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I read an electronic magazine in which there is a report on the controlling functions gets important in programmable logic. After reading, I became interested in programmable logic. So I searched some related materials and information about this topic from google. Today I come to this forum to turn to you to discuss my understandings are appropriate. Here are my personal understandings: The selection includes the selection of calculation functions, control functions, communication functions, programming functions, diagnostic functions and processing speed. 1, computing functions Simple programmable logic controller computing functions include logic operations, timing and counting functions; general programmable logic controller computing functions include data shift, comparison and other computing functions; more complex functions such as algebraic operations, data transmission; Large programmable logic controllers also have analog PID operations and other advanced computing functions. With the emergence of an open system, there are communication functions in the programmable logic controller, some products have communication with the lower computer, some products with the same position or the host computer communication, and some products also with the factory or business Network for data communication function. Design selection should be based on the actual application requirements, the rational use of the required computing functions. Most applications require logic operations and timing counting functions. Some applications require data transfer and comparison. When used for analog detection and control, algebraic operations, numerical conversions, and PID operations are used. To display data, you need to decode and encode operations. 2, control function Control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., should be determined according to control requirements. Programmable logic controller is mainly used for sequential logic control, therefore, most occasions often use single-loop or multi-loop controller to solve the analog control, and sometimes also use a dedicated intelligent input and output unit to complete the required control functions, Programming logic controller processing speed and saving memory capacity. For example, the use of PID control unit, high-speed counter, with speed compensation of the analog unit, ASC code conversion unit. 3, communication function Large and medium programmable logic controller systems should support a variety of fieldbus and standard communication protocols (such as TCP / IP), and should be connected to the plant management network (TCP / IP) when needed. The communication protocol shall conform to the ISO / IEEE communication standard and shall be an open communication network. Programmable logic controller system communication interface should include serial and parallel communication interface, RIO communication port, common DCS interface, etc .; large and medium-sized programmable logic controller communication bus (including interface devices and cables) should be 1: 1 redundant configuration , Communication bus should meet international standards, communication distance should meet the actual requirements of the device. Programmable logic controller system communication network, the higher the network communication rate should be greater than 1Mbps, communication load is not greater than 60%. The main form of communication network of programmable logic controller system has the following forms: 1) PC as the main station, more than the same type of programmable logic controller for the slave, composed of simple programmable logic controller network; 2) 1 programmable logic controller as the main station, the other with the same type of programmable logic controller for the slave, constitute the master-slave programmable logic controller network; 3) Programmable logic controller network through a specific network interface to connect to a large DCS as DCS subnet; 4) Dedicated programmable logic controller network (each vendor's dedicated programmable logic controller communication network). To reduce the CPU communication tasks, according to the actual needs of the network composition, should have different communication functions (such as point to point, field bus,) communication processor. 4, programming function Off-line programming mode: Programmable logic controller and programmer Common a CPU, programmer In programming mode, CPU only for the programmer to provide services, not on-site equipment control. After programming, the programmer switches to run mode, and the CPU controls the field device and can not program it. Offline programming can reduce system costs, but the use and debugging is not convenient. Online programming: CPU and programmer have their own CPU, the host CPU is responsible for field control, and in a scan cycle with the programmer for data exchange, the programmer to the online program or data sent to the host, the next scan cycle, The host will run according to the newly received program. This method is costly, but the system debugging and easy operation, often used in large and medium-sized programmable logic controller. Five standard programming languages: sequential function chart (SFC), ladder diagram (LD), function block diagram (FBD) three kinds of graphical language and statement table (IL), structural text (ST) two kinds of text language. The selected programming language should comply with its standard (IEC6113123), and should also support multiple language programming forms, such as C, Basic, etc., to meet the control requirements of special control situations. 5, diagnostic function The diagnostic functions of the programmable logic controller include hardware and software diagnostics. Hardware Diagnostics Determines the hardware's fault location, hardware diagnostics, and diagnostics by hardware logic. Through the software on the PLC internal performance and function diagnosis is the internal diagnosis, through the software on the programmable logic controller CPU and external input and output information exchange function is diagnosed outside the diagnosis. The strength of the diagnostic function of the programmable logic controller directly affects the technical requirements for the operation and maintenance personnel and affects the average maintenance time. 6, processing speed The programmable logic controller operates in a scan mode. From the real-time requirements, the processing speed should be as fast as possible, if the signal duration is less than the scan time, the programmable logic controller will not scan the signal, resulting in signal data loss. Processing speed and the length of the user program, CPU processing speed, software quality and so on. At present, the programmable logic controller contacts fast response, high speed, each binary instruction execution time of about 0.2 ~ 0.4Ls, it can adapt to the control requirements, the corresponding requirements of the fast application. The scan cycle (processor scan cycle) should be satisfied: the scanning time of the small programmable logic controller is not more than 0.5ms / K; the scanning time of the medium and large programmable logic controller is not more than 0.2ms / K. Ps: Excuse me if I was wrong in words or expressions as I am a green hand in the field of programmable logic. I need continual learnings. What is your idea ? Do you agree with my ideas ? Any of your ideas would be highly appreciated. May someone would like to help ? thanks in advance