According to principle that writing a Boot Loader program in single-chip microcomputer can realize on-line self-programming updating of ladder diagram of PLC by user,the paper introduced a design of Boot Loader program of ATmega128 based on PLC application.The program writes object code of ladder diagram of PLC into FLASH of single-chip microcomputer through RS232 interface with imitating mitsubishi communication protocol.The experiment showed that it makes design of PLC application software be more flexibl...