I've borked it

Discussions about Sinclair ZX80 and ZX81 Hardware
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siggi
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Re: I've borked it

Post by siggi »

Hopefully you have plugged in the CPU and ULA the right direction, NOT as shown in the ZX81 manual chapter 25:

https://www.manualslib.com/manual/12017 ... =71#manual

;)
My ZX81 web-server: online since 2007, running since dec. 2020 using ZeddyNet hardware
http://zx81.ddns.net/ZxTeaM
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msknight
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Re: I've borked it

Post by msknight »

siggi wrote: Sat Feb 24, 2024 4:00 pm Hopefully you have plugged in the CPU and ULA the right direction, NOT as shown in the ZX81 manual chapter 25:
https://www.manualslib.com/manual/12017 ... =71#manual
;)
Good grief. Who on earth let that past quality control!

No, I've just got a problem in that I've been pulling chips in and out of sockets for testing combinations that the pins are getting about as weak as my will to continue.
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Dim_75
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Re: I've borked it

Post by Dim_75 »

This "tech-guy"' may need some of these :D
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msknight
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Re: I've borked it

Post by msknight »

I'm trying to figure this out and getting nowhere.

For the HM65256B, looking at the read cycle, both the Chip Enable Access time and the Column Address Hold time, which both run from Chip Enable running low ... are both 150ns. With an "Output hold time from column address of 5ns... it's as if there's no time at all for the processor to actually read any valid data from RAM.

This is making no sense to me.
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1024MAK
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Re: I've borked it

Post by 1024MAK »

Reading the diagrams on a datasheet is an art :lol:

Keep in mind that the diagram itself can be misleading if taken literally. In the real world, the "width" of the various features on the diagram depends on a number of variables, including the clock/bus access speed of the system. Which in the ZX81 are more complex than normal Z80 systems due to the way that the ULA and the Z80 work to generate the data stream for the video.

Okay, the site that I am going to link to is for the 6502, but keep calm, and do have a look, as it does make the timing diagrams make a little more sense. Here's the link.

Back to the Z80 and 32k byte SRAM chips. Most of these all work the same. After all, they were (mostly) all being used for the same function.

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

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msknight
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Re: I've borked it

Post by msknight »

That's a comprehensive document, but while I can understand what its saying, I'm having trouble identifying the mismatches because some of the chip diagrams are different enough that I can't then make the leap to identify what I need to do, to solve the problem... if that makes sense.
Moggy
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Re: I've borked it

Post by Moggy »

Having exhausted all other repair possibilities a more drastic solution is found!
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msknight
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Re: I've borked it

Post by msknight »

OK - after much scrumpled pieces of paper, I can't work it out.

Help! Please!
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1024MAK
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Re: I've borked it

Post by 1024MAK »

"These aren't the droids you're looking for".

If, and it's a big if, I can find a little time at the weekend, I want to try something which may or may not help.

Far too tired now. Dogs woke me at 04:50 just before my alarm was due to go off, as I had to catch an early train to take me to London. Now on the train on the way back.

I'm gonna be going to London next week as well...

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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msknight
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Re: I've borked it

Post by msknight »

It would be wonderful if you could... it's the last piece in the jigsaw for me now.

But I fully understand. Life is difficult enough without commuting to London on top.
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