Sinclair ZX80 / ZX81 Forums

[Chad]

Hi All,

Newly signed up, but a ZX81 user of old. I had one back in the day which is sadly long gone, but recently purchased one from EBay and have the bug again Although I also purchased a 16k RAM pack, I know only too well the terrors of RAM pack wobble, so having a few spare 6264 SRAMS from some old arcade boards, I fitted myself an internal 16k upgrade which is working nicely.

Having now obtained myself a couple of 62256's, I'm quite keen to expand to 64k. I've seen a few different schematics around and was wondering which is the preferred for maximum cabability (HRG etc) and compatability with existing hardware/software? The only "grey area" seems to be the 8-16k region, which is expected to be RAM for some apps but also seems to be the next logical area for additional ROM.

I'd ideally be looking to fit this internally, space permitting - though a homemade keyboard and perhaps new case may be next on the agenda which should alleviate the cramped conditions.

Thanks!

Chad

[sirmorris]

If it were me I'd insist on having RAM at 8k for the hires goodies.

Perhaps have the 8k region switchable, giving the best of both worlds?

C

[siggi]

And if you want to run machine code programs above 32K, you need to install a M1-NOT-circiuit.
And if you also want to run big BASIC programs (> 16K), you need to disable the M1-NOT-circuit.
So install also a switch to enable/disable it.

Siggi

[ksng36]

Hi Chad,

Why don't you head over to Wilf's website (http://www.user.dccnet.com/wrigter/). I think you'll find more than enough circuits, including the 64K RAM, to play around with.

ksng

[Chad]

Hi ksng,

I've seen Wilf's pages and there are indeed a plethora of very interesting projects there! It was after seeing his 64k RAM and comparing it with other RAM expansions that I wondered if there was a "standardised" config at all.

The point raised by Siggi is very valid and something I may not otherwise have considered. May I ask for some clarification at this point - I've seen a few references to "the M1 not circuit" but no actual circuit per se. Is this just the gating of A15 and /M1?

RAM between 8-16k is looking like the preferred option, though I may look at switchable ROM here too.

Thanks for the comments!

[sirmorris]

What about a small production run of circuit boards to make an add-on comprising:

64k RAM
Paged ROM in 8k region
m1-not
GPIO/MMC card interface

This is what I'm working (slowly, slowly) towards, more or less. The think is I get demoralised when I think of how many wires I'm going to be soldering. Not that I'm unable to do it - it's the sheer amount of time that's involved.

http://arduinonut.blogspot.com/

Thinking about it - perhaps home-made boards would be sufficient? I can solder the larger SMD components which means less drilling...

Anyone here able to make PCBs?

C

[Chad]

Now that sounds like a great project! I'm very interested in your MMC stuff, the Speccy interfaces with ResiDos are very slick.

I was thinking of doing my RAM/ROM expansion internally - hooking into the CPU and RAM sockets should provide most of the signals required so was thinking of a plug-in daughterboard which doesn't require much (if any) soldering direct to the motherboard - though I understand some models have all the chips soldered in place and sockets are needed. De-soldering the 1k SRAM was a chore enough!

I was edging away from an external add-on as I have horrible memories of RAM-pack wobble, though obviously this path provides an upgrade path available to everyone, and you needn't worry about re-housing your '81 as there is very limited space within.

Personally, I'm having fun playing around with my current machine for development - next up I think will be a proper keyboard and more roomy case. I'm thinking a BBC keyboard looks the part, they're proper switches, have a nice action and the keys are quite clean having only letters on them - just need to get hold of one now

I've had PCBs for arcade projects made up in the past by Olimex who will accept Eagle files and they've been very professional. I'm not sure how they'd stack value-wise now though given the Pound-Euro rate - it's been 3-4 years since I last used them.

Thanks for the feedback, good sir!