PLC is programmable controller, its control circuit diagram in addition to electrical wiring diagram, mainly its internal ladder diagram. Ladder diagram and general electrical control wiring diagrams are not very different. It is "coil energized, contacts change state; coil de-energized, contacts return to their original state".
PLC programming is an electronic system of digital operations designed for use in industrial environments. It employs programmable memory, which is used to store instructions within it for performing operations such as logical operations, sequential control, timing, counting, and arithmetic operations, and to control various types of machinery or production processes through digital and analog inputs and outputs.
Programmable controllers and their related equipment should be easy to make the industrial control system to form a whole, easy to expand its function of the principle of design.
With the rapid development of microprocessors, computers and digital communication technology, computer control has been extended to almost all industrial fields.
Modern society requires the manufacturing industry to respond quickly to market demand, the production of small quantities, multiple varieties, multi-specification, low-cost and high-quality products, in order to meet this requirement, production equipment and automatic production line control system must have a very high degree of reliability and flexibility, PLC programming is precisely in line with this requirement, it is a microprocessor-based general industrial control device. It is a microprocessor-based general-purpose industrial control device.
Principle of operation
PLC is the use of "sequential scanning, continuous cycle" way to work. That is, in PLC operation, the CPU according to the user according to the control requirements of the program prepared and stored in the user's memory, according to the instruction step number (or address number) for cyclic scanning, such as no jump instruction, the first instruction from the beginning of the sequential execution of the user's program one by one, until the end of the program.
Then re-return to the first instruction to start the next round of new scanning, in each scanning process, but also to complete the sampling of input signals and the output state of the refresh and so on.
PLC a scan cycle must go through the input sampling, program execution and output refresh three stages.
PLC in the input sampling stage: the first scanning mode in order to read all the temporary existence of input latches in the input terminal on and off state or input data, and will be written to the corresponding input status register, that is, refresh the input, and then close the input port, enter the program execution stage.
PLC in the program execution stage: according to the user program instructions stored in the order of scanning the execution of each instruction, after the corresponding operation and processing, and then write the results into the output status register, the output status register of all the contents of the program with the implementation of the change.
Output refresh phase: when all instructions are executed, the on/off state of the output status register is sent to the output latch during the output refresh phase, and output through a certain way (relay, transistor, or inter-crystal tube) to drive the corresponding output devices.