ZX 81 questions

Discussions about Sinclair ZX80 and ZX81 Hardware
rune
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ZX 81 questions

Post by rune »

Hi,

I'm an old time ZX user and have got some ZX81s recently. I've a couple of Issue 1s and 3s. I've been asking questions over on WOS and the guys have been great helping out. But I think ZX81 stuff gets lost amongst the Spectrum related discussion. So I looked for a dedicated ZX81 forum and here I am...

I'll try to keep my questions to one subject at a time. Here is the first.

The TV I'm using can tune into the RF signal but it is rubbish. I got a couple of ZX8 Modules from Pokemon and these are fantastic with the Issue 3/210 ULAs

However, when I try on the Issue 1s (one ULA is marked 8113 and the other is a 184) I cant get the ZX8 module to give a decent cursor. I get a black block 4 or 5 characters wide, its slightly skewed and the screen back ground is poor. I'll try to post a pic or link below...

Image

What are your opinions? Is it the motherboard or ULA versions.

I bought a new 210ULA and tried it in one of the Issue 1s, but I cant even get to the cursor on either RF or ZX8. Can ULAs be swapped between different ZX81 issues?

All, for now.

Dave
sirmorris
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Re: ZX 81 questions

Post by sirmorris »

It looks like the ROM is not being selected during character data reads. Not something I've seen before.
rune
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Re: ZX 81 questions

Post by rune »

Its weird, any problems Im having seem to happen on all similar machines. This on both Issue 1s. The save problem on both Issue 3s.

That black line does have the "K" for the cursor in it, its just stretched... and black.
I take it that pre 210ULAs work OK with the ZX8 module?
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1024MAK
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Re: ZX 81 questions

Post by 1024MAK »

Hello Dave

Welcome to ZX World, home to ZX80's and ZX81's and their clones :-)

The different ULA's produce a slightly different output (voltage levels and timing). Have you tried adjusting ("tuning") the ZX8 module?

Here are links to the WoSF posts:
Question for ZX81 Gurus
ZX81 ULA question

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

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Moggy
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Re: ZX 81 questions

Post by Moggy »

Having read the manual for the ZX8 module Pokemon only mentions the two later ULAs in the fitting instructions so I thought that might have some bearing on it as far as the early ULA is concerned.
But the fact that the character "stretching" is apparent on two different ULAs , the one date marked 8113 is the early 2C158E the other 2C184E, would point to something else other than the ULA.

Like Mark suggests some twiddling of the pots may be in order.

As for the ROM I'm i sure saw on the forum somewhere that the ZXpand doesn't play ball with the earliest incarnation so the the video module may react the same way, a quick way to test is to type PRINT SQRT .25 as a direct command anything other than .50 for the answer means early ROM.
Apart from the CPU the only commonality I can see is the ROM.

These are just suppositions on my part I would follow all and any advice from Mark and Sir Morris as I have found them seldom wrong in most if not all cases, an interesting puzzle never the less.
rune
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Re: ZX 81 questions

Post by rune »

Thanks Mark/Moggy

The ROMs are the same as those in the Issue 3s so I guess these have been upgraded at some point.

Yes, I did retune the ZX8 using the pots, that black bar is the best I can get to the cursor. Its possible to see part of the letter K in it but the photo doesn't show this. (I did the same on the Issue 3s and got a perfect cursor in the two I fitted).

I've got a couple of modded Issue 3s that I'll use for the ZXPand and day to day use. I will maybe try the other composite module when i have spare cash.
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PokeMon
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Re: ZX 81 questions

Post by PokeMon »

Hmmm - not sure what the picture says.
Could be a problem when tuning the trimmers on the ZX8CCB (probably swapped name/function of the trimmers).
What happens if you press on "P" after startup which should result in more characters thank just the "K" ?
rune
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Re: ZX 81 questions

Post by rune »

Hi Karl

That black smudge if the K cursor. I couldn't get a decent photo but you can see the "<" part of the K. Pressing keys gives all the correct responses. It's just BLACK and about 5 times wider.

I've already tuned the ZX8 modules on two Issue 3 ZX81s. But I get that on both my Issue 1s. One is an early 158 and the other the 184 ULA. I think you had that ZX8 back for testing and it was fine. Both Issue 1s give a picture on RF, although its poor.
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PokeMon
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Re: ZX 81 questions

Post by PokeMon »

What I mean is, when you press "P" the ZX81 will printout the keyword PRINT plus Cursor L instead of K.
So what does it look like as the keyword is not inverse and give additional 5 chars plus space in front.
So there must be a difference in the picture.

Does the cursor move 6 chars to the right as expected, is the cursor still 5 black chars long, are the first 6 chars printed, are they maybe inverse or what ?
That information would help a little bit.
rune
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Re: ZX 81 questions

Post by rune »

Does the cursor move 6 chars to the right as expected, is the cursor still 5 black chars long, are the first 6 chars printed, are they maybe inverse or what ?
That information would help a little bit.
I'll double check and grab some pics tonight.
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