strange behaviour with Eproms

[Marius]

Hi.

 

tonight I have been tweaking my Atari 800XL, and I was implenting a dual-OS Eprom (27C256). I had burned a few of them, and installed them with no problem in two of my XE computers.

 

then I upgraded my 800xl, and guess what... it did not work. My atari seemed to be dead.

tried another... again no luck...

The third one worked fine.

 

So I had two not-working 27C256's eproms, which gave no error on the burning process. And when I compared them afterwards, they appeared to be 100% ok too.

 

what could be the problem here?

are there some restrictions I don't know? speed?

 

Any suggestions would be appreciated.

Marius

[Bryan]

Look at the requirements for the VPP pin (1). Some manufacturers require that it be held in a certain state (usually Vcc) during use.

[Marius]

Look at the requirements for the VPP pin (1). Some manufacturers require that it be held in a certain state (usually Vcc) during use.

 

I don't understand this. Pin 1 has +5V ... isn't that on all 27c256's?

 

Marius

[Bryan]

Sometimes people forget to tie it to 5v and with some makes of EPROM it will still work. What are the speeds? I know some of the slower ones (like 450ns) won't work.

Edited February 19, 2009 by Bryan

[Marius]

Sometimes people forget to tie it to 5v and with some makes of EPROM it will still work. What are the speeds? I know some of the slower ones (like 450ns) won't work.

 

Hmm it was 200ns... strange is it.

 

I have been thinking your pin 1 comment. I was wrong. I was thinking of pin 28. that is +5v. So that might be the issue indeed.

 

How can I find out what to do?

 

Is there a difference between XE motherboards and XL motherboards?

 

Thanks

Marius

[+Larry]

Sometimes people forget to tie it to 5v and with some makes of EPROM it will still work. What are the speeds? I know some of the slower ones (like 450ns) won't work.

 

Hmm it was 200ns... strange is it.

 

I have been thinking your pin 1 comment. I was wrong. I was thinking of pin 28. that is +5v. So that might be the issue indeed.

 

How can I find out what to do?

 

Is there a difference between XE motherboards and XL motherboards?

 

Thanks

Marius

 

Did the eproms in question verify OK against the buffer when you burned them? I've certainly had some bad eproms from time to time, but never seen one that would work in one Atari and not another. That is weird. 200 ns. is fine. 250 is still fine.

-Larry

[Marius]

Sometimes people forget to tie it to 5v and with some makes of EPROM it will still work. What are the speeds? I know some of the slower ones (like 450ns) won't work.

 

Hmm it was 200ns... strange is it.

 

I have been thinking your pin 1 comment. I was wrong. I was thinking of pin 28. that is +5v. So that might be the issue indeed.

 

How can I find out what to do?

 

Is there a difference between XE motherboards and XL motherboards?

 

Thanks

Marius

 

Did the eproms in question verify OK against the buffer when you burned them? I've certainly had some bad eproms from time to time, but never seen one that would work in one Atari and not another. That is weird. 200 ns. is fine. 250 is still fine.

-Larry

 

Yeah. They are verified and even verified AGAIN. Even after I removed them out of the failing 800XL.

Perhaps the problem is related to the switching part. I might better have chosen another location for the +5 -> resistor. I took this from pin 28. Perhaps the eprom does not like that?

 

Marius