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Hi Guys,
Using my best announcer voice....I interrupt this program to bring you some FYI.... The pin out for A14 are different between the 27C256 and 28F256. The 28F256 models a RAM chip. You can use the 29F256 as a drop in flash ROM replacement for the 27C256. If designing a circuit, you can leave pin 1 of a 27C256 floating, it's not connected. BUT do NOT leave this pin floating if you are using 29F256. I learned this the hard way. I don't know why, but the chip will continue to work but with an access time of a few milliseconds. Drove me nuts, because I would view content via the front panel of the IMSAI and it was correct, but the program did not work, my logic analyzer showed the slow access time. Once I tied pin 1 high, problem solved. PS. It is tied high on the ATAIR/IMSAI CPU card for this reason, so no mods are required. I did not add any jumpers to the ALTAIR/IMSAI CPU card to accept a 28F256. I know Rich Cini's designs usually include those... which in hind sight is a good option I over looked. I'll put it down on the V2 wish list. Cheers, Josh Bensadon |