Turns out I was using the 28 series EPROM (AT28C256) on my 8080 CPU board,
which as you pointed out is not pin compatible with the 27C256 EPROM.
I did the 28' mod (lift pin 1 & 27, bridge 27&28, jumper pad 27 to lifted pin 1) but sadly
my shadow ROM LED doesn't go off when RUN switch lifted. Is it possible to damage any
of the components on the board or the EPROM by using the 28C256
without the mods?
I don't have any 27C256's EPROM's to try (ordered and on there way).
On 11/15/14 07:35 AM, Crusty OMO wrote:
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
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