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

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

Post by banman »

Hi eslapion,


Very useful information. :D

This reminds me of something interesting you said to me in another forum:


"Every wire represents a parasitic inductance (remember the telegrapher's equation ?) and this can induce ringing in the signal you're checking. See: https://en.wikipedia.org/wiki/Ringing_(signal)"

I feel I may have encountered it in some of the tests I performed.

You are right an oscilloscope is in order..





I have looked hard at making your PLA glitch detecting circuit less awkward than the one I have been using on the breadboard.


I have made a Vero board version of your circuit.

The circuit is as close as practical to the C64's PLA socket. It has reduced / more direct wiring to lessen parasitic inductance.

It has additional takeoffs for !ROML and !ROMH for oscilloscope measurement.


Image

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It seems to work OK.


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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 »

banman wrote: Wed Jun 26, 2019 9:54 amThis reminds me of something interesting you said to me in another forum:


"Every wire represents a parasitic inductance (remember the telegrapher's equation ?) and this can induce ringing in the signal you're checking. See: https://en.wikipedia.org/wiki/Ringing_(signal)"

I feel I may have encountered it in some of the tests I performed.

You are right an oscilloscope is in order..
Yup!

Nice little test circuit. The wires between your board and your probe are still waaaay too long.
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by banman »

Hi eslapion,


eslapion wrote: Wed Jun 26, 2019 1:05 pm Nice little test circuit.


Thank you very much! That means a whole lot coming from someone such as yourself. :oops:

eslapion wrote: Wed Jun 26, 2019 1:05 pm The wires between your board and your probe are still waaaay too long.
Yes, they are..... I will definitely remedy this....





Here are some initial observations using eslapion's PLA test circuit. I feel I have some of the wiring wrong due to some of the observations I am seeing. I have had this issue before. I cannot track down the faulty wiring.



The board under test which is pictured in the above post is a C64 Assy no. KU-14194HB board.









**************Winbond W27C512 45ns EEPROM (electrically erasable PROM).


The green LED light comes on at start up of the C64. Pushing the reset button turns the light off.


The green light will come on at random intervals while the C64 is powered on. Quite frequently actually.





**************The Ultimate PLA.



There is no green LED light at start up of the C64.


The green light stays off while the C64 is powered on.








**************The Commodore Factory PLA.



There is no green LED light at start up of the C64.


The green light stays off while the C64 is powered on.
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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 »

I think a few of your pictures are missing.
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by banman »

Hi eslapion,



Oops.... Apologies for that omission...... :shock:





**************Winbond W27C512 45ns EEPROM (electrically erasable PROM).


The green LED light comes on at start up of the C64. Pushing the reset button turns the light off.


The green light will come on at random intervals while the C64 is powered on. Quite frequently actually.







**************The Ultimate PLA.



There is no green LED light at start up of the C64.


The green light stays off while the C64 is powered on.








**************The Commodore Factory PLA.



There is no green LED light at start up of the C64.


The green light stays off while the C64 is powered on.



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

Post by banman »

Hi eslapion,


Yesterday I had a stroke of luck...

I thought I'd go down to a very old local electronics store in my area who was shutting down. He had been operating the business since the early 70's so I discovered while talking to him.

The gentleman there happened to have 60 or so 74LS279 chips..all different manufacturers.

I ended up getting 11 different variations of 74LS279 chips. I tried 3 different brands I think. Then I tried a forth one out. This one behaves as expected on C64 startup with The Ultimate PLA and Commodore Factory PLA. Sometimes it will not trigger the LED to go on when the C64 is started.

It is a Hitachi brand HD74LS279P.

Image


I am happy because this means that I have constructed eslapion's PLA test circuit correctly.

These are 10 of the 11 chips I purchased.......


DSCF4024 cropped.jpg
DSCF4022 cropped.jpg
DSCF4021 cropped.jpg




I am not sure why the others did not perform as expected. I believe them to be original and not fake copies as does the owner of the shop. The stock this gentleman had I would say was from the 80's.



The logic probe (now that is connected to correct voltage...eh hmm! ) actually does pick up the C64 power on glitch (probably not the right word in this instance) as described for the correct operation of eslapion's PLA test circuit.

The only explanation I can offer is that the 74LS279 chips that don't light up on C64 startup may have added characteristics to reduce the chance of a false trigger on initial power up...

OR in other words

The transition to a low state is so breif and/or not low enough to activate these 74LS279 chips and don't display the intended LED light ON sequence at C64 startup.








These are the original ones that I used. These do not perform as expected....

Image



I will test the remaining chips to see how they perform.

I will upload these new videos soon.......
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by banman »

Hi eslapion,


I have been testing these chips....

These ones work as expected in your PLA test circuit as expected
DSCF4027 cropped.jpg

DSCF4028 cropped.jpg



These chips don't seem to work as expected........

DSCF4029 cropped.jpg


Image
https://postimg.cc/bdjWq8MB
https://i.postimg.cc/T2y6q3cf/DSCF4026-cropped.jpg


Notice that this one is a Motorola that doesn't operate as expected, however It looks slightly different to the other Motorola ones that do work..... (no little dot below the orientation mark...)

Image



I am not saying that these are faulty.

In fact they work very well when testing the bad Winbond W27C512 EEPROM PLA substitute.....

Maybe these can be made to work correctly. Would maybe pre-powering these separately by a 5v power supply allow it to check for the very quick start up glitch on the C64?
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by radron5809 »

This may be off topic but seeing test circuit made me remember I want some of these connectors. They used to come with JiffyDos, it's a little connector that can hook to a leg on a chip. I think they would be great for testing things. Does anybody know what the connectors are called?
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Re: The Dos and Don'ts of using an EPROM as PLA in a C64.

Post by eslapion »

radron5809 wrote: Sun Jun 30, 2019 4:24 pm This may be off topic but seeing test circuit made me remember I want some of these connectors. They used to come with JiffyDos, it's a little connector that can hook to a leg on a chip. I think they would be great for testing things. Does anybody know what the connectors are called?
Logic analyzer probes.
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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 »

banman wrote: Sun Jun 30, 2019 4:42 amMaybe these can be made to work correctly. Would maybe pre-powering these separately by a 5v power supply allow it to check for the very quick start up glitch on the C64?
There are 74LS279 chips made by all sorts of different manufacturer and as such they all have slightly different response speeds and their exact threshold voltage will vary (TTL is considered from 1.25 to 1.5V).

Using a 74LS279 as a means of detecting undesirable spurious logic lows from a PLA will allow you to tell for sure if a PLA has problems but the opposite is not true. If a PLA will not trigger the test schematic I have suggested then it MAY be a good PLA replacement but using an oscilloscope, as I did is the only way to be perfectly sure.

The Rigol DS1052E is probably one of the least expensive scope you can get which will do the job but there are many others that may be even less expensive.
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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