Electronics > Projects, Designs, and Technical Stuff
Cloning a Commodore PET-2001
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GK:

--- Quote from: MK14 on June 11, 2017, 07:08:42 am ---I see.
Yes, long term (decades) uncovered UV windowed EPROM's, are suppose to be in significant risk of dataloss, I agree!
--- End quote ---


The funky UV-EPROM behaviour has now been solved! When an EPROM is erased all bytes attain a value of $FF. It appears that all those years of storage in almost complete darkness without the window covered brought my EPROM to the very precipice of total erasure. What I was seeing on the screen (and you are literally seeing the individual bits of whole bytes here), with those bytes randomly alternating between $FF and the programed values were bytes that couldn't make their mind up if they were erased or not! It's curious to note that there was a hysteresis-like effect, where, initially, the application of light through the window for a short time actually restored the unstable data, but guess what? I went out for dinner this evening neglecting to stick that blob of Blu-Tack back over the window and now my EPROM is giving me nothing but $FFs! Well F me, now I'm screwed until my programmer turns up.
   
SeanB:
Tip for reading those EPROMS on the edge of erasure is to vary the supply voltage. If it is 4V5 it might read a degraded cell as a logic 1, but if you raise the supply voltage to 5V5 it will likely read it as a logic 0 instead, as the discrimination between logic levels is done with simple comparators and these are sensitive to supply voltage.

Old proper EPROM programmers did have a facility to verify the devices on both 4V and at 6V5, ensuring that the cells were well into the correct region, and would not slowly degrade with time leading to this erratic supply voltage dependancy.  You could either pop out the Vcc pin and supply it from an external supply, using a DIP socket stack for this, or supply the whole programmer from a USB hub and use an external power supply for the hub to up the voltage to give 5V5 on the Vcc pin when enabled.
boffin:
When you get it all working, you can display the easter egg by typing

WAIT 6502,20

(changing the last number changes the quantity)

As for RAM in the PET, the later 2001s (without the builtin cassette) used 4116s,  as I remembering soldering the 2nd row of them in mine.

Lastly, there's a pin on the GPIO that when held low or high (don't remember) and you fire an NMI, it takes to you to a simple ROM monitor.  You should be able to find details about that on the net.  Again, this might be a 2nd series option.
BrianHG:

--- Quote from: GK on June 11, 2017, 03:24:00 pm ---
--- Quote from: MK14 on June 11, 2017, 07:08:42 am ---I see.
Yes, long term (decades) uncovered UV windowed EPROM's, are suppose to be in significant risk of dataloss, I agree!
--- End quote ---


The funky UV-EPROM behaviour has now been solved! When an EPROM is erased all bytes attain a value of $FF. It appears that all those years of storage in almost complete darkness without the window covered brought my EPROM to the very precipice of total erasure. What I was seeing on the screen (and you are literally seeing the individual bits of whole bytes here), with those bytes randomly alternating between $FF and the programed values were bytes that couldn't make their mind up if they were erased or not! It's curious to note that there was a hysteresis-like effect, where, initially, the application of light through the window for a short time actually restored the unstable data, but guess what? I went out for dinner this evening neglecting to stick that blob of Blu-Tack back over the window and now my EPROM is giving me nothing but $FFs! Well F me, now I'm screwed until my programmer turns up.
 

--- End quote ---

Are you sure you don't have the out enable, or chip select, or upper address IO pin on your bread board open?  These CMOS eproms inputs can behave like 74HC logic and my just begin working due to charge on such an IO.  Try partially re-seating your eprom, bend the pinst and re-seat the wiring around it.

Such lemon breadboarding has caused me such headaches in the past.  Usually holding 1 finger on VCC and stroking the eprom IO pins, then once again with you finger on GND might reveal the true nature of your problem which I bet isn't a sensitivity to a little light entering the eprom window.

GK:

--- Quote from: BrianHG on June 12, 2017, 12:39:29 am ---Are you sure you don't have the out enable, or chip select, or upper address IO pin on your bread board open?  These CMOS eproms inputs can behave like 74HC logic and my just begin working due to charge on such an IO.  Try partially re-seating your eprom, bend the pinst and re-seat the wiring around it.

Such lemon breadboarding has caused me such headaches in the past.  Usually holding 1 finger on VCC and stroking the eprom IO pins, then once again with you finger on GND might reveal the true nature of your problem which I bet isn't a sensitivity to a little light entering the eprom window.

--- End quote ---


Totally sure, all the connection are fine. I tried Sean's suggestion of ramping up the supply voltage. 74HC logic is speced to 6V and in some cases won't blow until 9V or so, but I'm not too sure about my Fox crystal oscillator module, so I went to 6V, but it didn't make a difference unfortunately. It looks like the several hours under my strong bench light finally wiped this one clean.
 
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