

Vulcan-74 – A 6502 Retro MegaProject - cmrdporcupine
http://forum.6502.org/viewtopic.php?f=4&t=3329

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beneater
This looks like a lot of fun. A couple years ago, I built an 8-bit CPU from
7400 series logic gates on a giant breadboard and learned a ton. And it still
works today! I strongly recommend a project like this to anyone who might be
interested.

I made a few videos demonstrating it:

[https://www.youtube.com/watch?v=9PPrrSyubG0](https://www.youtube.com/watch?v=9PPrrSyubG0)

[https://www.youtube.com/watch?v=35zLnS3fXeA](https://www.youtube.com/watch?v=35zLnS3fXeA)

I'm slowly working on putting together a more detailed tutorial for building
such a thing. I actually think with the right guidance this is a very
accessible project for someone without much background in electronics or
computer architecture to learn a lot from.

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cturner
@beneater - if I wanted to wire-wrap my own system like this, what's a
sensible way to think about managing the wires, and what sort of boards to
get? These are naive questions - I've done some electronics theory but hardly
anything practical. I've now read a couple of tutorials about wire-wrap pen
tools, but I don't yet have a feel for the boards. Maybe I'm googling for the
wrong things. Also, what is the name for the thing that you mount a microchip
in, in order that you have a lead to wrap around?

~~~
pjc50
"Wire Wrap DIL sockets" are the thing you want. Obviously this requires DIL
chip packages too. The boards are unplated veroboard with 0.1" hole spacing.

It will be tempting to gather wires next to one another in neat rows, but this
worsens crosstalk. You can probably achieve speeds of several MHz with
wirewrap. With a small amount of practice it's the fastest _prototyping_
technology and easier and cleaner to rework than soldering.

~~~
cturner
This is very helpful, thank you. In particular thanks for the tip about not
lining things up in rows, because that's just what I would have done. I could
probably write a script so that I could design things in neat rows on a
screen, and then it could fit it for me in a tight bunch, and then I could
wrap it.

Are there smart ways to use decoupling capacitors to get more?

How do you get a feel for how much room you have to play with as far as MHz?
Is it just a case that you start with something small, and then go up until it
breaks?

Are there practicalities of what chip generations is possible with wire-wrap?
6502 would be possible with wire-wrap. What about Motorolla 68k? (Also - what
is the most sophisticated 68k chip that's still commercially available? Speed
and memory addressing are important, MMU is not.)

~~~
pjc50
You can get built in decoupling capacitors: [https://www.mill-
max.com/products/socket/122-XX-XXX-41-80100...](https://www.mill-
max.com/products/socket/122-XX-XXX-41-801000)

You could certainly estimate the clock frequency in advance. The things that
are going to cause trouble are transmission line effects (ie pulse length no
longer >> wire length), and the usual digital logic critical path constraints:
wire and gate delays; clock skew and drive strength; fanout constraints.

More decoupling doesn't help you once you have enough to prevent crosstalk
through the power supply. Star-routing the power and ground may be a good
idea.

I've done 20MHz. Coincidentally this is the limit of cheap scopes. This page
claims 33MHz is achievable, and gives a good explanation of why:
[http://www.sigcon.com/Pubs/news/2_8.htm](http://www.sigcon.com/Pubs/news/2_8.htm)

Some good tips: [https://www.pjrc.com/tech/8051/dev-board-
wirewrap.html](https://www.pjrc.com/tech/8051/dev-board-wirewrap.html)

Reccomendation to use the kind of board with built-in ground plane (sounds
good if you can find it): [http://www.williamson-labs.com/prototype-
lt.htm](http://www.williamson-labs.com/prototype-lt.htm)

For IC availability, check your local Digikey. Note that 68k is still being
manufactured as "Coldfire" by Freescale (with slight backwards
incompatibility).

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tlb
Sounds like fun. Around the mid-80s I tried to build a custom GPU with
dedicated raster op hardware, all with hand-wired 74LS chips and VRAM, and
connecting to an Apple ][ bus. Parts of it sort of worked -- it at least
generated a valid video signal and it could copy 16x16 memory blocks, but it
was damn hard to debug with just an analog scope.

You should decide what the rules will be for using logic analyzers. I won't
think less of you if you allow modern debug technology.

~~~
pvg
They had logic analyzers in 1984, right? I think that's the general rule the
author is trying to stick to.

~~~
jacquesm
Logic analyzers definitely existed but cost stopped them from being present
anywhere but professional r&d facilities.

Scopes were omnipresent, if you had a logic analyzer in 1984 you were a local
god and people would regularly offer you their firstborn for access.

~~~
kwhitefoot
We had Intel hardware logic analysers before 1984. I used them at the Mullard
factory (Philips semiconductors) in Southampton to debug Signetics 2650
embedded controller problems around about 1980. Of course that's a long time
ago so my memory might not be 100% precise. I wish I had kept a copy of the
drawings for some of those projects, still look back fondly on those things,
hardware was so much more fun in some ways than software which is what I do
now.

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foobarge
One YT view leading to an other, I ran into this:
[https://www.youtube.com/watch?v=0jRgpTp8pR8](https://www.youtube.com/watch?v=0jRgpTp8pR8)

7400 logic CPU, running a port of Minix, C compiler, assembler, TCP/IP stack,
apps, etc...

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loser777
What are the typical functions/acceleration that this type of GPU supports
other than the blitter mentioned in the post?

I'm very surprised to see so few chips being used; doesn't 7400 logic only
give you ~4-8 gates per chip?!?

~~~
niklasni1
The 74-series is not just plain gates, there are multiplexers, latches,
counters, adders, and even complete ALUs.

~~~
peterfirefly
\+ shift registers, amplifiers ("bus drivers"), tri-state thingies, carry
generation circuits (for faster ALUs, especially when you connect several ALU
chips together for wider additions).

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fr0styMatt2
This is the most epic hardware-related thread I think I've ever seen.
Fantastic! :)

I have an FPGA dev board and have wanted to do something like this; start with
VGA generation and work my way up to sprites.

~~~
blue1
I believe this guy [http://c65gs.blogspot.it/](http://c65gs.blogspot.it/) is
doing something like that, recreating the Commodore 65 that never was.

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icedchai
can anyone recommend a good electronics book ? i'd like to build one of these
things but would like some more basic understanding first.

~~~
k2enemy
I highly recommend Nerdkits
([http://www.nerdkits.com](http://www.nerdkits.com)). It is a kit, but the
real value is in the excellent ebook that comes with it.

~~~
davegauer
I second that recommendation. The real selling point for me was that they
didn't try to make some sort of "friendly layer" for the beginner to work
with. Instead, you build the kit from commonly-available components on a
breadboard and learn how everything works from the ground up.

Reading that ebook was a real revelation: exciting and a real boost to my
confidence with electronics.

