Embedded PLC Design

Programmable Logic Controller (PLC) is a kind of digital arithmetic operational electronic system. It is specially designed for the application of industrial environment. It use programmable memory to store inner program, execute logical arithmetic, order control, timing, count and arithmetic operation and such as user oriented instruction. It controlled different type of machines or product process by digital or analog Input/Output (I/O) system.Embedded system is a special computer system based on computer technology and take the application as the first important thing, their software and hardware can be cut out, and they also fit for application system that have strict request with function, reliability, cost, power waste and bulk. As very important basic equipment, PLC is called one of the three major pillars in Automation Industry.Embedded system becomes more and more important as the development of computer technology, semiconductor technology, and electronic technology. Especially from 1970s, embedded system based on application was well developed, and varies of microprocessors and micro-controllers were applied on many fields for the success of the large scale integration technology. Embedded system is based on the application of single chip, so far, many application systems based and single chip also widely applied. This kind of embedded system was used to control varies of equipments such as automobile electronic equipment, home electronic appliances, industrial equipment, communicational equipment, and so on.C8051F04x series is completely integrated mix signal micro-programmed control unit (MCU), integrated analog components, digital peripheral equipment and functional components,well compatible with 8051 kernel. CIP-51 adopt assembly line structure, compared with standard 8051, the instruction execute speed improved very much. C8051F04x series based on CIP-51 increased peripheral equipments, improved integer capability and much more adapt to real application. The digital switch net allow assigning inner digital system source ports to P0,P1,P2 and P3 port. It is different with standard multiplexing I/O micro controller, this kind structure can support all of the functional assembly. By set across switch control register, we can do the changing startup input of counter/timer, serial bus and hardware interrupt, output of comparator and set micro controller inner digital signal as the output of I/O ports.Embedded PLC kernel construct PLC function on the software platform in controller. Main functions including ladder diagram language, communication and user drive interface. The kernel structure is fulfill real time response as the precondition, design special structure according to functional plan, combine design the real time kernel and system function to get optimized code efficiency and good real time effect.In the thesis, under the developed project"Embedded systems development based on C8051F040 single chip", the paper presents design plan for the small PLC. The hardware is based on the high efficient 8 bit embedded microprocessor C8051F040, its input and output function is extended with its plenty of source on chip. The embedded PLC core programmer is taken as the center, and the needed application is developed, and is extended to a full and applied PLC system. Studied the kernel software of PLC core, including basic function, structure and realization of PLC core, managed and dispatched by PLC kernel.The basic components of PLC include CPU, memory, I/O system and communication 4 parts. The system programmer and user programmer of PLC was put in memory, the locale input signals were transmitted to CPU through I/O system, then CPU execute logical or arithmetic operation according to the instructions which were put in the user program memory. The instructions were transmitted to locale by I/O system and drive corresponding execute organ to finish the control task. The I/O system of PLC is the output channel of process state, parameter input and real time control signal. It provided varies of operation level and I/O interface template to fulfill level changer, electric insulation, serial/parallel changer, A/D and D/A changer function between controlled process and PLC I/O interface. Studied the PLC core hardware circuit based on the micro-controller C8051F040, including external RAM for power dropping, hardware watchdog circuit and the interface circuit for extent. The PLC core hardware including the input/output and the communication, and the hardware circuit for the small PLC is designed.The 14 points PLC software development of embedded PLC kernel mainly need programming user driver and user interruption program. User drive interface is user program embedded interface designed for DI/DO, AI/AO and special user function. It included 7 basic embedded interface and several interrupt entrance. Data store space including C8051F040 CPU memory and extend 64K power dropping RAM. These memory cells divide into system occupation, user embedded and PLC source 3 regions, the data store space is the 64K flash of CPU.CAN net is a subordinative construction, main node initiatively transmit commands to other nodes and get the information of nodes. 485 net is also subordinative construction, by ladder diagram set network, including net equipment address, net communication data content, net communication speed, and so on.By applied PLC kernel program, designed detail PLC application software. At last, combining real application system, basic function and basic instruction set is tested. By control a simple dolly, given the system ladder diagram program, results indicating sufficed for the basis function demands of PLC. Meanwhile, introduced how to extend and connect CAN net and 485 net.