I included it in the original post describing the strange behavior, a little upthread from the one you replied to. Here it is again:



I have expanded the ISR since then, but I never did figure out exactly what was causing that problem. I "solved" it by moving a couple of modules out of the bus slots near the power supply; I'm guessing it was EM interference or power line noise from the voltage regulator.

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"The key is not to let the hardware sense any fear." - Radical Brad

Well, I did that. I tried to make a better ground / vcc network, and be more conscientious about making every wrap be a good quality one. I also rebuilt the whole board, including the LCD part that already worked. Here it is:



Once again, it only kinda works. Just like before, the LCD part works fine, but the keyboard circuit is wonky, as though the RC circuit managing the clock has bad timing, or there is noise on the data line. So, to troubleshoot, I reverse engineered the wire-wrap board. Going point by point with my multimeter, I reconstructed it on a breadboard:



Interestingly, this breadboard version has exactly the same problems as my wire-wrap version: lots of repeated keys, and intermittent incorrect scancodes.



You can see that there is something conspicuously missing from the breadboard reconstruction, though - bypass caps! My wirewrap board does have a bypass cap for each IC, and I did put them on the breadboard during reverse engineering, but I took them back off again for dramatic effect. They don't seem to make any difference. But this one does:



I can put that cap anywhere on that side of the breadboard, and everything immediately starts working. If I put it on the other set of power rails (which is where I initially had all the per-IC decoupling caps) it has no effect.



If I only I can figure out the equivalent "magic spot" on my wire-wrap board and add a capacitor there...

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"The key is not to let the hardware sense any fear." - Radical Brad

What I'd probably do is set the scope trigger to vector pull (on the 816) or the interrupt line and then scope the address bus for what is happening.

André

PS sorry if I state what has been said already I'm only occasionally scanning the forum

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Author of the GeckOS multitasking operating system, the usb65 stack, designer of the Micro-PET and many more 6502 content: http://6502.org/users/andre/

No worries; I appreciate you taking an interest in my project!

I think a scope would make my life much easier. But so far in the last year I've only spent in the $300 - $600 on my hobby electronics workshop. Even used, a good scope will be that much again or more.

I have been thinking about getting one of those DIY kits like this - but the Amazon reviews don't inspire great confidence, and I'm not sure it would actually be able to read the signals on my machine. I'm running in the 4Mhz - 8Mhz range.

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"The key is not to let the hardware sense any fear." - Radical Brad

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Ed's post is here: Semi-OT: "The Secret World of Oscilloscope Probes"

As for used analog 'scopes, about 2 years ago I got a very nice Tektonix 465B on E-Bay for USD 120. (I paid the asking price.) It had a deflection problem but was still usable in spite of that. I repaired the problem anyway, and I've been very pleased with my purchase.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Every so often I troll eBay for one of those used Tektronix analog scopes. I think they're just what I want; the problem is that they're usually "for parts / repair only" and I am not even close to knowledgeable enough to try fixing one. "EEEVBlog" Dave has a talk about people just giving them away, or finding them for $50 on eBay. That doesn't seem to be case any longer, unfortunately.

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"The key is not to let the hardware sense any fear." - Radical Brad

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

OK well I said that, and then I found this!

I guess I'm getting an oscilloscope.

It doesn't seem to have any probes, which given Ed's post doesn't seem so bad, since I'll want to get decent ones anyway.

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"The key is not to let the hardware sense any fear." - Radical Brad

Congratulations!

However, I suspect the problem you described earlier is entirely due to poor power/ground distribution (icluding poor use of bypass capacitors). I hate to tell you, but the 'scope probably won't be helpful in correcting this. In fact, an inexperienced 'scope user could easily end up misinterpreting what's on screen and end up going on a wild goose chase.

So, I don't wish to discourage you. The scope will certainly be an asset in future. But if your first priority is to fix the problem at hand, I suggest you review and improve the power/ground situation (rather than trying to analytically "pinpoint the problem" using the scope).

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

You might need to be quite careful and selective: to see trouble in a system running at 4MHz or 8MHz I think you'll need a lot of bandwidth (and good probes too.)

Broadly, I might say that with logic analyser you can see what your 6502 system is doing, and with a good enough scope you might be able to see why it's doing it. Without the good enough scope, you'd use the 'what is it doing' together with analysis and experiment, based on 'how could that happen' reasoning.

I can't overemphasise how useful Hoglet's trace analysis tactics are, for so much debugging. With a very cheap 16-way logic capture box, and Hoglet's open source analysis converting bus activity to a cycle by cycle machine trace, one can get an exhaustive log of the system behaviour, and rapidly narrow down to the first cycle that things have gone wrong. One can also share the log and get help from others.

Originally he wrote a Sigrok plugin but it's now a standalone program.
https://github.com/hoglet67/6502Decoder
viewtopic.php?f=4&t=4963

No need for hate! I expect it will be a while before I can do anything useful with the scope. It has to get here first; then I have to see what, if any, reconditioning it needs, and then I'll have to learn how to use it! Getting a scope right now wasn't really part of the plan; I just happened to get lucky this particular eBay run.

Scope or no scope, if I can get the keyboard circuit to work on a breadboard, I think I ought to be able to get it to work on perfboard. Since the two are logically identical and the only difference is in the construction, I think you certainly must be right about power / ground. The circuit is a pretty simple one - not counting decoupling capacitors, it has only 3 ICs, a diode, a capacitor, and 4 resistors. The protoboard I'm using is specifically for the RC6502 - it has the bus pins along the bottom, a GND rail across the top, and a VCC rail down the right hand side. The majority of the board is a simple grid of plated through-holes; no strips or busing.

My first try I just ran one wire directly from each power pin to the appropriate power rail. My second try I daisy-chained all the VCC pins to each other and all the GND pins to each other, and than ran a few wires from the rails to make connections in various points. I saw some ads in an old magazine (and an old thread here) that showed some IC sockets with integrated decoupling capacitors, so I tried to make some like that - I put the sockets on the board, then soldered the capacitors diagonally across the power / ground pins on the underside. That doesn't seem to be working though...



Thanks for those links Ed. I will peruse them in detail this afternoon!

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"The key is not to let the hardware sense any fear." - Radical Brad

You mean didn't fix the problem?

What happened to me was mixing up the sides of a chip when looking at the underside. I always look multiple times nowadays.....

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Author of the GeckOS multitasking operating system, the usb65 stack, designer of the Micro-PET and many more 6502 content: http://6502.org/users/andre/

Yes, my hardwired version still didn't work. I'm sketching ideas on graph paper now before I try a third one. If all else fails, I can always get one of those breadboard PCBs and just solder the working breadboard circuit onto it.

Here's a kind of basic question; on breadboards, the ICs are usually lined up in column, like soldiers on the march. On some boards I've looked at, especially old PCBs, the ICs are lined up phalanx style, like a line of battle. Is this something that matters in terms of performance, or is it just a matter of convenience when doing trace layout (i.e., go phalanx style when you have to cram on a lot of ICs)? I've only got three 74-series logic ICs in this circuit, and I'm not sure if it matters which way they line up.

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"The key is not to let the hardware sense any fear." - Radical Brad

How they line up won't make much difference. But what will is the the bypass capacitors and the power and ground connections.

(Of course it's possible you have some other problems as well. For example, the intermittent incorrect scancodes you reported could indicate a logic design flaw, rather than a power supply issue. Or it could be caused by both. But my point is, the scancodes are not the only symptom. And proper power distribution and bypassing will be required in any case, so that's the logical place to start.)

Here is a post you might find helpful, based on the following theme: "It's best to avoid thinking of the Power & Ground Distribution network as something for connecting the chips to the power supply. Where high frequencies are concerned, its real job is to connect the chips to one another."

And I could load you up with lots more reading material but here's another alternative. Share with us a photo or a diagram that reveals the route taken by the ground connections, the route taken by the power connections, and where the chips and caps are attached. And we'll suggest improvements.

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html