White screen of death
Re: White screen of death
/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
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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Re: White screen of death
I was thinking that this circuit would be operationally equivalent:-
MarkZX81 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.
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.
Re: White screen of death
No need for the refresh signal, the zx81 ula neither has it or requires it...
Andy
Andy
what's that Smell.... smells like fresh flux and solder fumes...
Re: White screen of death
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
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
Re: White screen of death
Well that is very odd indeed.... most puzzling
what's that Smell.... smells like fresh flux and solder fumes...
Re: White screen of death
Yes, very strange indeed.
Here is a pic of the cursor without the GND line attached to the 74LS32.
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
Here is a pic of the cursor without the GND line attached to the 74LS32.
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
Re: White screen of death
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.
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
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.
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
Re: White screen of death
Me again,
I tried 2 things:
1) plugged in the ZXpand. It looked perfect...
2) plugged in the keyboard with no ZXpand...
I tried 2 things:
1) plugged in the ZXpand. It looked perfect...
2) plugged in the keyboard with no ZXpand...
Re: White screen of death
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
Have a look at this...It's even more dynamic
https://www.youtube.com/watch?v=zg1Gymf ... e=youtu.be
Phil
- 1024MAK
- Posts: 5131
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: White screen of death
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
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.
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.