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 »

/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
what's that Smell.... smells like fresh flux and solder fumes...
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1024MAK
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Re: White screen of death

Post by 1024MAK »

I was thinking that this circuit would be operationally equivalent:-
/ROMCS circuit generator
/ROMCS circuit generator
IMG_5221.JPG (80.57 KiB) Viewed 4132 times
Mark
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Andy Rea
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Re: White screen of death

Post by Andy Rea »

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

Andy
what's that Smell.... smells like fresh flux and solder fumes...
nama
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Re: White screen of death

Post by nama »

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 »

Well that is very odd indeed.... most puzzling
what's that Smell.... smells like fresh flux and solder fumes...
nama
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Re: White screen of death

Post by nama »

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 4099 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 »

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 4094 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 »

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

IMG_6159.JPG
IMG_6159.JPG (29.1 KiB) Viewed 4088 times
2) plugged in the keyboard with no ZXpand...
IMG_6160.JPG
IMG_6160.JPG (28.43 KiB) Viewed 4088 times
nama
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Re: White screen of death

Post by nama »

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 »

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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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