Next EPROM Question - Sizes

The only real difference betwen those 2 parts is that the former has twice as many locatiuons, and therefore 1 extra address pin. There's nothing to stop you holding any of the address pins in a contant logic state for all time. So if you program the 16K byte image in the first half of a 27256 and hold the A14 pin low, it'll behave as a 27128. Or program the image in the second hald and hold A14 high. Or, of course, program 2 images into the chip and select between them with a jumper conencted to the A14 pin. But please don't just program the same image into both halves of the 27156 and leave A14 floating, assuming it'll always apprar as a 0 or a 1 to the chip, and since the 2 halves contain the same data, it doesn't matter which it appears as. You _will_ have hard-to-find glitches if you do somehting like that. Other than the A14 input, the pinouts of the 2 devices are much the same in read mode. So there's not even mouch PCB modification to do. Also, IIRC, the pinout for programming a 27128 and a 27256 are rather more different, with Vpp moving to a different pin. But if you use a commercial programmer, that's taken care of. -tony