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Old 28th Dec 2018, 9:26 pm   #1
Karen O
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Default Retro computer homebrew: Orton Karenbak-1 computer

I present here a homebrew computer that has no microprocessor (and no humongous ROM look-up tables!)

It is comprised of SSI and MSI CMOS logic, with the single exception of a shift register for memory.

It won't break any benchmark records, but it might provide some insights into computer design.

Enjoy!
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Old 28th Dec 2018, 10:14 pm   #2
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Fantastic stuff Karen
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Old 28th Dec 2018, 10:17 pm   #3
Ed_Dinning
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Great Project Karen. If you want to go a little further back there was the WW computer with discreet transistor flip flops etc.
Schematics still on line somewhere

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Old 3rd Jan 2019, 1:22 am   #4
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

I love it - reminds me very much of a PDP8-S machine in so many ways. Still a wonderful design - I have enjoyed reading the documentation.

Nice how you have achieved so much with such a few gates and yet made a computer that is capable of performing arithmetic.
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Old 3rd Jan 2019, 9:02 am   #5
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

I really like this! Whether it'll get to the top of my 'things I'm going to do list' is another matter but it's definitely on there. I have two medium sized food containers full of old CMOS and various families of TTL chips and no idea what I'll ever do with them.

John
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Old 3rd Jan 2019, 1:42 pm   #6
Karen O
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Thank you all for your kind comments.

It was some time in the making, and the objective changed several times.

Originally it was to be a computer using 100 transistors or less.

That might still happen, and looks possible given that there are only 30 flip flops.

Initially, the delay memory was going to be cassette tape.

I was going to use a combined record/play head, which would provide a delay of some 90 milliseconds.

Trouble was, that'd result in only six instructions per second.

With the BBD delay it can achieve a wopping 100,000,000 microinstructions per second
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Old 3rd Jan 2019, 1:45 pm   #7
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

When I was at university in the mid 1960's one of the computers I had regular access to was a PDP8-S and as JohnBHanson has pointed out there are similarities in operation.
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Old 3rd Jan 2019, 1:57 pm   #8
Karen O
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

I actually admire the drum machines like the LGP-30. All basic timing signals and address information could be put on dedicated tracks thereby avoiding a lot of hardware. In theory, and ignoring the diminished read head signal that would result, you could hand crank the drum and the computer would still work - just more slowly.

I also looked at using glass PAL delay lines as storage. I quickly got 32 bits recirculating in one of these. I'm on the look out for a means of delay of around a few milliseconds so that I can build a version without ICs. Continuous contact magnetic media would doubtless wear out too quickly.

Any ideas?
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Old 3rd Jan 2019, 2:25 pm   #9
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

I have quite a bit of mercury if you want to do a delay with that.
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Old 3rd Jan 2019, 2:27 pm   #10
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Another thought, use the PAL delay line transducers on a long bit of glass.
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Old 3rd Jan 2019, 2:31 pm   #11
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

The EDSAC reconstruction team looked at recreating mercury delay lines. The transducers have to be very precisely aligned, the delay is temperature sensitive, and they'd need special facilities, not to mention the 16,000 cost of the mercury! In the end they decided to use nickel wire delay lines.
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Old 7th Jan 2019, 12:39 am   #12
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Great work Karen. For my nbsecam experiments my next step was going to be an attempt with various tensioned wires but i never progressed beyond thinking about it. The other "idea" was to try an old hard disk drive but that would require parking mechanisms and would no doubt end up more complicated than the mother machine! Maybe an old floppy disk and just move the head on a touch when you wear the surface out though that would maybe be too slow?
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Old 7th Jan 2019, 1:25 am   #13
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Wow, what a wonderful project, congratulations.

I have recently been repairing a vintage SOL-20 computer from 1976 and it is amazing what they could do. The one I have is fitted with three 16k memory cards.

What astonished me was the number of IC's that Processor Technology (PT) used to create a 16k S-100 style memory card. About 71 IC's . That is more than the total number of IC's used in Atari's Arcade Pong video game from 1972, which has 66 TTL's, for a whole video game.

These old PT cards are really worth a look, they also contained digital delay lines, there is a photo of them here:

http://worldphaco.com/uploads/PROCES...A_RAM_CARD.pdf
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Old 7th Jan 2019, 2:14 am   #14
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

I did look into using LC delay lines. These were used in some valve calculators to store a single number. The trouble is you need 3 or 4 LC elements for every bit you want to store. Also, the circulating signal has to be amplified and reconditioned every 12 or so elements.

Actually Dominic, a 3.5" floppy would only have to spin at a little over 300rpm (i.e. normal speed) to provide a 5msec delay, provided I used a dual head of the sort used in cassette decks with off-tape monitoring. It's worth considering! A stereo head would also afford a clock channel which would ease the data recovery task.

Thank you for your kind comments Argus25. You know the feeling when you suspect something is possible...? I just had to build it to find out!

The Karenbak-1 NEARLY worked first time, but the problem wasn't the logic. The multi-position switch I chose was make-before-break, and this shorted my 5V supply! Adding pull-ups on the high-tied connections fixed that, and it worked exactly as planned after that.
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Old 7th Jan 2019, 2:47 am   #15
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

I'm not sure if this has any application in your project:

One very effective simple memory, to make with discrete parts, which acts as a memory for an analog voltage, which could be derived from a DAC as one source, is to use a single mosfet in conjunction with a reed relay, with a small capacitor of a low leakage type, 0.01uF to 0.1uF or similar in the gate and use the mosfet in a follower mode.

You can then get the output of that and ADC it back to a digital byte etc. Due to the insignificant gate current this can remember the voltage value for many days, even with a small capacitance. This trick was used in early color video cameras to remember the offsets for a white balance adjustment. When I first saw it I thought it wouldn't be any good as a memory, but it really was. It could just be used for a single bit storage too.

The capacitor, reed relay connection and Fet's gate connection are lifted free from the pcb to reduce leakage. Small reed relays, coil driven, easily operate at 300Hz too, and the size of the capacitor can be reduced to allow quick charging.
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Old 11th Jan 2019, 9:20 am   #16
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Karen, How are the components on the card wired? Is it a custom made PCB or is it done with a wiring pen?

John
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Old 12th Jan 2019, 8:16 pm   #17
Karen O
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Hi John,

Sorry for the slow reply.

I've been out and about, attending the funeral of a much respected and admired NBTVA member.

The board is wired with 36swg point-to-point enameled copper wire.

The enamel can be stripped back simply by heat from an iron.

I'll probably pay for breathing the fumes
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Old 13th Jan 2019, 10:49 pm   #18
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Thanks! I have one of those Vero Wiring Pens and a couple of spools of pink wire. Heat from the soldering iron takes the enamel off whilst you solder.

I once used it to wire up an 8080A, clock chip, bus controller, RAM and an EPROM - worked great despite people telling me it would be unreliable!

John
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Old 14th Jan 2019, 6:47 pm   #19
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Hi John,

The power rails are more important than how you wire the signals. I used a Eurocard prototyping board with interlocking 'fingers' for power distribution.

I remember something called 'Nylese' from many years ago. It too was stripped by the heat of the iron. Burning plastic fumes - now that's GOT to be unhealthy!
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Old 15th Jan 2019, 6:30 pm   #20
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Default Re: Retro computer homebrew: Orton Karenbak-1 computer

Thanks Karen.

John
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