Re: What type op eeprom

[<markus@infoscape.com>]

Lasse Lafjell writes:
 > Hi,
 > I'm new to eprom programming, and I've just aqquired a Hi-Lo SEP-81AE
 > eprom
 > programmer.
 > So to get started, what type of EEPROM should I buy?
 > The purpose is to boot 3com 503's and 509's, and NE2000's.
 > 
 > The catalog of the dealer I use lists something called: X28C256S-25,
 > 32Kx8 EEPROM.
 > 
 > Would that fill the ticket?

Common EPROM types are "27xxx". The number following the "27" is the
size in kilobits. To get the more common size in kilobytes, you have
to divide by 8. So a 27256 is a 32kB EPROM. There might be additional
letters before, in, or after this number. They usually refer to the
manufacturer and to the manufacturing process. As most of these chips
are built in CMOS technology, you often see something like 27C256. If
there is another number appended, that is usually the access speed in
nanoseconds. You might have seen references to 27c256-120 or some
such. Common access speeds are 250ns and 120ns. More modern chips
might be even faster. For most NIC's 120ns is perfect; 250ns will
usually work, but some people have reported problems in the past.

One of the nice things about the 27xxx series of EPROM chips is the
fact, that it has been designed with backwards compatibility in
mind. As far as technically possible, all of the chips in this series
are pin compatible to each other. This means that if you cannot find
the exact size chip, that you need, it usually be replaced with the
next bigger size with minor or no changes to the schematics. For more
information, you should take a look at the datasheets that are
available from various chip manufacturers (most of them can be
downloaded from the web).

While EPROMs have been around for ever and while the 27xxx series of
chips is fairly standardized, this is not neccessarily true for
EEPROMs (the acronym means "electrically erasable programmable read
only memory"; some of them are also refered to as FlashROMs). Every so
often manufacturers come out with new EEPROMs that have various
subtleties. These chips usually try to mimick one of the 27xxx chips
and that is why they are often called 28xxx or 29xxx. Usually (but
this is not always the case), you can directly replace a 27xxx with
one of these EEPROMs. Things get more difficult, if you want to
program these chips. Each of them has their own anachronisms and you
will have to check with the manuals for your NIC or EPROM burner to
figure out whether a chip is supported. As there are so many slightly
different chips out there, your best bet is to always read the
datasheets first.


Markus

-- 
Markus Gutschke                         Internet: markus@infoscape.com
Infoscape, Inc                          Phone:    +1-415-278-0137
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San Francisco, CA 94105

Disclaimer: The above message represents my personal opinion; It does
            not constitute an offical statement by Infoscape!

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