Reversible 16K RAM

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

Reversible 16K RAM

Post by Lardo Boffin » Thu Dec 06, 2018 8:43 am

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
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
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

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1024MAK
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Re: Reversible 16K RAM

Post by 1024MAK » Thu Dec 06, 2018 11:34 am

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: 862
Joined: Sat Nov 26, 2016 2:42 am

Re: Reversible 16K RAM

Post by Lardo Boffin » Thu Dec 06, 2018 3:42 pm

Thanks Mark. Good points.
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
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

McKlaud
Posts: 196
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: Reversible 16K RAM

Post by McKlaud » Thu Dec 06, 2018 11:33 pm

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 » Fri Dec 07, 2018 10:17 am

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
http://zx81-siggi.endoftheinternet.org/index.html

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