Replacement ZX80 Cases

[PokeMon]

For me, the big difference between the zx81 Vs zx80 is that the zx81 supports 32-bit floating point math whilst the zx80 only supports integer numbers. Sure, the zx80 is faster than than the zx81 but that is understandable, I'd rather have slightly slower machine with fp than without.

As Mark stated, this is just software. The ZX81 ROM is fully compatible to ZX80 and allows floating point math but only in FAST mode due to missing NMI generator - talked above. So the hardware is identical but smaller as all discrete logic was pushed into one chip (ULA) plus adding the NMI circuit. There is one more difference which isn't that major - the keyboard input is realized as open collector with pull up resistors for the ZX81. This allows easy parallel connection of internal and external keyboard.

[commie]

As Mark stated, this is just software. The ZX81 ROM is fully compatible to ZX80 and allows floating point math but only in FAST mode due to missing NMI generator - talked above. So the hardware is identical but smaller as all discrete logic was pushed into one chip (ULA) plus adding the NMI circuit. There is one more difference which isn't that major - the keyboard input is realized as open collector with pull up resistors for the ZX81. This allows easy parallel connection of internal and external keyboard.

Hi PokeMon,

Are you suggesting new zx users should first build the zx80 and then modify it's hardware/software such that the end result is a functional zx81? The hardware circuitry for this transform is given on Mr G. Searle's web site, which isn't trivial, I can't imagine any zx80 collector wanting to modify their zx80 for zx81 functionality either. There's no need for the latter approach given zx81's are available on ebay for around £20/30, also, in the zx81 favour, are spare keyboards and new ULA chips, all at reasonable cost.

Granted, when the ULA chip supply dry's up there is going to be a problem which is my whole point to get the ULA into a workable cpld.This has already been done using a XILINIX XC9572 84plcc, however the problem here is the latter 5V part is no longer available, the jedec file doesn't seem to be available and 84plcc programming adapters are extremely expensive, therefore 44plcc is my limit. Note, zx81 ULA has i/o 38 pins available whilst a atf1502 44plcc has i/o 36 pins available, it's quite challenging to get 38 i/o pins into 36.

Cheers
Commie.

[PokeMon]

Hi PokeMon,

Are you suggesting new zx users should first build the zx80 and then modify it's hardware/software such that the end result is a functional zx81?

No - I didn't suggest that. First you have to distinct between collectors and other users. Collectors mostly aren't interested in other stuff than original hardware and add ons. They wouldn't modify it as same as they are not target group for a CPLD project.

Second it is not necessary to do complicate integration of a NMI generator circuit in original ZX80 hardware and I didn't propose it. Grants project is not the only one reproduction of a ZX80 board, there are more solutions available (for example the board of Lee Hart). All I was talking about is, that there is no big hardware difference between both calculators and the ZX81 was made to be compatible to the previous ZX80. This covers the usage of floating point ROM as well as several hardware add-ons like memory extensions and more.

You maybe missed the ZX80CORE thread as you are new at the board which followed another approach, combining easyily both of ZX80 and ZX81 in one system. There is a ROM switch on the board which allows to simply change between ZX80 and ZX81 and back while turning a switch (hex code switch with 16 positions for 16 different ROM images). The optional NMI board works together with either ZX80 and ZX81 ROM while it is turned off after power-on and activated through the ZX81 ROM (or other ROM images supporting SLOW mode of ZX81).

And of course there are more previous solutions to pack the ULA code into a CPLD.
One is from RetroTechie "Houston, we have an image!"
viewtopic.php?f=6&t=653&hilit=houston

The other one I know is from Andy available as commercial product as well ZX-ULA2.
The development thread was "ULA revisited."
viewtopic.php?f=6&t=445&p=3620&hilit=ZX+ULA2#p3620

I am sure there are more attempts in realizing the logic into CPLDs.
Anyway a new well documented project would be appreciated here as well.

[Paul]

Grants website is a fantastic source for infos about zx80.
It's not intended as a howto build a zx80 for beginners.
Pokemon zx80core is a good start for beginners. PCB available, just as the parts.
With a great manual. And he also offers the nmi generator as addon.
There is no shortage to ULA.
Sold by hundreds new.
Do you know Andy Rea's ULA? It's really great. Source is available and programming via normal incircuit cables.

Kind regards
Paul

[Paul]

Another approach was holmatic and his hybrid computer
It unites a zx80, zx81 and Jupiter Ace in a single CPLD.
Less documented and not commercial.
Just a single computer because it's possible.
And with his ZXNU he made the opposite approach.
A ZX81 in ZX81 board form factor based purely on standard TTL.
Kind regards
Paul

[commie]

Hi PokeMon,

Are you suggesting new zx users should first build the zx80 and then modify it's hardware/software such that the end result is a functional zx81?

No - I didn't suggest that. First you have to distinct between collectors and other users. Collectors mostly aren't interested in other stuff than original hardware and add ons. They wouldn't modify it as same as they are not target group for a CPLD project.

Hi PokeMon,

Phew.., I thought there was some kind of conspiracy or something going on, Okay thanks for the threads, two of them I know very well but the third 'ULA revisited' made very interested reading, so thanks for that.

Cheers
Commie

[Paul]

No conspiracy at all
You are welcome to make anything you like.
And please report about your progress.
Most of us are planning or working something more or less usefull stuff.
My projects are useless. For example I made a three chip zeddy instead of four.
Just because it's possible.
And I want to make a zx81 based on plcc chips only
Kind regards
Paul

[RWAP]

This thread reminds me that not everyone coming back to the ZX80 / ZX81 or even looking at it afresh will have read through the 100s of threads on this board (even if it is only a few years old) .

People wanting to re-invent the wheel are always welcome to come onboard and discuss their projects - because sometimes, not only do they learn by re-inventing the wheel, but sometimes they come across a different approach which could be an improvement on an earlier project (or help overcome some obstacles).

[commie]

No conspiracy at all
You are welcome to make anything you like.

Hi Paul,

Okay Paul, I will let the board know how I get on. I'm currently about 65% complete, the thing is, when I write software for the avr, I do a little and get that small amount of code working, then code a little bit more and get it working, taking little steps forward and so on, in same way nature does. Some call it evolution, our sun works the same way. Unfortunately, I'm finding I can't use this technique with this project, which means I could run into trouble later on when I've completed coding.

Cheers for now
Commie

[PokeMon]

For example I made a three chip zeddy instead of four.
Just because it's possible.

Really ?
I didn't see any pictures yet.