Can i use AT28c256 or AT29c256 to replace 27256? and what is the different between AT29c252 and AT28c256?
AT29 is flash; AT28 is (parallel) EEPROM. AT27 is OTP EPROM. Given the similarity in part numbers, I'd expect the three devices to be reasonably similar other than the differences you'd expect from the different technologies. Programming them all will be different. Of course, you ought to check the data sheets to be sure. http://www.atmel.com/products/