(Topic ID: 221615)

Oscilloscopes and Pinball


By oldschoolbob

1 year ago



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  • 49 posts
  • 11 Pinsiders participating
  • Latest reply 1 year ago by barakandl
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    #1 1 year ago

    I just got a new oscilloscope. I don’t think I really need it and I don’t really know how to use it but I find them fascinating and want to learn more.
    I have an old MPU 100 that I rebuilt a while back. It boots up on the bench, it boots up in a game and plays fine so I thought I’d try some tests to learn something.

    I started my testing on the CPU (U9). D0 – D7 (pins 33 – 26). The wave forms looked about what I expected.

    d0a (resized).jpg

    Then I looked at A0 – A14 (pins 9 – 24). Again the wave forms looked pretty expected.

    a0a (resized).jpg

    Except A3 and A7 don’t show any dips like the other wave forms.

    a3a (resized).jpg

    Also A4, A8, A11, A13 and A14 all show garbage.

    a4a (resized).jpg

    And the voltage looks way down – 200 MV.

    Why are there no dips in the A3 or A7?

    Why do I get low signal on A4, A8, A11, A13 and A14?

    Is there anything else I might want to look at to learn something?

    Thanks

    (Student) Bob

    #3 1 year ago

    These measurements were taken on the bench after 6 flashes. Would the seventh flash make a difference?

    Bob

    #7 1 year ago

    Would being in attract mode make a difference? I have a transformer that I often use to get the seventh flash. If I connected that and did the bench test would that be the same as attract mode? Would that get me out of the failed test and out of the loop?

    Quoted from CactusJack:

    Therefore, while you may see what look like normal waveforms on most of the address lines, others may look totally dead if your scope can't catch and display a single fast change (a single read or write to a port).

    My scope has single sequence - Would that help to catch a fast change?

    Most of this stuff is way over my head. I'm not an electrical engineer or a computer programer but I find this stuff very interesting. One thing I've learned is that I need to learn a lot more.

    Thanks

    Bob

    #18 1 year ago

    The ROMs are from a Stern Magic.

    Zacaj, You were right. I finally got a little shop time and connected my transformer for the seventh flash. Seems I’m getting regular wave forms on all the address lines now.

    a0a (resized).jpg

    Even A3 and A7 look more like a wave form.

    a3a (resized).jpg

    Also A4, A8, A11, A13 and A14 are showing 4.8 volts and a regular wave.

    a4a (resized).jpg

    Which brings up another question – Are those wave forms always changing? If they are, then why? I thought the ROMs sent the information to the CPU to play the proper game. Why wouldn’t they just send the information once.

    And for those of you that are just starting to mess with your oscilloscope, Stick with it. Maybe these guys that have the knowledge will teach us all something. I’m 72 years old but still learning every day.

    And a special thanks for all the knowledgeable guys here for taking the time to teach us.

    Bob

    #20 1 year ago

    I understand the CPU has no memory but I thought that’s where the RAM came into play. I guess not.
    So if the ROM keeps sending data to the CPU, is that data changing? I thought every wave form at a certain pin would all be the same. But that doesn’t seem to be the case.

    I experimented today – using only CPU pin 9 (A0) - I took three measurements. Because the wave was jumping around I had to use the RUN/STOP to get a steady measurement. I got three different results. Even the frequency and peak to peak was different.

    pic_24_1a (resized).jpg
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    Then I thought my method was fraud. Maybe I wasn’t capturing enough wave form. So then I used Single Sequence to capture the wave. (Thinking Single Sequence would capture the beginning of the wave.) Again I get different results.

    pic_24_5a (resized).jpg
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    Am I doing something wrong? Is there a way to capture the whole wave form?

    Thanks
    Bob

    #23 1 year ago

    WOW, you guys are so far over my head you’re not even a blip on my radar. But bear with me to see if I’m beginning to get the idea. Let’s say the ROM wants to send the word “cat” to the CPU over the A0 line. The first time it sends the letter ‘c’. The next time it sends ‘a’. Then ‘t’. It’s the CPU’s job to put it all together.

    This is why when I watch the wave on the scope it looks like it’s not triggering – just a bunch of waves dashing across. And when I hit the run/stop it is only showing me one of those letters. And when I hit the run/stop again it may be showing me a different letter.

    If it’s sending thousands of letters – then capturing the same wave form twice would be near impossible.

    I understand that it’s not sending letters – just 1’s and 0’s. But I’m trying to put this in terms that I can understand. Am I on the right track?

    There’s no way I’d ever understand how to write the code. I just want to understand how it works. And you guys are very good teachers.

    Thanks

    Bob

    #27 1 year ago

    You just answered my next question - what’s the difference in Address lines and Data lines. So, the address lines are requesting information and data lines are the information going to the CPU. So when I was getting garbage wave forms on A4, A8, A11, A13 and A14 (without the seventh flash) it was because the CPU wasn’t requesting data.

    Looking at the schematic I’m guessing the CPU is requesting, receiving, and sending data to the RAMs the same way. If I probe the address lines and data lines on the 6810 and 5101 I should see a similar wave form – correct?

    I think I’ll try that next.

    Then I’ll get into PIA’s.

    And I’ll get into Quench’s post shortly – I see some very interesting experiments in there.

    I don’t know who is following this thread but I find this one of the most fascinating and learning subjects in a long time. I just want to thank all you guys for being such great teachers.

    Bob

    #30 1 year ago

    Today I looked at the address and data lines on the RAM chips. As I suspected they are similar to the wave forms from the CPU.

    The R/W makes perfect sense. How else would it know if it’s supposed to send or receive data? A very complicated system – and extremely fast. I noticed all the major IC’s (except the ROMs, of course) have a R/W pin. Including the PIAs.

    Please correct me where I’m wrong but the PIAs read and write similar to the CPU – it’s reading the switches, sends the data to the CPU, then the CPU tells the PIA to fire the coil or light a lamp or change the score display.

    If my theory is correct when I probe the PA(0-7) pins on U10 and U11 I should see activity but the PB pins should be blank because nothing is connected. (bench testing). I check those next.

    There’s a lot going on inside the pinball game. I’d hate to think what’s going on inside my computer. And it all happens so fast.

    #31 1 year ago

    That's not the Ben Heck I always see on youtube - great show. I read your post but I need a clear head to make some sense of it all. And I'm too tired tonight. I'll digest it in the morning. Glad to have you comment on this thread.

    For some reason this thread is bringing in a lot of great talent.

    Bob

    #37 1 year ago

    Thanks for the article on PIAs. I just skimmed it over and can tell it will be very helpful. I'll study it tonight.

    I need just a little help with the basics. I understand frequency, pulse width, rise time and period. I understand minimum voltage, maximum voltage and peak to peak. But what is mean voltage?

    I did some more testing today. I looked at the PA's and PB's on U11. As I expected I got wave forms on the PA's (trying to send data to the displays). I got no wave forms on the PB's (trying to send data to the driver board - no solenoids firing in attract mode)

    Then I looked at U10. I got wave forms on the PA's (trying to send data to the light driver and switch strobes). But what surprised me I'm getting wave forms on the PB's (at a slightly less voltage). The PB's are switch returns. There's no switches connected. Could the wave forms be from the PIA calling for data?

    Bob

    #38 1 year ago

    I always thought logic analyzers were super expensive. But that don't look bad.

    #40 1 year ago

    Now that you say that, I do remember reading somewhere that when diagnosing problems on a MPU you should turn off all dip switches. I guess that's the reason why.

    Would it I see a difference if I turned off all the dip switches?

    #41 1 year ago

    I turned off all the dip switches and re-probed. No difference. You’re right, U10 is a busy place.

    I tested the remaining pins on U10 (pins 18 – 40) and screen captured each one. I read the PIA article several times and compared my captures to the info in the article. All this helped me to better understand what’s going on in a PIA. My goal isn’t to fully understand binary numbers and code but to just get a general road map.

    I did see some interesting frequency’s – D0 was over 1.1 MHz and the IRQ’s were in the Hz.

    Next I think I’ll try some of the experiments that Quench suggested. That should be interesting.

    Bob

    #43 1 year ago

    Quench, a quick story about binary – years ago a friend of mine (computer geek) built a binary clock. He forced me to read binary because it was the only clock in his workshop. With some thinking (and maybe pencil and paper) I can read some binary.

    I removed the ROMs and started checking the CPU address lines (A0 – A14).

    A0

    cpu 9 (resized).jpg

    A1

    cpu 10 (resized).jpg

    A2

    cpu 11 (resized).jpg

    A3

    cpu 12 (resized).jpg

    At first I didn’t think it was working – then I noticed the frequency wasn’t doubling but it was half-ing (each frequency was half the one before). Don’t know why mine is counting backwards.

    Then I checked the 555 pin 3. It’s close – 305.2 Hz.

    u12 p3 (resized).jpg

    The zero crossing was spot on.

    u14 p4 (resized).jpg

    This new oscilloscope didn’t have any problems reading the CPU clock signals

    Pin 3

    u9 p3 (resized).jpg

    Pin 37

    u9 p37 (resized).jpg

    I may be looking for your help soon on a sound card. I just started working on a Stern Lectronamo that needs a lot of work – including no sound.

    This has been a very educational and enjoyable few days. I know I’ve learned a lot. I want to thank everyone for their suggestions, comments and mostly their knowledge.

    Thanks

    Bob

    #46 1 year ago
    Quoted from CactusJack:

    00000000
    00000001
    00000010
    00000011
    00000100
    00000101
    00000110
    00000111
    00001000

    0
    1
    2
    3
    4
    5
    6
    7
    8

    not sure about address line

    #47 1 year ago

    Thanks Zac, I think I have that in my file already. But I need to get a bunch of other stuff going first. I know I have power problems then connectors. It's really tired electronically.

    Bob

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