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
Sparkly pixels
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Sparkly pixels
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: Sparkly pixels
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.
I guess it’s some sort of timing issue with the ZXpand, as there’s no issue when a regular Sinclair 16K is used.
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Re: Sparkly pixels
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...
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...
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
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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.
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Re: Sparkly pixels
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.
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)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: Sparkly pixels
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?!
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?!
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- Joined: Sat Nov 26, 2016 2:42 am
Re: Sparkly pixels
Clearly there are multiple sparkling factors to be considered!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: Sparkly pixels
Nice one Lardo.
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.
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.
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- Joined: Sat Nov 26, 2016 2:42 am
Re: Sparkly pixels
Thanks Moggy!Moggy wrote: ↑Thu Jan 23, 2020 2:42 pm Nice one Lardo.
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.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA