Next EPROM Question - Sizes

Tony Duell ard at p850ug1.demon.co.uk
Mon Sep 17 17:41:05 CDT 2007


> So the question of the minute is this, is it possible to substitute a 
> 27C256 for a smaller 27C128?

The only real difference betwen  those 2 parts is that the former has 
twice as many locatiuons, and therefore 1 extra address pin.

There's nothing to stop you holding any of the address pins in a contant 
logic state for all time. So if you program the 16K byte image in the 
first half of a 27256 and hold the A14 pin low, it'll behave as a 27128. 
Or program the image in the second hald and hold A14 high. 

Or, of course, program 2 images into the chip and select between them 
with a jumper conencted to the A14 pin.

But please don't just program the same image into both halves of the 
27156 and leave A14 floating, assuming it'll always apprar as a 0 or a 1 
to the chip, and since the 2 halves contain the same data, it doesn't 
matter which it appears as. You _will_ have hard-to-find glitches if you 
do somehting like that.

Other than the A14 input, the pinouts of the 2 devices are much the same 
in read mode. So there's not even mouch PCB modification to do.

Also, IIRC, the pinout for programming a 27128 and a 27256 are rather 
more different, with Vpp moving to a different pin. But if you use a 
commercial programmer, that's taken care of.

-tony



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