached the file used in the test.
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 5:10 pm
by sirmorris
This is from the ZXpand RAM select logic.
RAM = true when RAM is required.
assign IFRAMOE = !nRD | !nREFSH;
assign IFRAMCE = (!nMREQ | !nREFSH) & RAM
Also some WRX programs require RAM at 8k.
C
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 5:17 pm
by Andy Rea
Go back to using the second circuit above and put a small 33pf ( or maybe 100pf) cap from ~CE to GND some newer and faster memory chips release the data lines a little bit too quick for the poor ula to fully capture the data. The image above of the zx81shows that sometimes it gets the right data and sometimes it doesn't
Hth regards Andy
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 6:29 pm
by Luciano
Hi All
-Sirmorris, about RAM at 8k ,exist some way to test this in my setup?
-Andy- I tested using the capacitor.. tested 33pf, 100pf and 470pf.. all same results as above
No ULA .. it is a "discrette" 74ls logics..
(from Grant page ..schematic
http://searle.hostei.com/grant/zx80/zx80.html)
Attached the wrx1k file.. this one works fine.. i dont know why.
Do you have some Hires files to send for test..? ( like 2k, 4k and 8k in size..)
I am using the NMI generator VERSION 4.1.. maybe using a another NMI circuit?
Thanks a lot!!
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 6:31 pm
by sirmorris
I've only just noticed the band where the image looks correct, you have highlighted it in your original images. Is this static on screen or does it sweep from top to bottom?
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 6:54 pm
by Luciano
Hi
The "white" stripe is static on screen.
The image 2 is like a "wallpaper"
like a background for image 1
Re: Help with Homemade ZX80-81- HIRES problem
Posted: Mon Nov 20, 2017 7:36 pm
by sirmorris
Yes but do you see that the image is correct in that stripe? It looks to me like there is a problem with memory decoding. It seems that for a small region of the display it is correct, for the rest there is something happening. It would be interesting to see what address is correctly displayed.