ZX81CCP

[PokeMon]

Now have the new board available at sellmyretro.com ZX81CCB (basic version).
For GBP 4.90 only (plus shipment).
http://www.sellmyretro.com/offer/detail ... -ZX81-2666

[TMAOne]

As a satisfied customer let me offer my recommendation for this neat little board. It:

• - is inexpensive
  - is easy to install with just a tiny bit of soldering
  - does not require removal of the circuit board from the top shell (i.e. no stress on the keyboard ribbon)
  - is small and mounts conveniently under the keyboard with 2-sided tape
  - allows the original RF modulator to remain unaltered
  - optionally provides inverse video, which is kind of cool, (except when playing chess)
  - outputs, of course, a crystal clear picture

Thanks PokeMon!
Ian

[zx81user]

[quote="PokeMon"And plan a new luxury version which keeps the picture content - not just an empty synced screen.
That would be fun and that would be a nice add on for all ZX80 around to have a stable picture during running program. [/quote]

Hi PokeMon,

You seem really into this ZX81 video thing and looks like you got the time to work on it as well . If you want to keep the picture content on the screen while in FAST mode or any other unsynced situation, you would have to store all the picture data into a buffer memory and read from this buffer to re-create the screen, right? Probably with an FPGA and some external memory. If you go this far, you may as well take the grabbed picture and re-sync it to a VGA output. That would come in handy.

I worked on it for a bit but I got no time to work it out completely. I thought of using two IDT7205 or 7206 asynchronous FIFOs. You need at least 8K; probably 16k is better to support odd hires programs like 320x240 pixels. You could send display information to this circuit by encoding the data in one line of a frame, for example the exact screen dimension HxW in pixels; the refresh rate; or even information concerning border/text colour.

How much would such an interface cost?

Michel

[PokeMon]

How much would such an interface cost?

Michel

Thanks for your thoughts.
I can not say how much such an interface would cost exactly, my idea is not more than maybe 20 or max 30 EUR. Something like that. It's my summer project with no stress at all. In the moment I work an the ZX81XRAM (finished today) and on the ZX80CORE (mainboard redesigned from ZX80 schematic, lets say a replika). After I stop developing hardware and concentrate on software several weeks (ZX-IDE) and then go on with my summer project.

I will let you know about progress then.

[PokeMon]

Thanks PokeMon!
Ian

Thanks for this nice feedback.

[zx81user]

How much would such an interface cost?

Michel

Thanks for your thoughts.
I can not say how much such an interface would cost exactly, my idea is not more than maybe 20 or max 30 EUR. Something like that. It's my summer project with no stress at all. In the moment I work an the ZX81XRAM (finished today) and on the ZX80CORE (mainboard redesigned from ZX80 schematic, lets say a replika). After I stop developing hardware and concentrate on software several weeks (ZX-IDE) and then go on with my summer project.

I will let you know about progress then.

If you re-design the ZX81 main board, add a small series resistor between NMI-out from ULA and NMI-in from Z80 (680 ohm or so). The idea is that you can then externally disable the NMI interrupt. Or maybe not really disable as the ULA will still produce them but they won't be seen anymore by the Z80. If you do this, you can hookup the VGA hardware to the NMI interrupt and only demand a certain amount of frames per second. Say you allow 5 frames per second, that means the left over 45 frames will be used to run the user (BASIC or assembly) program resulting in an almost 4 times higher speed! Slow mode becomes almost Fast mode! The amount of frames per second can be user programmed through a frame line as described before.

You can either design a circuit like this as a card you plug in the back of the ZX81, but nicer would be to have it inside the ZX81. If you re-design the main board, you can actually already incorporate a VGA output that works in this way, and at the same time use a 20MHz Z80. Total speed increase will be close to 24 times!! It's all very simple stuff, if I only had the time to work on it!

Michel

[PokeMon]

Maybe you overread this - I am not working on a redesign of a ZX81, it is for the ZX80 which doesn't have an NMI generator.
By the way, I do add on's very moderate. There are many discussions what a ZX80 or ZX81 is and what's elementary.
For the example speed, this is unwanted when you play games as most of them would be unplayable if they are running to fast.
Same is for sound.
It's not always a matter of speed.

[zx81user]

Maybe you overread this - I am not working on a redesign of a ZX81, it is for the ZX80 which doesn't have an NMI generator.
By the way, I do add on's very moderate. There are many discussions what a ZX80 or ZX81 is and what's elementary.
For the example speed, this is unwanted when you play games as most of them would be unplayable if they are running to fast.
Same is for sound.
It's not always a matter of speed.

No, I am not mistaken, I mean the ZX81. If you can externally disable the NMI you can control how much time the ZX81 consumes for building up the screen and you can control how many frames per second the ZX81 will be making. Currently you cannot externally disable the nmi because the ULA NMI OUT pin is directly connected to the Z80. If you add a series resistor, you CAN externally disable the NMI without damaging the ULA output pin.

If you have a ZX81 that runs very fast, of course you can add options to let it run at the same speed as an unmodified ZX81, however, for programs demanding a lot of processor speed (I am thinking about a web browser or some other demanding software) you CAN speed it up very easily. Would be a great feature. Also for real time processing, rather than having 50 frames per second you could have 50 times 1/5th of a frame resulting in 10 whole frames per second BUT only processing 1/5th of the time every 20ms, the rest of the time the processor is free to do high speed data transfers for example.

You can think it over, it is relatively simple to implement and the advantage is huge.

Michel

[PokeMon]

No you overread. I am not planning to develop a ZX81 board. That's your mistake. If you want to have you have to develop yourself.
For me it makes no sense as there are many boards cheap available in working condition to acceptable prices (about 20 EUR / 30 USD).
For ZX80 this is different. This model is very rare and if working you have to pay high price for it (300 EUR / 450 USD and more up).

By the way there is already existing a 20 MHz "Uber Zeddy" from Andy Rea.
viewtopic.php?f=6&t=961

And by the way, this is not a general discussing thread. You may open a new topic under development for going deeper into your project.

[zx81user]

No you overread. I am not planning to develop a ZX81 board. That's your mistake.

Oh yes, I see now!