Reversible 16K RAM

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
Posts: 1344
Joined: Sat Nov 26, 2016 2:42 am

Reversible 16K RAM

Post by Lardo Boffin »

Hi all

Has anyone done this mod on the issue one zeddy?

http://blog.tynemouthsoftware.co.uk/201 ... n.html?m=1

It looks slightly less invasive than the full blown cut the tracks option and was interested in giving it a go.

Are the address lines in the same place on an issue one?

Many 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 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Toastrack
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1024MAK
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Re: Reversible 16K RAM

Post by 1024MAK »

On the RAM chip, yes. I can’t remember if the layout of diodes is exactly the same, but it’s not hard to work out.

Bending pins of chips out was popular in the past. Just be careful with regards to static, ensure you use an iron that has an earthed bit (unless it’s a gas or battery iron).

This method does have one disadvantage. It’s not easy to swap chips around when fault finding.

Mark
Lardo Boffin
Posts: 1344
Joined: Sat Nov 26, 2016 2:42 am

Re: Reversible 16K RAM

Post by Lardo Boffin »

Thanks Mark. Good points.
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 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Toastrack
McKlaud
Posts: 290
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: Reversible 16K RAM

Post by McKlaud »

Yeap,

To be honest you need 3 wires only. A10 line is connected via L2 (link) to the new memory chip.

Image

And don't forget about 10k resistor between pins 27 & 28.

Image
Claudius
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siggi
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Re: Reversible 16K RAM

Post by siggi »

McKlaud wrote: Thu Dec 06, 2018 11:33 pm And don't forget about 10k resistor between pins 27 & 28.
.. which is not necessary for this use case ;)

Maybe it has been necessary for ram extensions, which had a battery (to write-protect the ram during power-off state) ...

Siggi
My ZX81 web-server: online since 2007
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diskman
Posts: 20
Joined: Tue Apr 02, 2019 9:23 pm

Re: Reversible 16K RAM

Post by diskman »

Now to find a source of HM62256ALP chips that wont take a month to arrive.. :)
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Reversible 16K RAM

Post by 1024MAK »

The currently available modern DIL 32K byte SRAM chips should work ;)
E.g. this type LY62256PL-55LLI available from Farnell.

Not that I have actually tried this make/type yet.

Mark
diskman
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Joined: Tue Apr 02, 2019 9:23 pm

Re: Reversible 16K RAM

Post by diskman »

I ordered some HM62256ALP-10s this AM with 28 pin dual-swipe sockets. If there is interest, I will sell them in the states as I ordered a few. Now I get to wait.. And wait... And wait...
diskman
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Joined: Tue Apr 02, 2019 9:23 pm

Re: Reversible 16K RAM

Post by diskman »

Wellp.. My "RAM" arrived from China today. WRONG chips. I am not the proud owner of 10 "ATMEGA328P" chips.. wheeeeeee No 16K for my T/S 1000s.. No sir.

lame
ClarkWGriswold
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Joined: Sat Aug 03, 2019 2:47 am

Re: Reversible 16K RAM

Post by ClarkWGriswold »

Sorry if this is a Captain Obvious question, but I want to make sure I'm absolutely correct before starting - Link L2 also needs to be joined as well?
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