The Dos and Don'ts of using an EPROM as PLA in a C64.

Disk drives, Monitors, SuperCPU etc.
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eslapion
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by eslapion » Thu Aug 01, 2019 6:22 am

Wow! What a stockpile! These sockets are worth money!
banman wrote:
Wed Jul 31, 2019 9:28 am
There is a varied selection of what I strongly believe are original 27512 EPROMs.

This will provide a good test reference for checking out the Glitch when I manage to get the oscilloscope up and running....

I think having a genuine ST 27C512 very similar to yours on hand will be a benefit when comparative testing on the same oscilloscope is run against the newer Knockoffs (fake) ST 27C512 90B1..B6's..FX1 from China.

My way of thinking is even if the oscilloscope isn't dead actuate at least all the results will out by the same margin. That is my guess anyway.....
I can't read the whole code on your photos so I don't know the speed code for most of the ST PROMs you have in there. I can tell you -15B6 means 150ns so these are much too slow to be used as PLAs.

They do have the correct form factor and country of origin.


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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by banman » Thu Aug 01, 2019 6:52 am

HI eslapion,

Thank you for the information about the meaning on the ST PROM ..15B6 being 150 ns.... I just checked them unfortunately the 4 are 150ns speed.


Damn... I thought I had a similar ST PROM to yours ..

They do look striking like your ST PROMs in printed and physical markings. That's what got my attention..

The windowed ST EPROMs in that same photo are ST M27C512 10F1. Would that be 100ns speed?

I understand that the slower E/PROM speeds would not be suitable as a working PLA substitute. With these slower E/PROMs do you think that the Glitch will still be present in a similar way that the ST M27C512-B6 running at 90ns on C64 start up?

Maybe not a loss after all. These may make good oscilloscope comparative test subjects to observe glitches....



Not to worry.... I will go thru what I have and document the E/PROMs for future reference. There is a lot to sort thru.

Basically it was like buy a full box and take your chances....

The turned socket holders look like good quality. I think they are made by Thomas & Betts...There is more still at the shop.....

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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by banman » Thu Aug 01, 2019 9:03 am

Hi eslapion,


I was just brainstorming....


Maybe I am looking at this the wrong way.

Would it be possible to make a stand alone circuit that would trigger an E/PROM in such a way that we could observe the output on an oscilloscope (i.e. the Glitch)?

I would be more than happy to give building the circuit a go. :)

I know it is not the spirit of this thread which is focused on the C64 PLA E/PROM substitute.

It would give a wider net to test different manufacturers who no doubt use different fabrication processes and circuit designs.

I have a good range of E/PROMs now....


My suspicion lies in the circuit design of ST E/PROMs whether they are original or knockoff (fake). I could be wrong . In fact I'd very much like to be proven wrong.

I think further observations with the oscilloscope is essential.

Just a thought....

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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by eslapion » Thu Aug 08, 2019 5:10 pm

banman wrote:
Thu Aug 01, 2019 9:03 am
Would it be possible to make a stand alone circuit that would trigger an E/PROM in such a way that we could observe the output on an oscilloscope (i.e. the Glitch)?
That's not as easy as it sounds. The reason why a glitch on outputs will appear depends on a combination of things.

The first one is there was an address change while the !CS !OE! both remained low - when a CPU reads data normally, one of the two will go high between each access, not so when using a memory chip as a logic chip. The second one depends on what the values are at both the initial address and the new one. It can also depend on the selected addresses themselves as the EPROM could temporarily 'see' an address where the stored value is 00 or any value that is 'equivalent' to a glitch. Finally, the fundamental electronics of the EPROM, for which the manufacturer will not provide a schematic is at play.

The C64 itself and the truth table of the PLA appear to be an ideal circuit for determining the presence of spurious logic low in the output. In other circumstances, there may be spurious logic highs as well but these are not an issue when it comes to using an EPROM as a PLA in a C64.

Sidenote:
If you happened to have found some of those super low profile Robinson-Nugent sockets with 40 or 48 pins in your recent find, I would be interested in buying.

Any 48 pins sockets with machined pins could be of interest too.
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by banman » Sat Aug 10, 2019 10:53 am

Hi eslapion,

Thank you for the clear explanation on the reasons for not creating a simple standalone circuit for testing the C64 PLA. :-)


Absolutely regarding the ic sockets. :-)

I will look out for them.

I have come across some sockets that have super long legs on them.

I plan to go back on Monday to have another look. There is more stuff to come up from the service centre.

I have seen an old BK Precision digital counter and a BWD (I think they might be made in Melbourne) sine wave generator that might be handy to have as a beginner.
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by eslapion » Sun Aug 11, 2019 7:43 am

banman wrote:
Sat Aug 10, 2019 10:53 am
I have come across some sockets that have super long legs on them.
It's wire wrapping sockets.
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders

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