EPROM tricks-of-the-trade Query

DarrenF

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I understand that one may generally substitute a 2732 in place of a 2716, by doubling up the image data. This works because the Vpp pin (21) on the 2716 is generally tied high in circuit; it's the A11 line on a 2732, so as long as it has a valid status (high or low) it will provide the proper data when programmed with a double-image.

Question is, is this same concept generally applicable to the 28-pin EPROMs? For example, can I use a 27128 in place of a 2764 (double-image, of course)? Or a 27256 in place of a 27128, or 2764?

Seems like it should work, as the added address lines in the bigger ROMs replace (in order) a non-connected pin, the PGM pin, and the Vpp pin. I'm thinking those would generally be held high or low in use... but I'm here to defer my theory to your experience.
 
Just make sure that the new addr lines are stable and it's fine. I suspect you'll find that these pins are nc on some boards, pulled low on others. You'll have to evaluate each board, because it's not impossible that they are valid addr lines on some boards (manufacturers sometimes plan for expansion).
 
If you find an address line that is NC on the board, you can just tie that pin high or low (whichever is easiest) with a little jumper wire on the back side of the board.
 
Or a little jumper wire on the chip itself. You want to avoid letting the unused address line float - even though the same data is in both banks, you still can get unusual problems if the unused address line is not tied high or low. A little 30 gauge wire soldered to the top of the pin and over to the ground pin or 5v will do it. You can do the same to the back of the board instead. Just be sure you don't do both tie one high and one low...

-Ian
 
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