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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 11th May 2022, 4:51 pm   #101
SiriusHardware
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Default Re: Perth PET

OK, let us know when you have gone through those checks.

Just to clarify, the situation you are now starting from is, what...

-Random characters on screen no matter whether you start the machine with...

Slothie EPROM in UD9
Daver2 EPROM in UD8 and original UD9 in UD9 position
Ram / ROM gizmo in place

No matter which of these options you try the switch on state is the same, just random characters on the screen?
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Old 12th May 2022, 12:39 pm   #102
ScottishColin
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Default Re: Perth PET

Who knew that using a meter for hours and hours could be so much fun.

Nothing jumping out at me yet, but I'm not finished (bearing in mind how many chips were replaced on here, there's a lot of socketed chips to be tested).

To answer your questions:

1. Random characters as post 92 with the original 6502 in place.
2. Same with Slothie EPROM in UD9
3. Same with Daver2 EPROM in UD8 and original UD9 in UD9 position
4. ROM/RAM board in place with a different 6502 - I get the attached screen with BASIC 4.0 selected, ROM and RAM replaced with those on the ROM/RAM board and no keyboard function.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
OK, let us know when you have gone through those checks.

Just to clarify, the situation you are now starting from is, what...

-Random characters on screen no matter whether you start the machine with...

Slothie EPROM in UD9
Daver2 EPROM in UD8 and original UD9 in UD9 position
Ram / ROM gizmo in place

No matter which of these options you try the switch on state is the same, just random characters on the screen?
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Old 12th May 2022, 1:01 pm   #103
SiriusHardware
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Default Re: Perth PET

Looks like the RAM/ROM is managing to do more because it doesn't depend on so much of the system working, but even then something is upsetting it.

Let us know when you've either had that Aha! moment, or you're ready to carry on.
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Old 12th May 2022, 2:13 pm   #104
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Default Re: Perth PET

OK - I can continue but my eyes are crossing now.

I have got the NOP tester out - should I start at post 650 from here?

https://www.vintage-radio.net/forum/...op#post1345717

Colin.
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Old 12th May 2022, 3:14 pm   #105
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Default Re: Perth PET

Yes, give yourself a rest and proceed as from that post in the original thread with the NOP test. For a quick go / no go indication that the NOP test is working, you should see a steady squarewave waveform on the 6502 'A0' pin if it is working.

When you get to the point where what you see is not what should be, flag it up.
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Old 12th May 2022, 6:04 pm   #106
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Default Re: Perth PET

To be more precise that would be the 'AB0' pin of the 6502, (pin 9) that you would be looking for the initial activity on when the NOP rig is in place.
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Old 12th May 2022, 6:11 pm   #107
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Default Re: Perth PET

Ok. Every single pin has a square wave, but they are all moving between 166khz and 250khz (ie flipping between those two values).

The scope is showing the voltage as between 0and -5v for the max/min values.

Colin.
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Old 12th May 2022, 6:41 pm   #108
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Default Re: Perth PET

The most likely reason for that odd minus offset is that your scope GND is not actually on GND where it should be, but rather on 5V. Either that or there is something not right about the way the scope is set up, possibly it is set to a mode in which it inverts the signal. Check that the scope GND really is connected to circuit 0V/GND and then put the probe on a known 5V, the reading should indicate +5V. If it still shows a minus reading you may need to untangle your scope settings before proceeding further.

When you say every single pin, you mean all of the AB0 to AB15 pins, or you really mean every single pin of the 6502...?

Last edited by SiriusHardware; 12th May 2022 at 6:48 pm.
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Old 12th May 2022, 7:28 pm   #109
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Default Re: Perth PET

OK - I found a god earth and the voltages are now 0 to +5v which is better.

I have only measured the address lines and they all have good square waves, but the frequencies are not what they should be. The frequencies are tricky to pin down but the seem to be 166 or 250 or 333 Mhz.

Would you like pictures of each of the Address lines from the scope or is that enough to know where to go to next?

Colin.


Quote:
Originally Posted by SiriusHardware View Post
The most likely reason for that odd minus offset is that your scope GND is not actually on GND where it should be, but rather on 5V. Either that or there is something not right about the way the scope is set up, possibly it is set to a mode in which it inverts the signal. Check that the scope GND really is connected to circuit 0V/GND and then put the probe on a known 5V, the reading should indicate +5V. If it still shows a minus reading you may need to untangle your scope settings before proceeding further.

When you say every single pin, you mean all of the AB0 to AB15 pins, or you really mean every single pin of the 6502...?
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Old 12th May 2022, 8:01 pm   #110
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Default Re: Perth PET

No, there is something fundamentally wrong at the moment, possibly the processor is not really executing NOP instructions but some other instruction instead.

Could you check and report the voltages / logic levels measured on the DA0 pin (pin 30) through to DA7 pin (pin 26). No need to show scope traces as we are expecting steady logic levels imposed by the pullup / pulldown resistors.
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Old 12th May 2022, 8:58 pm   #111
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Default Re: Perth PET

Correction, pins 33 through to pin 26 (but you knew that).
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Old 13th May 2022, 10:20 am   #112
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Default Re: Perth PET

If the current NOP investigations don't reveal anything it might be worth just verifying that the CS1 & CS2 signals to the PIAs & VIA are present and correct using the 'scope (pins 23 & 24 for all three chips). This would be after removing the NOP generator of course.

Alan
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Old 13th May 2022, 10:30 am   #113
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Default Re: Perth PET

pin 26 - no detectable voltage (actually reads at 471mv)
pin 27 - as pin 26
pin 28 - only one with a square waveform - 5.49v
pin 29 - straight line with a slight variation between 5.33-5.49v
pin 30 - as pin 26
pin 31 - straight line with a slight variation between 5.02-5.33v
pin 32 - as pin 26
pin 33 - straight line with a slight variation between 5.02-5.33v

Colin.
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Old 13th May 2022, 11:24 am   #114
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Default Re: Perth PET

Pin 28 shouldn't be jiggling like that, it should be in a steady state. Check that it is properly connected to either 0V or 5V (I forget which) by its pulldown / pullup resistor, also check that NONE of CPU pins 33-26 are connected to the motherboard DA0-DA7 lines.

The only thing any of those CPU DAx pins should be connected to is either +5V or GND via individual resistors. All other CPU pins should be connected through to their relevant pads on the motherboard.
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Old 13th May 2022, 11:33 am   #115
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Default Re: Perth PET

Since 6502 NOP is hex EA, that translates to 1110 1010, where pin 33 is the rightmost of those bits (DA0) and pin 26 is the leftmost (DA7).

Therefore, pin 28 (DA5) should be held steady high by a resistor going to +5V.
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Old 14th May 2022, 3:14 pm   #116
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Default Re: Perth PET

My plan is to make another (third) NOP tester to see whether I get the same results. I have to leave for several days tomorrow to see ill old people down south so it may be Thursday or so before I report back (unless I get time before I fly down tomorrow).

Colin.
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