ZX81+35 is my clone of the Sinclair ZX81, started in 2014, and finished in 2016, it uses modern (SMT) components on a small (10x10cm) PCB, and only uses commercially available parts, so no ULA, and works with the original ZX81 ROM code, and has the maximum possible of 32K of RAM buit in. it supports the back-porch patch which makes it better compatible with modern TV's for clear bright and sharp video.

Its called ZX81plus35 here because github doesn't allow for the plus sign in project names.

Previously (in 2014) I named it ZX14 but after two years working on it, I changed my mind and renamed it to first to ZX81+34 and later to ZX81+35 to clarify that my copy was first designed 34/35 years after the original.

There are three versions of my clone, the first prototype I designed was called ZX81+34 (Rev 1.0) and was build and tested. This PCB needed many patches, for example the 5V and GND signals on the micro-USB port were inverted, and the footprint for the crystal oscillator I chose was wrong.

The second PCB I made and tested was called ZX81+35 (rev 2.2) and was tested in 2016, and only needed a minor patch to work, and some patches to improve cassette loading. This version fully worked as a ZX81 clone, and was compatible enough to run everything I threw at it, including high resolution graphics.

The third version (revisions 2.3 and later) were never tested, but all improvements found were implemented in the layout, and patches tested on the Rev 2.2 PCB. Rev 2.3 contains the patches I found when testing rev 2.2, the latest version was rev 4.0 which has an 180 degrees rotated expansion header, for full compatibility with the real ZX81, and my PSG board.

It was not released when rev 4.0 was ready, because I could not get the turbo (overclocking) to work with a stable picture, so I stalled its release, and eventually I gave up on getting turbo mode to work, as I started to work on a color home computer of my own design called RhoCoCo, information on that one can be found here: https://revspace.nl/Designing_the_RhoCoCo_Retro_Home_COlor_COmputer_hardware

So revision 4.0 of ZX81+35 has an expansion port that is corrected and should work with ZX81 peripherals, (using an adapter consisting of a strip of PCB fingers soldered to a female pinheader connector) and directly on my own peripherals (I designed a version of the ZON PSG board for it) https://revspace.nl/Zon_X-81_Programmable_Sound_Generator_expansion_for_ZX81PLUS35.

The pin-header edge connector should eliminate the infamous unreliability (wobble) of the expansion connector. Its location on the side of the board (instead of the back) is chosen so that like Sinclairs Quantum leap computer expansion packs can reside within the enclosure of the computer.

The ZX81+35 also has an onboard extra 8-bit output port, that can be used to drive an LCD display, and adds 4-bit sound (although these latter options were never tested, but they should individually work with a suitable software driver). The 8-bit output can also be used for general purpose outputs, and there is also one general purpose input.

It also has both acceleration features, which enabling can be combined using a dual pole switch located on the back of the board, or can be individually enabled using two jumpers. When loading original cassettes turbo mode should be disabled.

The keyboard supports a reset input, (pressing two buttons simultaniously) as well as turbo and power on LED's, but should otherwise be fully compatible with a real ZX81.

A complete step by step description of the design process with all improvements I did for Rev 4.0 up-to september 2016 can be read here: https://revspace.nl/ZX81PLUS34_ZX81_clone

release

Recently (june 2019) I became aware there is interest in my project, so I'm willing to release the latest schematics, BOM, gerbers and drill files here, so anyone can pick this up and create his own version. For now I uploaded the files I have, some are on an old laptop, so will take some more work.

I updated the older BOM so its compatible with revision 4.0

This design was developed with Ultiboard, (now National instruments NI Ultiboard) the schematic is released as a .UTSCH file, and the layout as .ewprj, hopefully they can be opened with a recent version of NI Ultiboard. I used a version of Ultiboard before they were aquired by NI, but I did not try newer versions. Its one reason why I switched to KiCad.

I opened the earlier uploaded ZX81+35 REV 4.ewprj file with ultiboard, which was an older backup, so I uploaded a newer version, although this one is missing copper files, but this should not influence how well the PCB works.

The "ZX81plus35rev 4.0 no copper fill.ewprj" File and the "ZX81plus35rev 4.0 production files no copper fill.zip" I uploaded on July 31, 2016 actually contain revision 2.3 files. Rev 2.3 does the patches, and will work fine, but they are not real rev 4.0 versions, that is their expansion ports are mirrored and are not compatible with a real ZX81, for that you will need a real revision 4.0 board. Unfortunately github does not allow me to rename these files to make it clear they are rev 2.3. I will replace these with real rev 4.0 versions later.

I also designed a keyboard overlay for a keyboard with 40 6x6mm buttons mounted in a 1/10" raster. I designed a (single sided) keyboard PCB for it, but never tested it, as my protoype keyboard was simply build up on veroboard, Iḿ reasonably sure though that the keyboard PCB is compatible with rev 4.0, if I can find the old layout and check it for compatibility with Rev 4.0

Licence

Unless otherwise specified, everything in this repository is covered by the following licence: