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Junior Computer
Posted: Thu Oct 05, 2023 3:21 pm
by fornext
Hi All,
Lately I've been busy with the restoration of a 'Junior Computer' (Elektor/Elektuur).
I decided to build an emulator for it (still in beta development).
So for those interested you can find it here (open source):
https://github.com/ForNextSoftwareDevel ... orComputer
Best regards,
Dirk.
Re: Junior Computer
Posted: Thu Oct 05, 2023 3:48 pm
by BigEd
Welcome! Looks like an excellent idea. You might want to test your emulator's accuracy with the Dormann tests (or others.)
(I'm assuming this is a Windows project? That will certainly have an audience, although there are those who don't use it, including me.)
Re: Junior Computer
Posted: Thu Oct 05, 2023 4:07 pm
by drogon
I built one, back in the day...
Sadly I no longer have it - long story involving my usual disregard for "obsolete" hardware back then... Hindsight, etc. ... Ah well.
But like Ed, I don't use windows so can't check it out, but don't let that stop you developing it. (although making it work under WINE might be nice)
Cheers,
-Gordon
Re: Junior Computer
Posted: Thu Oct 05, 2023 4:33 pm
by BigEd
Good point, WINE usually works - I use it on my macbook relatively often.
Re: Junior Computer
Posted: Thu Oct 05, 2023 4:51 pm
by fornext
Thanks for the replies.
Yes, it is a Windows project using c#
Problem with c++ is that it would take much more time to achieve the same functionality.
I did make several programs under linux but that is usually quite time consuming.
Re: Junior Computer
Posted: Thu Oct 05, 2023 5:48 pm
by gfoot
C# can actually work well in Linux, in a way it provides its own portability layer. However I'm not sure about the UI side (WPF etc) - I've only run console applications this way.
Re: Junior Computer
Posted: Fri Jan 12, 2024 12:35 pm
by tanteju
Cool project. Have a Junior in real and the successors. But as other slaready mentioned, I could not test the software, as I do not have Windows. With graphic UI MacOS would be preferred for me.
Re: Junior Computer
Posted: Sun Aug 03, 2025 11:25 am
by frankdh
I built mine when it appeared in the issues of Elektuur and after the books came out.
After some time I got married, moved and bought an ATARI ST. My Junior went in a cardboard box and into storage. Now that I'm retired and starting to feel nostalgic I dug up the cardboard box and am now in the process of restoring it. It wil have a 28C16 EEPROM instead of the old 2708 ( done! ), and run on a single 5V power supply. Also I plan on replacing the processor with a 65C02 ( done! ) and the 1 MHz crystal, by the clock circuit of Ben Eater (
https://www.youtube.com/watch?v=kRlSFm519Bo), - so I can use an Arduino Mega like described in another of his video's (
https://www.youtube.com/watch?v=LnzuMJLZRdU) - and a crystal oscillator. So far I've progammed the EEPROM and replaced the processor, checked all necessary modifications (already done way back in the '80 s), and fired it up. But ... nothing...
So I gueuss I have my work cut out for me. I'll be back.
Re: Junior Computer
Posted: Sun Aug 03, 2025 1:24 pm
by Dr Jefyll
So far I've progammed the EEPROM and replaced the processor, checked all necessary modifications (already done way back in the '80 s), and fired it up. But ... nothing...
Welcome, frankdh. Nice to hear about another vintage project garnering attention again after decades in a cardboard box!
I hope you've been doing the mods one by one, and testing each one before proceeding to the next. At what point was your most recent successful test?
-- Jeff
Re: Junior Computer
Posted: Sun Aug 03, 2025 7:28 pm
by BigDumbDinosaur
Now that I'm retired and starting to feel nostalgic I dug up the cardboard box and am now in the process of restoring it...by the clock circuit of Ben Eater...
First, welcome aboard.
I should caution you that Ben Eater’s videos will, at times, ignore design and/or construction details that are important in producing stable operation. Quite often, we have new members sign up here who tried to build something that was depicted in an Eater video, only to not have it work due to omission of an important detail that was not brought up in the video. If you want to get an accurate view of what goes into the design and construction of a functioning 65C02-based computer, you may find Garth Wilson’s 6502 primer an interesting read.
Having been designed for use with the NMOS 6502, the Junior’s glue logic is not entirely compatible with the 65C02, and may explain the troubles you are experiencing. It all depends on whose 65C02 you are using. The only new source for the 65C02 is Western Design Center’s static-core product (W65C02S6T...), which is, on paper, electrically incompatible with the TTL logic in the Junior. There are also a couple of pin differences that must be accommodated, although those are readily handled.
Logic voltage levels are the important thing to consider in retrofitting an old unit such as the Junior, especially with regard to clocking. In particular, the WDC 65C02 requires that Ø2 swing at CMOS levels, which means a minimum high voltage of VCC × 0.7 and a maximum low voltage of VCC × 0.3. Also, the rise/fall time of the clock signal should not exceed 5 nanoseconds. The Junior’s original clock generation circuitry cannot meet those requirements.
Lastly, as implied by Jeff, changing more than one thing at a time will make it difficult to troubleshoot. If it were me doing this, the replacement of the 6502 with the 65C02 would have been the first step. If you made all the changes you mentioned at the same time, you have no idea which modification is causing the unit to not work.
Re: Junior Computer
Posted: Tue Aug 05, 2025 12:19 am
by BillO
Thanks for the replies.
Yes, it is a Windows project using c#
Problem with c++ is that it would take much more time to achieve the same functionality.
I did make several programs under linux but that is usually quite time consuming.
I don't know if I agree wit this. Once you get used to C++ it will go quickly, both in terms or writing code and in terms of execution. Let's not even discuss the standalone executable size. The big plus these days is that it's a more portable language.
Cool project and I hope it continues to go well.
Re: Junior Computer
Posted: Wed Aug 06, 2025 10:16 am
by frankdh
@ Dr. Jefyll & BigDumbDinosaur thanks for your input.
I've started reading through the primer (and downloaded some referenced documents). A very comprehensive primer indeed. It highlighted some things I hadn't considered or thought of.
Made me reconsider my course of action(s). I guess I'm going to start with the original, except for the (E)EPROM (I want to keep the single 5V pwr supply).
And see what happens.
The primer also inches me towards designing and building a new system based on the Junior design (or
https://wilsonminesco.com/6502primer/potpourri.html also looks like a good starting point), using modern components.
I'll keep you posted on any progress.
BTW I performed the mods before 1985 (except for the 65C02 and 28C16 - I didn't have a decent programmer back then), and can't remember when or whether I had a successful test. I'll soon find out though.
Re: Junior Computer
Posted: Wed Aug 06, 2025 3:40 pm
by BigDumbDinosaur
I've started reading through the primer (and downloaded some referenced documents). A very comprehensive primer indeed. It highlighted some things I hadn't considered or thought of.
Aside from the primer, Garth has a lot of other useful information on his site. For a bit of nostalgia, you could read his page about slide rules. Those of us of a certain age who were already into engineering and science by the time we reached high school always had a slide rule nearby. Mine was a Dietzgen log-log unit, which I used all the way into my early thirties. I designed the chassis for my first race car using that slide rule to do the calculations. I finally broke down and bought a pocket calculator when the TI-55 came on the market in the late 1970s. 
Made me reconsider my course of action(s). I guess I'm going to start with the original, except for the (E)EPROM (I want to keep the single 5V pwr supply). And see what happens.
That, I think, would be the most productive route for you.
Although the Junior’s design was well thought out, it was constrained by the technology of its time, especially the limitations of 74LS logic. With modern CMOS logic, good timing will be easier to achieve, noise margins will be better, and running much faster than the 1-2 MHz speeds common in the Junior’s day will be practical (one member, Plasmo, has cranked the 65C02 up into the 30 MHz range). Also, we now have a wider choice of I/O devices, especially much better UARTs, and more ways to interface to the outside world. So I would be inclined to look beyond the Junior’s design.
A number of us here have “build logs” that go into varying levels of detail on projects we’ve built. Reading some of those might give you design ideas, as well as insight on potential design “gotchas” that resulted in unstable or DOA units (my first build was partially DOA due to a trivial PCB layout error).
Re: Junior Computer
Posted: Wed Aug 06, 2025 6:27 pm
by GARTHWILSON
except for the (E)EPROM (I want to keep the single 5V pwr supply).
Re: Junior Computer
Posted: Thu Aug 07, 2025 8:54 am
by frankdh
All the (E)EPROMs I'm aware of are single-supply. EPROMs usually require a higher voltage to program them; but once they're programmed, operation in your circuit requires only 5V. Hopefully you'll find that to be a relief.
I meant to indicate I will replace the original 2708 - which needs +5, -5 and +12 V - by a single +5 V powered EEPROM. I can then do away with the -5 and +12 V.