ZX81CCP video output for ZX81

[stefano]

Forgive the very ignorant question: what is the simplest way to get a visible picture on a new-ish TV set with an old ULA ?
I'm afraid I have the earlier one because the picture is almost totally black (which did not happen with all the B&W TV I tried in the past). Mine is a Timex 1000, thus probably tuned at 60hz. I wouldn't want to totally open it because the keyboard membrane is almost broken. Is there a way to pick a composite signal from the rear and fix it enough to be able to just 'see' something ?
My main goal so far is only to do experiments by tweaking the load/save speed, so clear picture is not an issue.

In any case I remember it was very clean on the old Televisions, even the impedance adjustments were not so important.. this compatibility issue sounds so odd to me .

[PokeMon]

As you see in the first posting, you only have to remove the back cover. So the keyboard membrane will not be touched.



Well the board I made is not available now but I am developing a new low cost board with only necessary components for about 5 GBP all in all. No picture inversion, no sync generator. But could take 4 or 5 weeks to have it "ready".

If you are in hurry, you can solder your own board regarding the schematics. The simple transistor solution would work as well but wouldn't improve your dark picture which is probably caused through the missing back porch.

[Paul]

Hi Stefano!
If it is urgent I still have an unused board of version one (with back porch without sync generator) which I could send you if it keeps you updating the z88dk.
If you even consider some mode for zx80 with flicker free display without nmi generator I send it to you for free
(Which would also work on zx81 in fast mode).
If you are interested just send me a pm.
Best regards
ZX80 Paul.

[stefano]

Wow, thankyou for the warm welcome !
Well, I'd love to get the ZX80 in the list of the supported machines, but the flickerless video output with no extra hardware circuits is.. well, something I don't think I'm able to get. Probably it is possible to port z88dk to it, but believe me when I say I never thought that the zx81 port would have been so difficult, so.. no promises.

[Paul]

Well any ZX80 mode would be a great achievment.
I always wanted but never dared to contact you
And the board ist still available if you are interested.

[stefano]

Ok then,
I had a glance at the original zx80 ROM disassembly and its main peculiarity is not having the NMI handler at all.
This is both bad and a good news, because it is not possible to apply most of the tricks I used for the ZX81 but on the other side we do not have to worry to preserve the ix register.
So we have two possibilities:
- to stick to the classic zx80 ROM and try to keep the code as small as possible (maybe we could see some small program stay in the 3/4 KB RAM limit)
- to be more 'generic' and try to support also the retrofit ROM (as the original ADV_A game did), but since this is supposed to exist already in the zx81 '-startup=0' option, we don't need it, right ?

Which are the more common (if we can say so) memory expansion configurations with the ZX80 ?

P.S. well, ok.. I might me interested in the board, I'll send a PM.

[Paul]

I am afraid most common will be 1K of RAM with the ZX80.
There are some 3K extensions (giving a total of 4K) but I guess most ZX80 Users will be able to borrow a 16K Extension from a ZX81.
I guess there will not be many Users for ZX80 anyway, so the easiest approach will be to create 16K Programs.
I am not shure about preserving the IX register because the Video is also created by executing the Video memory and reading the ROM Characterset the same way as a ZX81 will do, just on a different base adress.
(it might even work with the same adress because you will find the first ROM shadow there )

We might have a talk about this, do you have skype?
Please PM me your adress so I can send the board to you. I guess its reall the easiest way to get a clear picture with a ZX81.
By the way, do you know the small hardware hack that disables the NMI generator on startup so you can use a unmodified ZX80 ROM in any ZX81? Just a few resistors needed. In case you would like to test some stuff on the ZX80 on real hardware and do not own one.

I just realized your question about the 8K ROM on the ZX80, no this is not needed at all because its already supported by ZX81 Fast mode.

[stefano]

IX register is not a problem on the vanilla zx81, since we're not worrying for the 8K ROM retrofit, then we're set.
I noticed that the SAVE routine on the ZX80 overwrites all the system variables, including the pointers to the various SP, so passing from a bigger to a smaller memory model could be a bit difficult.. let's first see how big the programs will be then we'll decide if being compatible with the 16K only or consider a 4K option (was it 3K added to the existing 1K, right ?).

[Paul]

let's first see how big the programs will be then we'll decide if being compatible with the 16K only or consider a 4K option (was it 3K added to the existing 1K, right ?).

I do not own one, but I had a look into its manual and this clearly states the memory range for each pair of 1Kx4 chips and yes, it's additional to the built in 1K.
I am really looking forward to this.
I had a quick look into my basement and found two of those boards. I am going to send one to you on Monday.
Best regards
Paul

[stefano]

So kind, there's little time to pick-up the solder in those days around Christmas but to me it is the best gift I could get !
There's a bit of work to do around the ZX81 port, hope I'll be able to tweak the corrent tools (at the moment I'm adapting appmake) with no hassle.