Build your own ZX-81 computer, from discrete pin-through hole components (schematics, gerbers and BOM on Github)

[mahjongg]

You did say that you are using a revision 1.9 PCB didn't you?

I ask before in a previous version I didn't isolate the area around JP1 from the copper fill, meaning that there were unwanted copper traces (to GND) to all parts of JP1. I'm not sure that version made it to gitHub, but I cannot be sure...

the correct version (rev 1.9) should look like this around JP1 seen from the solder (bottom) side, so it is mirrored and the traces on this side are in blue.:

Schermafbeelding 2023-01-12 om 00.23.29.png (125.45 KiB) Viewed 2756 times

the top side should look like this, traces are red, not mirrored:

Schermafbeelding 2023-01-12 om 00.44.57.png (52.74 KiB) Viewed 2756 times

If this isn't how the layout around JP1 looks, then JP1 is NOT functional, and you should NOT use it!

especially don't wire anything to the top PAD 1 (A13).

Instead bend out pin 1of U23, and wire that either low (for 16 K ROM space, or to A13.


after thinking this through, I'm quite sure there is a problem with A13, the best clue is that the keys connected to A13 do not work!
This means that the keys do not receive an A13 signal that goes low (either completely to GND, or perhaps halfway to say 2.5V).
If you have a multimeter then I suggest you remove the Z80 from it's socket, and measure the resistance to both GND and +5V measuring from PIN-3 (A13).

There must be something wrong with the connections of A13.

good luck!

[squelch41]

Made progress -- desoldered pin 1 of U23 and indeed, wiring it to A13 directly now gives me RAM at 8k -16k using the memcheck software I showed before (now get NO, YES instead of NO, NO)

I have a spare PCB and I think it looks like the pictures you posted.

On the unpopulated PCB, If I probe, even with no jumper connected, pin 1 of U23 is connected to the GND pin of the jumper.
Therefore, if I jumper it, doesnt that mean I am connecting A13 both to pin 1 of U23 and GND - therefore the jumper doesnt function?

I wonder if the same is happening at JP12? When I probe that, in the default position all is fine, with the three positions all open, all is fine, but if I jumper to enable 32k of ram, the system wont boot and A14 is grounded.

Some WRX software now does seem to function where none did before, but not all.

Could anyone tell me if they can run ZXTERM*80 - for me?
For me, it now doesnt crash completely as it used to, but I get just weird graphics patterns coming up one it tries to initialize the high res graphics (I choose W, then X then No to all other options)

[mahjongg]

On the unpopulated PCB, If I probe, even with no jumper connected, pin 1 of U23 is connected to the GND pin of the jumper.
Therefore, if I jumper it, doesnt that mean I am connecting A13 both to pin 1 of U23 and GND - therefore the jumper doesnt function?

I wonder if the same is happening at JP12? When I probe that, in the default position all is fine, with the three positions all open, all is fine, but if I jumper to enable 32k of ram, the system wont boot and A14 is grounded.

I hope you do realise that JP1 (and other jumpers with a default setting, like JP12) have a trace on the reachable bottom side of the board connecting two pins of the jumper together!

Therefore yes indeed, without placing a physical jumper (wire) pin 1 of U23 is still electrically connected to GND through this trace. you have to scratch away the trace if you want to change the default! In case of JP1 the trace is on the bottom side (so you can reach it) between pin 2 and 3, it is a bit thinner than a normal trace but it is clearly there (in the bottom = blue picture).

Perhaps I have not been too clear about this fact. If so I apologise .

Note that the second 8K section that now is RAM, that ram (8K to 16K) is a copy of ram elsewhere. It is just that BASIC ignores this RAM, just like it ignores the top 16K of RAM.

So expanding the RAM from 16K to 32K does NOT mean that BASIC will suddenly have 32K to work with! the top 16K will simply be ignored by BASIC!
That is why I, even though the RAM chip has 32K, I chose to use just 16K of it as a default.

[squelch41]

It's obvious now you mention it- excuse my naivety - first time building something like this!
It's been fun learning though

[squelch41]

Note that the second 8K section that now is RAM, that ram (8K to 16K) is a copy of ram elsewhere. It is just that BASIC ignores this RAM, just like it ignores the top 16K of RAM.

So expanding the RAM from 16K to 32K does NOT mean that BASIC will suddenly have 32K to work with! the top 16K will simply be ignored by BASIC!
That is why I, even though the RAM chip has 32K, I chose to use just 16K of it as a default.

That makes sense. Really, I am just trying now to get ZXTERM80 to work so I can use a high-res terminal emulator for the serial interface I built - works nicely with MTERM but that doesnt have a higher res mode or support for lower case characters

[mahjongg]

I'm trying to contact the canonical ZX-81 hobbyist Wilf Rigter.

the only e-mail address I have of him wilf@cafe.net no longer works.

Does anyone know anything what happened to wilf, is he still alive can't find anything about him post 2005.

[squelch41]

His old website (https://web.archive.org/web/20210601201 ... m/wrigter/) has address:
wrigter@dccnet.com

That email also comes up on https://www.facebook.com/wilf.rigter/

[mahjongg]

Thanks, I have just sent an e-mail to wrigter@dccnet.com. awaiting a reply (or a bounce message from the system).

[mahjongg]

I found out about the Canadian NABU computer which is creating a great interest in the US with retro computer fan's, and I decided to buy one, and also two others from the national video game museum in Zoetermeer the Netherlands, including the owner and director of the museum also bought one, and now I''m trying to convert it to a 220V 50Hz supply, and modifying it to get PAL (or component video) out of it, so that is a new hobby and I will spend less time on my ZX81+38 which I now consider finished.

I wrote about this new NABU project, and it can be found here: https://revspace.nl/Canadian_NABU_retro ... _220V_50Hz

[scruss]

I have a NABU sitting in a box here not doing much. One day - after my BBC Micro and tiny Altair-in-a-mint-tin are working successfully - I'll take a look at it. One of the local universities, York*, has a computer museum that has kept the NABU archive going, long before this recent surge in interest.

Have fun with your NABU: it's almost an MSX with no ROM. It is also a product of its time and Canada's lax environmental standards. That yellow plated metal that makes up most of the case? Cadmium. Yeah, your NABU is toxic waste.

---
*: first English name for Toronto. Man, those early settlers had great imaginations ...

[hippy]

One of the local universities, York*, has a computer museum that has kept the NABU archive going, long before this recent surge in interest.

While I do understand "vintage", "retro", "things I had as a kid" and other nostalgia, "because it's fun" and "because I can", I am surprised so many appear to have jumped into an adventure which it seems will ultimately only benefit a few people.

I have often thought "I'd like one of those to play with" but that's always been tempered by knowing it's a lot of effort for little real gain, when I can do so much more with modern off-the-shelf product, at lower cost, with a wider community to help and encourage me, which will benefit far more people.

But each to their own, and whatever rocks their boat is fine by me.

[mahjongg]

The gain (for me) is that I have fun converting a computer to work again in a country that never had one, which has a completely different power system (220V 50Hz) and video system (NTSC). so I'm replacing the PSU with a meanwell switcher, and will the replace the TMS9918A with a TMS9929 that generates component video (and through a converter box RGB (for SCART) and VGA.

I'm a big MSX fan, see the interview I did with Bas kornalijnslijper about my history with MSX see https://www.youtube.com/watch?v=3FzAum_ft1M&t=1444s (in Dutch)

so I will enjoy using it to play some MSX games, and the software that is developed for it now like cloud CP/M.

Simply said, its a continuation of my hobbies, which I will have plenty of time for, as I will retire in a couple of weeks.

I'm a RETRO computer collector too. At one time, a couple of years ago, I had collected around 180 home and game computers, these are now safely in 2 computer musea. and I have only a few left (the PC like computers went to Bonamy computer museum in Zwolle, and the game systems went to Stichting Awesome Space in Utrecht)

In my home town of Zoetermeer (near Den Haag) there happens to be a great computer games museum, for which I'm a volunteer, and one of the three NABU's we bought will be displayed in the museum. AFAIK these three are the only ones in The Netherlands, and in the whole of Europe there are only about 5 or 6.

[hippy]

Simply said, its a continuation of my hobbies

That's probably the differentiator. It's not that I am against doing it, but for me it would serve little purpose beyond the journey itself, leaving me in a 'so what was the point of having done it?' state. If one is going to get some use and continuing enjoyment out of it then it's a different matter.

BTW do enjoy your retirement and I'm wishing you well with getting the NABU working.

[mahjongg]

Simply said, its a continuation of my hobbies

That's probably the differentiator. It's not that I am against doing it, but for me it would serve little purpose beyond the journey itself, leaving me in a 'so what was the point of having done it?' state. If one is going to get some use and continuing enjoyment out of it then it's a different matter.

BTW do enjoy your retirement and I'm wishing you well with getting the NABU working.

well the fact that there were hundreds of people reacting to the announcement that NABU's were sold as new old stock when YouTuber Adrian's digital basement presented his latest video "The 80s computer you've never heard of: The NABU PC" (https://www.youtube.com/watch?v=HLYjZoShjy0) three months ago it created a storm of interest in the RETRO computer collectors community in this homecomputer that nobody knew about.
This made me have a look at the specs of it, because I too am a RETRO collector, and I saw that its specs were amazing for 1982. As there was only a limited supply (there were one thousand of them in new old stock) I immediately wanted to buy one, because they will become rare and sought after soon!

So now I have a new hobby. The National video game museum in zoetermeer also was interested for showing it in the museum, and so it has to work on 220V, and create a top notch video signal, which Frans (the main technician of the museum, who bought his own NABU too) and I will make sure to happen.

It will be fun!

I already have disassembled the PCB of the NABU, and found out it has many patch wires on the underside. And I have removed the 64K of DRAM for the Z80, and replaced the with IC sockets, so Frans can test the DRAM's in his universal IC tester.

de onderkant van het NABU mainboard, met patches en patchwires.

1000011092.jpeg (63.3 KiB) Viewed 1427 times

[mahjongg]

I built a new fan and new power supply into my NABU, and it works fine,


but I discovered that my NABU is defective!
booting only gives a blue screen! No "waterfall"GPU test, nor any text appears, and after some measuring I found that the VRAM chips (TMS4116) of the TMS9918A GPU missed its +12 supply voltage The proof is that the dropper resistor to power the keyboard with 8V from the 12V did have +12V on it, but neither the LM3900, nor the TMS4116 received 12V. Somewhere between the +12V power input and these chips there was a break, perhaps a PCB defect, or a broken patch wire. So because the VRAM did not work the TMS9918A could only produce a blank screen.
actually I have already socketed all the 64K RAM chips in his NABU, so he can test the chips in his RAM tester. (but this problem was not caused by a defect but by an unsound assumption by me on what the Power connector use was, the pins are NOT labeled!.

P.S. I found the cause of this problem by looking at my friends dissembled NABU PCB, and following the tracks on the bottom. I found that the power connector did not have just one +12V inputs but two! One is only used to power the keyboard, through a voltage dropper resistor (10 Ohm/5 Watt).
and another input was also +12V, and used for the LM3900 and the TMS4116 chips. next Tuesday I will make the necessary changes, and hopefully the TMS4116 chips won't have suffered, but I doubt that because the -5V substrate voltage was present. And most spectrums (which use the same chip) survive the absence of 12V, and often even the absence of the -5V substrate voltage. We will see. The separate +12V input is likely because they planned to use a supply that supplied +8 to +10V for the LM7805 regulator in the keyboard. If you look at the traces you can clearly see the dropper resistor is an afterthought.

[hippy]

three months ago it created a storm of interest in the RETRO computer collectors community

I do have to admit it has spawned a lot of diverse projects, hardware, software and emulators, and that's a good thing in its own right Some people are adding Raspberry Pi SBC's into the mix and I am sure the Pico and Pico W will find their way in too.

I'm wondering who will be the first to deliver a complete NABU emulation on a Pico ?

I guess there are worthwhile journeys to be had which I hadn't considered.

[mahjongg]

The NABU is already emulated with the MAME emulation system. but yes, if you can emulate an MSX-1 on a Pico, then it is just a small step to emulate a NABU too, more interesting is how to emulate the mainframe that sent the software to all the NABU's in Canada, via the cable TV network. but precisely that is already realised using USB to RS422 adapters on a laptop with some software. people like DJ Sures and others are working on that issue, and on creating special NABU system ROM's. (the NABU boot and system ROM) for example to run MSX-1 software. and to develop "cloud CP/M".
The native operating system on the nabu was called "DOS", no not "disk Operatings System" but "Downloadable Operating system"

I think the NABU is an almost perfect system to convert software to from other systems that use a Z80, a TMS9918A VDP and a AY-3-8910 PSG, and there are several of those, as they were very popular component to build a computer or game console around.

[mahjongg]

My NABU is working after I added a second +12V connection. I also now see a slight difference between Orange and red wires. So now I use Red for the +5V and orange for the second +12V,


Next task is to buy a suitable USB to serial adapter with an RS422 connection.
and solder a dual twisted pair cable to a DIN-5 connector. Also I will add a 180 Ohm terminal resistor on the R422 input channel, (or measure if one is already integrated).

[hippy]

Next task is to buy a suitable USB to serial adapter with an RS422 connection.
and solder a dual twisted pair cable to a DIN-5 connector.

Looking at the circuit diagram it seems to me that by tying HR- or maybe HR+ to 0V/GND you might be able to use a standard USB-to-UART, input to the ungrounded HR, output from HT+ or HT-

I have done similar with other differential I/O and that worked well. It may depend on voltage and in-line resistors always recommended.

That would probably make things easier for others if they can use a standard USB-to-UART or USB-to-RS232 and not have to buy a usually more expensive USB-to-RS422 adaptor.

Of course it's always best to have an official set-up which works so voltages and polarities are known.

Looks like I've accidentally joined the adventure

[mahjongg]

No, these are very high speed RS422 connections so "tricks" to use a normal USB serial adapter are NOT recommended.
we need an interface with true balanced transmitters and receivers, but thanks anyway.

[scruss]

Some folks I know have had success by moving the NABU's serial chip to a carrier board and routing the relevant pins to a USB-TTL Serial adapter. Modern interface chips are quite happy with the highly unusual rate that the NABU requests

[mahjongg]

My RS422 cable works fine!

[mahjongg]

I'm going to take my NABU to a Dutch MSX event in december 9th of this year, because MSX is almost 40 years old. It will be the largest MSX event of the last decade, and some important persons in the MSX scene will be there. Like Kazuhiko Nishi the Director of ASCII Corporation, the Japanese branch of Microsoft, he is the inventor of MSX. And Yuji Takenouchi the musician who wrote the music for a lot of popular games like Space Manbow, and many others.

The event is called MSXGOTO40
https://www.msxgoto40.com/en/

[mahjongg]

I finished a building up and testing document for the ZX81+38, you can find it as a "building and testing the ZX81+38.pdf" on my GitHub page (https://github.com/mahjongg2/ZX81plus38). I suggest reading it before you start. As faith would have it, mine (my revision 1.9 version) did not work first time, ultimately I found that I forgot to solder one end of a resistor in the video driver circuit . But the result was that I had a "black screen error", so now was the time for debugging, and that resulted in including a basic fault seeking part in the building up document. For many people this part wil not be needed as their ZX81+38 will work first time. But it seems my false confidence let me down, it all depends on how careful and precise you work, but as the build up document says, this is not a kit suitable for first time soldering persons. And I was a bit too careless.
Anyway, a basic set of fault seeking instructions are useful, or watch a lot of youtube repair video's.

I also just borrowed an Atari joystick, to test my joystick interface with. But I don't expect any problems (knock on wood)..
Another task is to test the cassette loading and saving function.

[mahjongg]

I have been asked to explain more about the jumpers on the ZX81+38, note that no jumpers need to be touched, they are all already set (with a copper trace) to default settings. But curious people want to know. Okay Ill write a document explaining all jumpers and what they do.
Us usual I will publish it on GitHub. (by the way, the schematic also explains all jumpers.