ZX81+35 Clone

[mahjongg]

This is what, according to Eurocircuits, the PCB will look like when I get it:

Top view

PCBPreview ZX81+35 rev 2.2 top.png (545.13 KiB) Viewed 3155 times

Bottom view

PCBPreview ZX81+35 rev 2.2 bottom.png (282.28 KiB) Viewed 3155 times

[mahjongg]

The people at the German ZX81 forum asked if my clone would accept original keyboards, and the answer was NO!

So I made a design change, and now the answer is YES!

Details about the change can be found at revelation space, here: https://revspace.nl/ZX81+34_ZX81_clone#2_February_2016

The connectors are spaced wider apart, so the foil connectors will now fit, and also I re-ordered the address line signals over the 8-pin connector, so they are in the exact same order as the ZX81 (and ZX80 and even the Spectrum).

So now you can use the original foil keyboard, and aftermarket products.
I will also in the future design a keyboard PCB that is fully compatible with the ZX81+35, including power and turbo LED's and reset button(s).

[RWAP]

I am pleased to hear that

[mahjongg]

I am slowly building up the board, now all the passive components have been done, it helps that we now have a reflow oven at RevSpace.

I am considering building another replica of a ZX81 kind of computer, the Galaksija from the former Yugoslavia, but getting reliable information about it is proving to be -very- difficult, so I have translated someones University diploma thesis about the Galaksija from Slovenian into Englisch, (a hundred pages or so) to clear up some points. Until now I only have some half related older schematics, (including a GAL based version without the description of the GAL) a youtube video of a talk, and a quarter of the schematics of a modern (CMOS) implementation. To get a full schematic I need to install a Linux CAD program to open the .SCH file of hopefully the full CMOS schematic. The original (74LS logic based) schematic is no longer compatible with modern Z80 processors.

[mahjongg]

finally acquired the full schematics of the CMOS Galaksija.

Galaksija_schematic.png

(36.7 KiB) Downloaded 294 times

as you can see its a -very- simple system, as simple as a ZX80.

[PokeMon]

Yes - looks interesting.
A bit strange is the memory layout.
So you have 8k ROM from $0000-$1FFF and 6k RAM from $2800-$3FFF.
It is strange that the first 2k RAM from 8k all in all has been unused but as seen from schematics there is no chance to write this region in RAM. This covers with information in wikipedia, stating this system with 2k up to 6k RAM.

It is announced that it could be extended up to 54k RAM which seems to be unpossible with this schematics revision.
After taking a look into the rom listing it corresponds to these details beginning the RAM test at address $2800.

[mahjongg]

ready with soldering component to the board, except part of the joystick logic.
Still need to program the FLASH ROM, then I can start testing.....



as for Galaksija, added some more info about it here: https://revspace.nl/Galaksija
As for conflicting addressing in the system, It seems the designer was aware, as he added some resistors in the data-bus in several occasions, to prevent damaging data-bus drivers during bus conflicts.


I added two other schematics, the original first publication, and a "GAL version".

I'm planning to (re-)design a single sided board for it with just PTH parts, so it will be easy to build.

[sirmorris]

PCB Porn. Looking good!

[1024MAK]

PCB Porn. Looking good!

Ere, careful you don't dribble all over it

But, yes, looking good. I'm looking forward to seeing it complete with all ICs in place and working

I must find time to have a look at the Galaksija details as well...

Anyone got a time machine?

Mark

[mahjongg]

its alive!
A "hello world" picture is here: https://revspace.nl/ZX81%2B34_ZX81_clon ... s_alive.21

I will keep working on it to iron out any problems I find.