The PLC System Program Design Based On IEC61131-3

As the standard of programming language of automation control system, IEC61131-3 standard is widely used and gains agreement. It's very important to develop products based on this standard.Based on the content about programming module in IEC61131-3, develop the system program of PLC and the communication function with PC.The users' programming environment is developed on Windows operating system and VC++, based on IEC61131-3 standard.The key technology has three parts. The first part is the design of physical hardware, which is finished by a hardware engineer. The second is the development of users' programming environment, including edit window development, algorithm of translating ladder diagram to executive codes, and serial port communication. The two parts are introduced simply.The main part is the system program of PLC, which is the bridge of physical hardware and users' program. The third part is the main point of this paper. The system program performed loading the users control program, users' program explanation and execution, error application, interrupt response and so on.The CPU of PLC used was Atmega64 singlechip. The development tools were necessary, and I used the ICCAVR compiler and AVR Studio emulator. After finishing the system program of PLC under ICCAVR complier, download it to flash memory of PLC through JTAG interface.The system program finished dynamic storage allocation, locating the memeory based on the number of variables users used. Most PLCs adopted fixed storage allocation, allocating the memory by supportable maximum. Compared with fixed storage allocation, the dynamic storage allocation avoided the limitation of sullortable maximum to users, and save a lot of memeroy. On the other hand, the system program expanded the instructs and converted the codes from four bytes to six bytes, which increased the program execution speed.To good comprehension, the paper first introduced the hardware and users' programming environment of PLC. Secondly, give a detailed introduction about the system program. The system programs were downloaded to the Atmega64 singlechip, and achieved the expectation effect in the emulation state.