My first try at EPROMs

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Floopy
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My first try at EPROMs

Postby Floopy » Wed Nov 08, 2017 3:27 pm

So I finally got some time and patients to try this out, here are the details...
I built the EPROM adapter so that I could try replacing ROMs inside my VIC-20. I found the wiring online and wiped out the soldering iron. After a few adapters had been made and some ROMs programed, I pop-ed open the hood and grabbed the character rom after installing the new rom I got a normal screen ( no garbled characters). After pulling it in and out I did not get a good screen, I kept getting a bad screen (see pictures), once in a while I got a good screen but the failure rate is 9/10. If I replace the EPROM with the original ROM I get no issues at all???A little bit of help?
And if your asking why I'm trying to replace the character ROM the answer is I'm not. This is only for testing purposes!
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Re: My first try at EPROMs

Postby norm8332 » Wed Nov 08, 2017 11:08 pm

The answer is in this thread http://sleepingelephant.com/ipw-web/bulletin/bb/viewtopic.php?t=7058#p76592 The charcter ROM had to be wired differently than the basic or kernal ROMs do. Apparently because of the way it is accessed by the VIC-1 directly. Also the EPROM apparently has to be faster than 300 ns. I haven't replaced a character ROM myself, It's usually the kernal that goes bad most of the time for some reason.

Norm

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Re: My first try at EPROMs

Postby Floopy » Fri Nov 10, 2017 11:18 am

I will try replacing the kernal and see if I get any issues. I'm just testing my adapters, this is only for testing purposes I have no reason to replace my character ROM at all.
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Re: My first try at EPROMs

Postby eslapion » Fri Nov 10, 2017 3:58 pm

Floopy wrote:...I pop-ed open the hood and grabbed the character rom after installing the new rom I got a normal screen ( no garbled characters). After pulling it in and out I did not get a good screen, I kept getting a bad screen (see pictures), once in a while I got a good screen but the failure rate is 9/10. If I replace the EPROM with the original ROM I get no issues at all???A little bit of help?

You're not providing any technical details that would allow helping you. The original VIC-20 character generator is a 2332 which uses 2 different independent chip select lines. Definitely the most complicated to replace and it usually requires an EPROM along with a logic gate.

To top the cake, a C64 CG will not work on the VIC-20 because CS2 is active high on the C64, while it is active low on the VIC-20.

And if your asking why I'm trying to replace the character ROM the answer is I'm not. This is only for testing purposes!

Using the kernal instead of the CG would have made your first experience much easier.
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Re: My first try at EPROMs

Postby Floopy » Sat Nov 11, 2017 12:04 am

eslapion wrote:The original VIC-20 character generator is a 2332 which uses 2 different independent chip select lines. Definitely the most complicated to replace and it usually requires an EPROM along with a logic gate.

To top the cake, a C64 CG will not work on the VIC-20 because CS2 is active high on the C64, while it is active low on the VIC-20.
.

Yea.....
I obviously have no experience in this, but this is why I'm trying.
norm8332 wrote:The answer is in this thread http://sleepingelephant.com/ipw-web/bulletin/bb/viewtopic.php?t=7058#p76592 The charcter ROM had to be wired differently than the basic or kernal ROMs do. Apparently because of the way it is accessed by the VIC-1 directly. Also the EPROM apparently has to be faster than 300 ns. I haven't replaced a character ROM myself, It's usually the kernal that goes bad most of the time for some reason.

Norm

I tried today and I had no issues at all, I think my ignorance is just the problem here. :oops:
-Floopy

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Re: My first try at EPROMs

Postby Floopy » Sat Nov 11, 2017 12:06 am

Well this confirms that my adapters work :) , I may try building something that look nicer later.
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Re: My first try at EPROMs

Postby Mike » Sat Nov 11, 2017 2:38 am

eslapion wrote:[...] The original VIC-20 character generator is a 2332 which uses 2 different independent chip select lines. Definitely the most complicated to replace and it usually requires an EPROM along with a logic gate. [...]

Nothing complicated about that. /OE and /CS of a 2732 with an adapter socket do the job, without any extra logic.

See the link to the relevant thread norm8332 pointed to in his post above.

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Re: My first try at EPROMs

Postby eslapion » Sat Nov 11, 2017 2:47 am

Mike wrote:Nothing complicated about that. /OE and /CS of a 2732 with an adapter socket do the job, without any extra logic.

Last time I checked, 2732 EPROMs were as rare as hen's teeth. Anyways, I got confused with the C64 CG which has one /CS active high and another one active low.

You can always use a 27256 or 27512 which are still being made, I guess.

A propos... I just checked Digikey and they now carry the 27C512 in surface mount only. Mouser still carries the DIP version but it's a clear sign this is going the way of the dodo.
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Re: My first try at EPROMs

Postby Mike » Sat Nov 11, 2017 3:14 am

eslapion wrote:Last time I checked, 2732 EPROMs were as rare as hen's teeth.

:lol:

I got confused with the C64 CG which has one /CS active high and another one active low.

http://sleepingelephant.com/ipw-web/bulletin/bb/viewtopic.php?t=8448&start=9

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Re: My first try at EPROMs

Postby eslapion » Sat Nov 11, 2017 5:47 am


Yeah, I remember. I must edit that post to specify the CS2 on the C64 CG is active high.
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Re: My first try at EPROMs

Postby norm8332 » Sat Nov 11, 2017 8:41 am

eslapion wrote:Last time I checked, 2732 EPROMs were as rare as hen's teeth.


I've been buying what I think are Chinese clones, like here: https://www.ebay.com/itm/172533017593 They have been working fine (time will tell). :D

Ebay is literally flooded with these "new" ST EPROMS right now.
Last edited by norm8332 on Sat Nov 11, 2017 3:21 pm, edited 1 time in total.

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Re: My first try at EPROMs

Postby eslapion » Sat Nov 11, 2017 2:23 pm

norm8332 wrote:Ebay is literally flooded with these "new" ST EPROMS right now.

Yeah, the datecode (1035) is quite interesting. There is no way SGS-Thomson was making more 2732 EPROMs in late 2010.

Also look at the size of the die. I have real 2732s here and they are 4 times bigger.
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Re: My first try at EPROMs

Postby Floopy » Sat Nov 11, 2017 5:27 pm

norm8332 wrote:
eslapion wrote:Last time I checked, 2732 EPROMs were as rare as hen's teeth.


I've been buying what I think are Chinese clones, like here: https://www.ebay.com/itm/172533017593 They have been working fine (time will tell). :D

Ebay is literally flooded with these "new" ST EPROMS right now.

What would you use 2K eproms for?
-Floopy

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Re: My first try at EPROMs

Postby Mike » Sat Nov 11, 2017 5:38 pm

Floopy wrote:What would you use 2K eproms for?

2732 EPROMs hold 32 kbit ^= 4 KB. That's entirely sufficient to store a small homebrew on cartridge, like I did with TRON Light Cycles.

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Re: My first try at EPROMs

Postby Floopy » Sat Nov 11, 2017 6:07 pm

:? I was dividing by 16 :oops:
I'm only making myself look even more ridiculous.
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