[Coco] EEPROM replacement for 27C256

[Gene Heskett]

On Sunday 28 June 2015 13:14:06 John B wrote:
> Thanks.  I was looking at the data sheet for the 29C256 and it seems
> to be pin for pin compatible with the 27C256.  Pin 1 is VSS on the
> 27C256 and W/E on the 29C256.  I ordered a few off eBay and when my
> TL886 gets here I will program one to see if it works.

Another piece of trivia folks.

As long as the data buss is the same width AND on the same series of 
pins, it doesn't matter one bit if data pin d0 is connected to the data 
line on the board named d0. Ditto for d1-d7 (or d15 for 16 bit wide 
chips) Because any sucn accidental or on purpose scrambling simplifies 
the board layout, scrambling of the data written, it will be unscrambled 
by the same "re-arrangement" on the subsequent read.  You will get back 
the same data you wrote in any event.

Exactly the same situation exists for the address lines, the operative 
again is that the same group of pins are being used.  The a0 thru a15 
(or a23, a31 in the case of wider chips) can be fed to the chip in any 
sequence.

As long as those conditions are met, it will work as expected.

I can think of a flash eprom that would need exact matching because they 
often have to erase and rewrite a whole 4k block to change one bit.  The 
cocosdc might, probably is subject to that exception, but Darren can 
correct me.

> On Sun, Jun 28, 2015 at 12:17 AM, Dave Philipsen <dave at davebiz.com> 
wrote:
> > There is a 28C256 that is as probably as close as you'll get in
> > terms of pinout.  It won't be an exact drop-in replacement, though,
> > as the A14 pin changes position and you'll have to come up with a
> > write enable as, obviously, the 27C256 which is a primarily
> > read-only device does not have. I have a 28C256 floating around here
> > somewhere that I used in a custom circuit I built one time but I've
> > never tried to wire it up to a CoCo3. One thing for sure, you'd have
> > to make sure that any changes written to the EEPROM contain no
> > undesirable code that could cause the CoCo to become unbootable. 
> > Otherwise you'd have to pull the 28C256 and re-program it out of
> > circuit.
> >
> > Dave Philipsen
> >
> > On 6/25/2015 2:43 PM, John B wrote:
> >> Does anyone know of a suitable EEPROM replacement for the 27C256
> >> found in the CoCo3?
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco

Cheers, Gene Heskett
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