Posted By
crock
on 2013-02-19
11:22:44
 |  Re: Using 27c256 to replace 27128
Very easily. The only difference between the two is pin 27, which is the program enable (/PGM) on a 27128 and A14 on a 27256. On the C16 and plus/4 boards pin 27 of the roms is tied to +5v, as you would expect.
So, to do a 1 for 1 swap, you don't need to do anything. Just make sure you put the image in the upper 16k of the 32k eprom.
If you want to be able to switch between 2 images, you need to have a method of switching pin 27 between ground and +5v. Unless you want to avoid taking a sharp instrument to the board and cutting the trace, I think the best way of doing this is to put the eprom in the socket with pin27 bent out of the socket.
Using a SPDT (single pole, double throw, aka "changeover") switch you need to wire it so the switch directs pin27 to either ground or +5v. If you have a SPST (single pole, single throw, aka "on-off") switch then you need to put a pull-up resistor (about 1k) between pin 27 and +5v and then wire the switch between pin27 and ground. When the switch is "on", the lower 16k will be selected and "off", the upper 16k.
cheers, Rob
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