27256 in place of a 2764

Dave Dunfield dave06a at dunfield.com
Mon Jan 28 07:59:48 CST 2008

> > Under what conditions can a 27256 be used in place of a 2764?  I have a
> > schematic that calls for the latter, of which I have none, but I have lots
> > of the former.

> Ground the extra address pins and write the data to the lowest 8k of the
> ROM. They are pins 1 and 26.
> Make sure the grounded pins aren't attached to anything else on the
> circuit. 

Basically the extra two address lines come from:

Pin 27 was PGM on 2764 is A14 on 27256
Pin 26 was N/C on 2764 is A13 on 27256

These pins are *supposed* to be tied to +5 on a 2764 socket.

Pin 27 is tied high as per specs on the chip, and this is
usually the case.

Pin 26 is tied high for backward compatibility with 24 pin parts,
and this is not always implemented, sometimes it's N/C, sometimes
it's grounded.

Check your board and if this is the case (pins 26 and 27 tied high)
you can use a 27256 without any cuts/straps by programming your
code into the TOP 8K of the device. If pin26 is grounded, you can
put the code at the 3/4 boundary $6000) - Or you can do what I often
do and simply program it 4 times at all 8k blocks.

If you have to cut/strap, consider putting in jumpers or a DIP
switch which lets you have 4 "virtual EPROMS" which you can select.


dave06a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:

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