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8-bit computer from scratch (ciernioo.wordpress.com)
94 points by danielam on June 4, 2016 | hide | past | web | favorite | 19 comments

Hint: when posting images of schematics, make them big enough that readers can zoom in and read the numbers.

It's interesting to see this done with so few ICs. Early Z80 machines had a much higher part count, because a lot of external support was needed to talk to anything. Today we have the "system on a chip", with lots of onboard peripherals, but a Z80 is just a CPU.

The predecessor to the "system on a chip" was the semi-custom "junk chip", which contained unrelated stuff the standard chips didn't have. He may be using some of those to get the parts count down. Can't tell; schematic too small.

One of the features of the Z80 was built-in memory refresh, so it needed a lot less glue logic than most processors, which had to have external registers and latches to handle the refresh cycles.

Way back I used to hand solder Z80 embedded controller board prototypes, and they never needed much in the way of chippery.

In an S100 system the biggest extras were the video (formerly VDU) cards, and the memory cards.

Both were limited by the RAM of the day, because RAM came in tiny sizes like 1 bit x 16k dynamic or 2k x 8 static. So you needed a lot of chips on a separate board for what was really not much RAM at all.

Also, money. 64k boards cost over $1000, which is > $3k in modern money.

Now you can get 8 bit x 256k static RAM on a single chip, so you can park that next to the processor without a separate card. With a bit of glue you can even share it with the graphics hardware, eliminating any need for separate graphics RAM.

Did you check the downloads? I think the schematics there are going to be more readable.

From the title, I assumed "from scratch" meant logic gate, or even transistor level. It's still very cool though.

Check out the Megaprocessor[0]. A guy made his own "processor" out of roughly a hundred thousand transistors, resistors, etc. Really interesting project.

[0]: http://www.megaprocessor.com/

Relatedly, a discrete 6502 built with a little over 3K transistors:

http://monster6502.com/ as seen on HN https://news.ycombinator.com/item?id=11703596

So impressive, but I am so happy to never have any reason to need to do anything like this myself. I used to fix IBM mainframes like 3033s back in the day, and debugging computers made up of gajillions of interconnected frames, boards and cards, under immense pressure, was truly the stuff of nightmares. After hours or even days, victory was finding a ringing signal with your scope, caused by a loose trilead. Then you went home and slept for a day.

"If you want to make an apple pie from scratch..."

Why stop there? From scratch should involve extracting the materials from the Earth.

Because that's the point where you take things that don't compute, and make something that computes.

>>Because that's the point where you take things that don't compute, and make something that computes.

Good point but I didn't get it quite right. What is your definition of "things that compute"? I am curious.

A device that can use a mathematical process to figure something out.

A single transistor definitely doesn't compute. A single gate? Eh, not really.

A design with a couple latches and an adder that can perform SUBLEQ? Yeah that computes if you attach a ram.

A non-turing-complete pipeline of math functions? Yes, that computes, though why make that when it takes more gates than a microprocessor needs.

You can make a razor blade diode or a pyrite diode, so I wonder if there's some way to make a make a transistor out of household junk...

Jeri Ellsworth has built working transistors at home.

Point contact germanium BJT: https://www.youtube.com/watch?v=vmotkjMSKnI

Etched silicon FET: https://www.youtube.com/watch?v=PdcKwOo7dmM

The rusty razorblade + pin point contact diode doesn't need anything special because it's a Schottky diode, with a metal/N-type semiconductor junction. If you probe lots of places you can find an N-type region where the dopant is random impurities. But the transistor needs both N and P-type regions, so I don't see any way to build one with common junk. Jeri's point contact example works by diffusing phosphorus from phosphor bronze into commercially produced N-type germanium using a high current to heat it. Phosphor bronze is arguably household junk but I don't think germanium diodes are.

I wouldn't mind actually. That toaster thing was interesting.

Yeah this is what I was expecting too

I am currently building my own Z80 based computer. It's been a great experience building up my electronics lab and I wish I had done it years ago. I recently received a Russian made Z80 that I ordered off ebay. Probably the only product of Russia I've ever owned.

Don't know why, but the CatChurn game jumped out at me as being written by Yahoo Software in 1982 (the year I was born). There does not seem to be a Wikipedia entry for this company, but the address appears to be a residential condo as per Google Maps - https://www.google.com/maps/place/10970+Ashton+Ave,+Los+Ange....

Interesting to see a little bit of history and how game writers really were working from home on an idea even in the early days.

Specially like the wooden case and keyboard. Wish the schematics were a bit easier to read, though. :)

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