3 up - 3 to go

[rune]

I've just resurrected another ZX81, an Issue 1 with Recom regulator, ULA heatsink, Pokemons ZX8CCB composite module and a new ZXPand keyboard membrane. Picture is rock steady.

I've got another three basket cases to look at and am thinking of including Pokemons switched keyboard next. Apart from the Recom, ZX8CCB and heatsink are there any other mods I can do to improve the basic computer?

Does anyone know the pitch of the casing screws?

[Moggy]

Colour?

viewtopic.php?f=7&t=1495


Plus the AY unit for the Xpand for speech and music.

http://www.sellmyretro.com/offer/detail ... odule-2331

[rune]

Cheers Moggy

very nice addons but maybe a bit too expensive for me at the moment, especially as it will be for ZX81 No.4.

I am thinking more along the lines off what can be built or modded my Mr Clumsy (me) on a budget.

[nollkolltroll]

If you have a 16k RAM-pack, make it HRG-compatible. Simple enough, and very cheap.
Lacking a RAM-pack, build it internally. Trick out the ZX81 with 32k in the 16-48k range (HRG compatible) and 8k in the 8-16k area (make it UDG compatible). This will allow you to run most programs.

[rune]

If you have a 16k RAM-pack, make it HRG-compatible. Simple enough, and very cheap.
Lacking a RAM-pack, build it internally. Trick out the ZX81 with 32k in the 16-48k range (HRG compatible) and 8k in the 8-16k area (make it UDG compatible). This will allow you to run most programs.

Thats the stuff, projects that dont cost the earth...

I've got lots of RAM packs and have converted one of the ZX81s to 16K internal - its got the 32K chip though. So the hi-res graphics conversions sound good. Where would I get details of these?

Would a RAM pack or the ZXPand override the internal memory after the 16K upgrade?

[1024MAK]

Would a RAM pack or the ZXPand override the internal memory after the 16K upgrade?

If you still use the ULA's /RAMCS (on the RAM chip side of the resistor) in the decoding for the internal 32k RAM chip, yes.

Mark

[rune]

If you still use the ULA's /RAMCS (on the RAM chip side of the resistor) in the decoding for the internal 32k RAM chip, yes.

Mark

Thanks Mark,

I'll try plugging it in and see what happens. The ULA/RAMCS stuff goes way over my head I can just about handle one syllable words and pictures.

[sirmorris]

Wilf's 'Why Wait' mod is also good for a few extra cycles of processing power!

[1024MAK]

If you still use the ULA's /RAMCS (on the RAM chip side of the resistor) in the decoding for the internal 32k RAM chip, yes.

I'll try plugging it in and see what happens. The ULA/RAMCS stuff goes way over my head I can just about handle one syllable words and pictures.

Here's a picture then

schema_ZX81_Fr_with_RAMCS_in_green.gif

(166.02 KiB) Downloaded 446 times

The /RAMCS circuit is coloured green. The boxes with the cross inside represents the edge connector pins. So if an external device (like a RAM pack) puts a logic high (logic one, +3V to +5V) level on this edge connector pin (2A), it overrides the ULA output pin and tells the on board RAM chip(s) that they are NOT selected (CS means Chip Select, so /CS means the chip IS selected with a logic low). When a RAM chip sees it's /CS input pin with a logic high level, it disables itself. So if the /CS line never goes low because an external device (like a RAM pack) connects /RAMCS to the +5V supply, the internal RAM will stay in standby mode and will ignore everything

The resistor (R2, 680 ohms) prevents the ULA output pin from being damaged by a direct +5V connection.

Note I modified the French version by XavSnap because this has some errors corrected and includes the edge connector connections as part of the schematic. The only significant difference to the UK version is in the TV modulator and associated wiring/components.

Mark

[1024MAK]

Wilf's 'Why Wait' mod is also good for a few extra cycles of processing power!

Yep, I've done this mod. It's not too difficult.

Mark