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Thread: Sick SuperPET

  1. #131
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    I would wait for the new ROM before proceeding.

    However, if you look at the four outputs from U36 (A, B, C and D) you can see what bank should have been selected.

    I note a little confusion in your posts above regarding what number to POKE to select a specific bank. Banks are numbered 0 through 15. This is the number you POKE. Therefore, to select bank 0 you POKE a 0, to select bank 7 you POKE a 7, all the way up to bank 15 when you POKE a 15.

    As the DRAM chips are 64K*1 bit; it is highly probable that if one bank of memory works OK, they should all work OK - but all of the addressing faults cannot be totally ruled out unless you test all banks.

    Dave

  2. #132
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    Hello everyone,

    I have an update on the SuperPET:

    The 2K EDIT ROM arrived from Dave_m. After installing this I am able to successfully switch between RAM banks in the 64K memory on the SuperPET daughterboard. I tested all this RAM and it checks out okay (if you were following this thread you will remember that one RAM chip was faulty and after replacing, all banks pass).

    Unfortunately, this did not solve the problem with the 6809 mode not functioning. Switching the computer to 6809 mode and powering on still results in a black screen. So there is something greater that is the problem here. The RAM is functional, the 6502 on the SuperPET board is functional (the computer works fine as a standard PET 8032). It's only when switched to 6809 CPU mode that it does not respond.

    So, as I see it I have several options:

    1. Determine if the 6809 is problematic. Probing? Just buying a new CPU chip?
    2. Replace the ROMs on the SuperPET board. Dave_m supplied me with replacements for two of the three ROMs, but it will require de-soldering the old ROMs and installing IC sockets at those locations.
    3. Probing the board to check for faults?


    Can anyone advise on the next best step to take? Again, I have a multi-meter, logic probe and desoldering iron at my disposal. No oscilloscope yet.

    I do feel encouraged. We are making progress. We have eliminated the 4K EDIT ROM memory conflict, and the faulty RAM chip as the problem, so we are narrowing things down.

    Any advice is appreciated.
    Last edited by Ral-Clan; February 17th, 2017 at 06:02 AM.

  3. #133

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    Check all the signals at the 6809. Don't assume the 6809 is bad.
    Be polite and I may let you live.

  4. #134
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    Quote Originally Posted by KC9UDX View Post
    Check all the signals at the 6809. Don't assume the 6809 is bad.
    Okay, now previously in this thread Daver2 had suggested to check the following:

    6809 pins to check are:

    34/35 two-phase input clocks.
    39 (TSC) should be 0.
    40 (/HALT) should be 1.
    37 (/RESET) should be 1.
    38 (LIC) should be pulsing high for every instruction that is executed.
    36 (AVMA) should pulse when a bus transfer is required by the processor.
    It may be worth checking /NMI, /IRQ and /FIRQ (pins 2, 3 and 4 respectively) to make sure they are high (and not jammed low in error).
    I have only just learned how to use a logic probe when you guys had me test the 6502 (which checked out okay). Because of my novice nature I want to make sure I do everything right. So, from my understanding the correct procedure is this:

    - I need to find the +5V pin on the 6809 and jumper the logic probe's power lead (red) on the logic probe to that
    - Logic probe's black lead (GND) goes to chassis of PET
    - Probe tip to the pins mentioned above

    Correct?

    Also, I do understand what to do when Daver2 says I should check to see if certain pins are high or low (0 or 1), but I am not sure what he means above when he says pins 34/35 are input clocks. What am I to look for on these pins?

    Lastly, I assume I should be seeing pulsing HIGH signals on pins 36 and 38, according the the information above? Correct?

    My apologies for the pedantic nature of my posts, but I do realize one misplaced Logic probe tip can ruin an otherwise good day (and a good PET), so I am trying to be very methodical.
    Last edited by Ral-Clan; February 17th, 2017 at 06:26 AM.

  5. #135
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    Glad to hear that Dave solved your 2K ROM issue and your RAM is now working. One (more) thing we can tick off the list that it is not!

    Please be as pedantic as you like - it is not only important that you understand what you are doing - but these fault finding techniques will help you in the future when it breaks again!

    The power supply (+5V and GND) for your logic probe can be obtained from anywhere on the PET main board or the SuperPET board. It doesn't have to be specifically from the 6809 CPU. Apart from that - yes correct.

    By 'input clocks' I mean they will have a square wave clock signal on them (i.e. they won't be a fixed logic '0' or '1' but will be oscillating between a '1' and a '0' constantly). The clock for the 6502 is on pin 37 - so if you put your logic probe tip on that pin you will see what a high frequency clock looks like from the perspective of your logic probe indicators. Putting the logic probe tip on pins 34 and 35 of the 6809 should yield a similar display on the logic probe indicators.

    Work through the signal list I posted and see what you get.

    I did have another thought though worthy of checking out. You have two CPU's on the SuperPET (a 6502 and a 6809). Only one of these should be operational at any one time. My above list of pins is looking at the 6809 when it should be operational - but just suppose the 6502 does not get shut down when the 6809 is active. Mayhem will ensue. I would also check the +5V pin on the 6502 CPU (pin 8) when the 6809 CPU is selected. The +5V should disappear from this pin... It should reappear again when the 6502 is selected.

    EDIT-1: Checking out the RAM has been a pretty good test of most of the 'boring' logic on the SuperPET board itself! We are trying to identify whether it is the 6809 or ROM that is at fault. You don't really want to be desoldering the ROMs and replacing them unless we know they are faulty. We should check all the other avenues first (in my opinion anyhow).

    EDIT-2: Just checking the 6809 instruction set for the NOP instruction. It is HEX(12). Just thinking whether a NOP generator for the 6809 would help diagnose this problem further? See http://www.6502.org/mini-projects/nop-gen/nop-gen.htm for an example of a NOP generator for the 6502 CPU if you have not come across one before. A NOP on a 6502 is HEX(EA) [hence the arrangement of '1' and '0' signals forced onto the data bus]. As a NOP for the 6809 is HEX(12) the data bus forcing arrangement will be different. And (of course) the +5V and GROUND pins will also be different. Let me think about this on the way home from work...

    EDIT-3: Yes, HIGH going pulses on the 6809 pins 36 and 38.

    Dave
    Last edited by daver2; February 17th, 2017 at 07:02 AM.

  6. #136
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    Hi Daver2,

    Okay, thanks for the clarification on procedure. I will check and report back.

    Please also note that I have indeed built a 6502 NOP generator, which I was going to use previously before diagnosis went in a different direction. So that tool is available to me when and if we need it.

  7. #137

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    daver2 beat me to it, but you can connect the 5V power for your probe anywhere there's 5V, even if it's some other power supply outside the PET, as long as the negative/ground on your probe is connected to 0V or chassis ground on the PET.

    My probe (don't assume yours will do this without consulting the manual) will operate on anything from (logic+1) to +16V (from memory, could be wrong about the 16V, it may be even higher). So I typically connect a 9V battery to it, and connect the 9V battery negative to chassis ground. The reason is that I find myself less likely to accidentally short something with the probe power connections, and, I get a little more freedom of movement with the 9V battery outside the case.

    Sometimes I go so far as to use a 14V bench supply, if I'm going to be poking around where there might be a 12V signal, just to make sure I don't damage my probe (probably wouldn't, but like to be safe).

    Be very, very careful with the logic probe. I like to wrap the probe tip with electrical tape, so that only the very tip is exposed. It's very easy to accidentally short two adjacent pins and fry something, otherwise. Sometimes though it's nice to be able to short adjacent pins intentionally, to force a line high or low. But, don't do that unless you really know what you're doing.
    Be polite and I may let you live.

  8. #138
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    Always better to have things in 'stereo' KC9UDX!

    So you've built your own 6502 NOP generator. You can use the same technique to make a 6809 NOP generator (but - as explained in my last post - the wiring will be different). You can then place the 6809 NOP generator in the 6809 socket and the 6809 CPU in the 6809 NOP generator. If the 6809 CPU is alive (and the support signals like the clock etc. are present) then the 6809 CPU should burst into life and start executing NOPS. We would then need to identify what was wrong elsewhere. Just a thought...

    Dave

  9. #139
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    QUESTION:

    When I am probing around for a +5V point on the SuperPET daughterboard with my multimeter, do I have to attach the ground probe of the meter to a "real" ground rail/point on the daughterboard, or is it safe to ground to the metal chassis of the PET?

    I seem to remember that at the very start of this thread, when I was testing for voltages on the various rails (to make sure the power supply worked) I had the black lead of the multimeter clipped to the negative terminal of the large capacitor near the power supply.

    This is to get a +5V for my logic probe (so I can test the 6809).

    Also, this will be relevant when I need to test if the 6502 +5V supply is cut off (or remains on) when the 6809 switch is engaged. Can I touch the red multimeter probe to the Vcc pin on the 6502 and ground the black probe to chassis? Or do I need to ground the probe to the Vss pin on the 6502?

    Don't want to shunt power the wrong way.
    Last edited by Ral-Clan; February 17th, 2017 at 04:43 PM.

  10. #140

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    Always keep your test gear negative lead connected to the 0V/"ground" nearest the power supply, or the chassis. There's a myriad of other places you could connect it, but that's the easiest and safest.

    If your multimeter is high impedance, you won't be shunting anything in any case. Even if it's a typical "low impedance" one, you have nothing to worry about.

    The only time you could damage something is if you are trying to measure voltage but have your meter set to resistance or current.
    Be polite and I may let you live.

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