Sparkly pixels

Discussions about Sinclair ZX80 and ZX81 Hardware
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Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Sparkly pixels

Post by Lardo Boffin » Wed Jan 22, 2020 11:46 pm

So I have now had a number of zeddies with ‘sparkly pixels’, by this I mean that the screen shows occasional flickering and small groups of pixels randomly appearing where they should not.

In the past changing the ROM has sorted this. I now have an interesting one:

1) With the original ROM and no ‘spand the picture is rock solid. With the ‘spand it goes sparkly.

2) With a different ROM it is sparkly without the ‘spand but rock solid with it plugged in!

The zeddy in question has a vLA81 ULA in it and 32K of RAM.

Is there a scientific explanation for this or is my house built on an old native American grave yard (unlikely given that I am in the UK)?

I assume that because the character information for the screen is being read from the ROM if there are any issues that would affect the output.

One thing I have noted is that I currently have the second ROM plugged in so the screen is not stable without the ‘spand. If I load one of Dr Beeps amazing 1K hi res wonders onto the zeddy the screen is fine! Is this because the ROM is mostly not being looked at?

I guess the next question is where can I get a reasonably priced ’new’ ROMs from on a regular basis? By this I mean a modern one or at the least a recently burnt EPROM.

Any information appreciated!

Thanks

Lardo
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

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mrtinb
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Re: Sparkly pixels

Post by mrtinb » Thu Jan 23, 2020 6:52 am

It sounds like the problem I had when connecting the ZXpand to a Lambda 8300. Then random chars would appear someplace on the screen. They weren’t wrong in RAM but only on screen.

I guess it’s some sort of timing issue with the ZXpand, as there’s no issue when a regular Sinclair 16K is used.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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1024MAK
Posts: 2691
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Sparkly pixels

Post by 1024MAK » Thu Jan 23, 2020 7:46 am

Timing I think is the issue.

Some of the ROM chips that Sinclair used were just about fast enough. But as soon as the expansion bus is loaded, the stray capacitance changes, hence the change on screen if the ROM chip is not turning off or on fast enough.

To be continued...

Lardo Boffin
Posts: 1135
Joined: Sat Nov 26, 2016 2:42 am

Re: Sparkly pixels

Post by Lardo Boffin » Thu Jan 23, 2020 10:06 am

Very interesting thanks Mark! Its a bit weird getting used to the idea that a supposedly digital process is so prone to interference like this.

Its a little strange that the ‘spand actually cures the issue on the zeddy with the current ROM installed i.e.

Zeddy + spand = no sparkles
Zeddy without spand = sparkles

Both ROMs I have tried are from different manufacturers.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

Moggy
Posts: 1909
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy » Thu Jan 23, 2020 2:22 pm

Just to add my 5 bobs worth.

I too had this happen with an original spand inside a Memotech case. I attached the stick on Velcro strips that came with these devices which meant the spand is never fully pushed home on the expansion slot.

In this format....sparkle sparkle!

With strips removed and spand pushed fully home no sparkle!

So who knows?!

Lardo Boffin
Posts: 1135
Joined: Sat Nov 26, 2016 2:42 am

Re: Sparkly pixels

Post by Lardo Boffin » Thu Jan 23, 2020 2:37 pm

Clearly there are multiple sparkling factors to be considered!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

Moggy
Posts: 1909
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy » Thu Jan 23, 2020 2:42 pm

Nice one Lardo. :lol:

Also I think Rich might do an EEPROM replacement as he supplies the 8k replacement for the ZX80 xpand conversion kit and I'm sure it could be programmed with the ZX81 code.

Barring that if you ever get hold of any of the old 21v 2764's I will happily bang an image of choice onto it for you. All my zeddies now have EPROMs fitted with the double speed BASIC ROM image and they work quite happily with the spand.

Lardo Boffin
Posts: 1135
Joined: Sat Nov 26, 2016 2:42 am

Re: Sparkly pixels

Post by Lardo Boffin » Sat Jan 25, 2020 10:37 am

Moggy wrote:
Thu Jan 23, 2020 2:42 pm
Nice one Lardo. :lol:

Also I think Rich might do an EEPROM replacement as he supplies the 8k replacement for the ZX80 xpand conversion kit and I'm sure it could be programmed with the ZX81 code.

Barring that if you ever get hold of any of the old 21v 2764's I will happily bang an image of choice onto it for you. All my zeddies now have EPROMs fitted with the double speed BASIC ROM image and they work quite happily with the spand.
Thanks Moggy!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

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