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The programmable controller as an important foundation of the industrial automation equipment , play an increasingly large role in the industrial field . However , the high-end ( communications , numerical control , data acquisition , and other functions ) PLC core technology has long been in Europe, America , Japan and other foreign-funded enterprises to master . They monopolized the huge domestic market of the PLC . Foreign PLC high prices , not open , universal very difficult. Therefore , it is in great need to develop with independent property rights PLC . In recent years , the development of advanced high-performance single-chip technology , PLC hardware developed material basis . The development of a computer and microcontroller master-slave structure , open , modular , low-cost PLC PLC product development in a direction . In this context , we summarize the basic configuration of the programmable controller , working principle and its characteristics , to establish the structural model of open programmable control system , and the selection of the 51 core microcontroller STC89C51 PLC processor designed a small PLC , the PLC logic input , output , and communication functions : on this basis . Study 51 core microcontroller and Mitsubishi FX2n PLC instruction set , C51 implementation of the the PLC integrated development platform , development tools, Borland C Builder design and development of the PLC PC software , the software with the ladder editor , logical operations , data processing and other functions, and can compile the ladder program can be 51 single-chip CPU running the executable file . Paper a detailed analysis of the the topic PLC system hardware design of the the host computer foreground ladder output method , gives the compiler to explain the principles of the PLC instruction , the the C51 compiler system data format , and how by calling the C51 compiler , linker / locator , converter , converting the the ladder language to machine language . Finally, the paper analyzes the USB communication principle and system structure , and complete USB communication firmware programming and data communications test , the success of the USB communication application to the PLC .
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