White screen of death

Discussions about Sinclair ZX80 and ZX81 Hardware
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Andy Rea
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Re: White screen of death

Post by Andy Rea » Sat Sep 02, 2017 9:01 am

/mreq to pin 1
A14 to pin 2
Romcs out on pin 3
5v pin 14
Gnd pin 7

I would gently bend pin 13 of the broken ula out the way incase it is holding the line in a fixed state.

Hth Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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1024MAK
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Re: White screen of death

Post by 1024MAK » Sat Sep 02, 2017 2:54 pm

I was thinking that this circuit would be operationally equivalent:-
IMG_5221.JPG
/ROMCS circuit generator
IMG_5221.JPG (80.57 KiB) Viewed 1774 times
Mark

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Andy Rea
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Re: White screen of death

Post by Andy Rea » Sat Sep 02, 2017 3:14 pm

No need for the refresh signal, the zx81 ula neither has it or requires it...

Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

nama
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Re: White screen of death

Post by nama » Sun Sep 03, 2017 12:12 am

Just racing out the door, but wanted to get this out there quickly.

Firstly thanks for the great ideas.

I quickly built Andy's circuit using jumper wires, and lifting pin 13 in the ULA.
At first it didn't work...still a blank white screen.

Then I accidentally knocked the GND wire off the 7432 and I was able to get a 'K' cursor on the screen!
However the 'K' cursor is a little funky, missing bits, and sometimes with some pixels flashing. Over time the imagine of the 'K' may degrade as pixels pop in and out.
But this is promising.

When I put the GND lead back onto the 7432, the image returns to a white screen!!!
Very weird.

Phil

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Andy Rea
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Re: White screen of death

Post by Andy Rea » Sun Sep 03, 2017 12:38 am

Well that is very odd indeed.... most puzzling
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

nama
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Re: White screen of death

Post by nama » Sun Sep 03, 2017 3:55 am

Yes, very strange indeed.
Here is a pic of the cursor without the GND line attached to the 74LS32.
IMG_6156.PNG
IMG_6156.PNG (115.72 KiB) Viewed 1741 times
Sometime it looks more formed than this, and other times worse.

As I mentioned previously, if I connect the GND line up then the screen become blank again.

Phil

nama
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Re: White screen of death

Post by nama » Sun Sep 03, 2017 5:34 am

Hi guys,
So I connected the ROM CS line to the 680 Ohm resistor instead of directly to the ROM pin 20.
This helped, now I can connect GND to the 74LS32 and still get the cursor.

In fact the 'K' cursor first appears perfectly formed, then after a few seconds pixels start to flicker off. Strangely this seems to start on the top and bottom rows of the 8x8 character grid, and work it's way inwards. Also it seems weirdly mirrored top and bottom.
IMG_6158.PNG
IMG_6158.PNG (128.32 KiB) Viewed 1736 times
Could this be a timing issue caused by using long leads?

Cheers

Update: looking at the screwed-up cursor image again. I now think that it's not so much mirrored but rather somehow related to the mirrored aspect of the 'K' letter form. I'll see if I can't plug the keyboard in and type something and see what happens.

Phil

nama
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Re: White screen of death

Post by nama » Sun Sep 03, 2017 6:29 am

Me again,
I tried 2 things:
1) plugged in the ZXpand. It looked perfect...

IMG_6159.JPG
IMG_6159.JPG (29.1 KiB) Viewed 1730 times
2) plugged in the keyboard with no ZXpand...
IMG_6160.JPG
IMG_6160.JPG (28.43 KiB) Viewed 1730 times

nama
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Re: White screen of death

Post by nama » Sun Sep 03, 2017 9:32 am

I just built up Marks circuit using a 74LS02.
Have a look at this...It's even more dynamic

https://www.youtube.com/watch?v=zg1Gymf ... e=youtu.be

Phil

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1024MAK
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Re: White screen of death

Post by 1024MAK » Sun Sep 03, 2017 10:04 am

There has been a lot of discussion about how the ULA generates the /ROMCS signal over the years. At first glance, Andy's circuit is all that should be needed. My circuit is based on the ZX80 design. But until someone de-caps a real ULA chip and works out the actual circuitry that Sinclair used, we are left to speculate based on measurements and experiments.

From that video that you posted, it looks likes like the signal timing is marginal, and as things warm up, it changes the timing enough that it affects the timing of when the picture data is put on the data bus from the ROM.

If you have another make of 74LS02 chip, or a faster version (74F02) or a CMOS version (74HCT02) it would be interesting to see if it makes any difference. Alternatively, if you have a different ROM chip, you could try swapping it.

Mark

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