[Coco] EEPROM replacement for 27C256

[Dave Philipsen]

Good point.  A couple of reasons for uniformity might be for ease of 
programming with standardized programmers and for certain internal 
page/block write operations operations that may require certain address 
lines to be assigned correctly.  Also, the 28C256 polls the D7 line to 
verify when a programming operation has finished although that could be 
taken care of through software.

As far as the 29C256 is concerned, according to the strictest sense of 
nomencalture  it is not an EEPROM but a FLASH.  I don't think you can 
write single bytes with the 29C256 like you can with the 28C256.  But 
the 29C256 might be just the ticket for you if you understand the way it 
works.

Dave Philipsen

On 6/28/2015 12:55 PM, Gene Heskett wrote:
> 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