Electronics > Projects, Designs, and Technical Stuff

Experimenting with TTL Cpu, 74LS chips, old vs New? Retro style switches?

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tggzzz:

--- Quote from: T3sl4co1l on June 04, 2020, 02:39:02 pm ---Honestly I'd stick with the TTL.  You have it, you can breadboard it today.  You may have more special functions on hand than are available in HC.  Do try to design some if you can, inventory what you have and make sure you have enough of various types to do it.  And you can always interface to HC and such with just a little buffering.

--- End quote ---

Agreed. And the interfacing is even simpler with HCT, of course.

duak:
One of the most interesting problems I ever solved was in a manufactured product.  On occasion, an output bit on a register would change even though it wasn't supposed to.  It turned out that the data lines to the register were routed near the clock line to the register but because it was a two sided board, it wasn't all that close to the ground.  It was an unusually shaped board because it was a control interface to some part of the assembly.  When enough of the data lines changed at once, the clock line followed and caused the register contents to change.  The register was LS and the bus driver (on the same board) was LS.  One solution was to put a small cap on the register's clock input.  The other is to solder a metal ground plane made from a thin single sided piece of PCB  over the extension connecting all the ground points together and also laying close to the data lines.

Moral of the story - watch out for short glitches causing unintended clocking.  These can come from the most mundane sources.

rstofer:
The PiDP-11 project uses a Raspberry Pi and the simh package to recreate a PDP-11/70.  Including the switches...

You might try to contact them for a lead on the switches:
https://obsolescence.wixsite.com/obsolescence/pidp-11

https://obsolescence.wixsite.com/obsolescence/contact

rstofer:
Ben Eater's project is pretty nice!  I built most of it on breadboards until I just got bored with jumpers.  Some kind of prototype board and wire-wrap would be a lot better.

As to higher order machines, just about nobody is going to use TTL or any other chip family when they can build the whole thing in an FPGA.  I built an entire IBM1130 in a Spartan 3 Starter Board (now on a Nexys 2 Board) that runs the factory software, unchanged.  You can define the chips in terms of some HDL and wire them up with a keyboard instead of a jumper.

Software is the thing.  Without an assembler and some high level compiler, what good is the hardware?  I remember well when the Altair 8800 came out and it had just 256 bytes of RAM.  Every program had to be toggled in and, of course, they had to be tiny.  Today, you could grab the T80 core from OpenCores and implement a Z80 machine that would run at 50 MHz, maybe more.  You could even run the ROMs for PacMan.  I have that on a Nexys 2 Board as well.

I have wire-wrapped projects up to 100 chips of original TTL (predating LSTTL) and they worked well.  To do this, you really need a Gardner-Denver cut-strip-wrap gun.  Using the pencil type tool is too grim to contemplate.

rwgast_lowlevellogicdesin:
I know this could be done in FPGA much easier, and at some point I may use 16v8gals or ATF1502 cpld's I have in the drawer. Really the point of this whole thing is to learn to efficiently use and think in logic gates (this way I can eventually move to an FPGA for some RF processing in other projects) and develop a software stack from the ground up and really understand implementing bare metal programming in assembler, which I feel would be more intuitive if im the one who built the hardware and understand its connections and mechanisms. The second goal is just to have a cool blinken lights piece of hardware sitting around doing simple tasks, which can easily be upgraded if I want to learn more advanced skills like predictive branching or graphics blitting.

You know I have a lot of copper clads I bought on the super cheap when RadioShack went out of businesses, not to mention the huge sheets of rogers I use for toner transfers when I need a quick way to build a comb-line filter or LNA for frequency's above VHF. I could actually dead bug the whole thing if i need too. As far as ringing, a square wave is full of harmonics and if the trace inductance is to high im sure a TTL chip can ring or spike on the rising edge, while this may not be an issue to logic threshold it will create a bunch of EMI, am I right about this? I mean a large enough harmonic spike somewhere could couple in to something causing a bit flip? At worse it will make a noisy ground!

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