Re: ROMs replacement

From: Gerrit Heitsch <>
Date: Tue, 09 Oct 2012 17:22:23 +0200
Message-ID: <>
On 10/09/2012 12:35 PM, wrote:

> 0. First 12 address bits change, A12 (13, 14) remains invalid
> 1. _CS goes LO, A12 (13, 14) remains invalid
> 2. Addresses are read and are being processed by the EPROM,  A12 (13, 14) remains invalid
> 3. A12 (13, 14) changes
> 4. Correct addresses are read and start being processed, all address bits valid
> 4. Data pops up on the data bits (as per state in point [2.]) --> ERROR!
> 5. Next data show on data bits (as per state in point [4.])..
> The question is how the EPROM behaves when addresses are changed within tACC - will it first deliver the data according to original addresses and then according to the new or will it discard the original and deliver the new only (albeit later)?

The EPROM will first deliver the invalid data since it doesn't know 
anything about the change at the input. tAcc of an EPROM is the delay 
needed for the input signals to filter through the decoder, memory 
matrix and output drivers. (Assuming _CE and _OE are LOW)

> I was not sure but I tried to take A12 processing out of the equations by removing LS00 and delivering A12 directly to the EPROM (sockets of pins 2 and 6 shorted). Result: machine even less stable (sic!)

And that's strange. If it was a too long delay, it should become more 

Do you have a scope? The interesting signal would be to trigger on _CS 
going low and display PHI0 and one of the data lines and see where the 
EPROM output changes state and what happens before and after.


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Received on 2012-10-09 16:00:37

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