Replace Eprom by Flash. Pinout.

[luiHS]

Some time ago I redesigned the PCB of an old board, and among other things I replaced the Eprom DIP memories with PLCC32. What I tried to do some time later is to use Flash memory instead of Eprom, both PLCC32.

But I got a surprise, the pinout is identical except for 2 pins that are swapped, A18 for Vpp/WE. I don't understand why the manufacturer decided to make this change, what use it has besides preventing Eprom or Flash from being used on the same board without any other change. This has forced me to redesign the PCB by changing only those two pins.

The Eprom is a 27c040 that I replace with a Flash SST39SF40. I already did something similar a long time ago in another old board, although in that case there were other changes to make and also the original Eprom was DIP and the Flash I put TSOP.

[DavidAlfa]

Seems strange to me, that's clearly intentional, they must have done it for some reason!
But go figure why! 

[SiliconWizard]

I wanted to check in the Jedec std, but unfortunately I don't have a full copy of it. (Should be JESD21-C.)

It's likely that one of them doesn't follow the Jedec std. Which is apparently not uncommon for EPROMS and Flash of that era.

[TomS_]

I had to make some adapter boards for some reverse engineering projects to swap A18 from the original socket/footprint to the other pin to suit a SST39SF040.

These use a Winslow Adaptics W9324-ZC-160 "PLCC plug", a PCB that is more or less 1:1 pin to pin, and a standard 32pin socket.

Would have been nice if they just followed the same pinouts, but I suspect I would have still needed these adapters anyway because the WE pin was connected to an additional address signal to permit the use of 8mbit ROMs in the original device, and during normal operation this would be low. So my adapter also has to pull WE high to prevent writing to the ROM.

[Gribo]

If you can manage with a smaller device, the 28F020 (Intel) or the AM29F020 (AMD) fit the original pinout.