The Siemens SIMATIC series PLC was born in 1958 and has gone through the C3, S3, S5, and S7 series, becoming a widely used programmable controller.   1. Siemens' product was first launched in 1975 as the SIMATIC S3, which is actually a binary controller with a simple operating interface.   2. In 1979, the S3 system was replaced by the SIMATIC S5, which widely used microprocessors.   3. In the early 1980s, the S5 system was further upgraded - the U-series PLC, with commonly used models including S5-90U, 95U, 100U, 115U, 135U, and 155U.   4. In April 1994, the S7 series was born, which has advantages such as more internationalization, higher performance level, smaller installation space, and better Windows user interface. Its models are: S7-200, 300, 400.   5. In 1996, Siemens proposed the concept of PCS7 (Process Control System 7) in the field of process control, integrating its advantages of WINCC (Windows compatible operating interface), PROFIBUS (industrial fieldbus), COROS (monitoring system), SINEC (Siemens industrial network), and control technology.   6. Siemens proposed the concept of TIA (Total Integrated Automation), which is a fully integrated automation system that integrates PLC technology into all automation fields.   The S3 and S5 series PLCs have gradually withdrawn from the market and ceased production. The S7 series PLCs have developed into the control core of Siemens automation systems, while the TDC system continues to use the SIMADYN D technology core, which is a further upgrade to the S7 series products. It is the most cutting-edge and powerful programmable controller in Siemens automation systems.

operational principle   After the PLC is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution, and output refresh. Completing the above three stages is called a scanning cycle. During the entire operation period, the CPU of the PLC repeatedly executes the above three stages at a certain scanning speed.       Collapse input sampling   In the input sampling stage, the PLC sequentially reads in all input states and data in a scanning manner, and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it enters the user program execution and output refresh phase. In these two stages, even if the input state and data change, the state and data of the corresponding units in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scanning cycle to ensure that the input can be read in any case.       Collapse User Program Execution   During the execution phase of the user program, the PLC always scans the user program (ladder diagram) in a top-down order. When scanning each ladder diagram, always scan the control circuit composed of each contact on the left side of the ladder diagram first, and perform logical operations on the control circuit composed of the contacts in the order of left to right, top to bottom. Then, based on the results of the logical operations, refresh the corresponding bit status of the logical coil in the system RAM storage area; Or refresh the state of the corresponding bit of the output coil in the I/O image area; Or determine whether to execute the special functional instructions specified in the ladder diagram.       That is, during the execution of the user program, only the state and data of the input points in the I/O image area will not change, while the state and data of other output points and software devices in the I/O image area or system RAM storage area may change. Moreover, the program execution results of the ladder diagram listed above will affect the ladder diagram below that uses these coils or data; On the contrary, in the ladder diagram below, the status or data of the refreshed logic coil can only be applied to the program above it in the next scanning cycle.       Collapse Output Refresh   After scanning the user program, the PLC enters the output refresh stage. During this period, the CPU refreshes all output latch circuits according to the corresponding state and data in the I/O image area, and then drives the corresponding peripheral devices through the output circuit. At this point, it is the true output of the PLC.       The same number of ladder diagrams, with different order of arrangement, result in different execution results. In addition, there are differences between the results of scanning user programs and the results of hard logic parallel operation of relay control devices. Of course, if the time occupied by the scanning cycle can be ignored for the entire run, then there is no difference between the two.

Reliable folding PLC does not require a large number of active components and connected electronic components. Its connections are greatly reduced. At the same time, the maintenance of the system is simple and the maintenance time is short. Plc adopts a series of reliability design methods for design. For example, redundant design. Power outage protection, fault diagnosis, information protection and recovery. PLC is a control device specially designed for industrial production process control, which has a simpler programming language and more reliable hardware than general computer control. Adopting a refined and simplified programming language. The programming error rate is greatly reduced. Easy to fold and operate PLC has high operability. It has the characteristics of simple programming, convenient operation, and easy maintenance, and is generally less prone to operational errors. The operation of PLC includes program input and program change operations. The input of the program can be directly displayed, and the operation of changing the program can also be directly searched or searched by the program based on the required address number or contact number, and then changed. PLC has multiple programming languages available for use. Used for ladder diagrams that are closer to electrical schematics. Easy to grasp and understand. The self-diagnosis function of PLC reduces the requirements for maintenance skills of maintenance personnel. When a system malfunction occurs, maintenance personnel can quickly locate the location of the malfunction through self diagnosis of hardware and software. Flexible folding The programming languages used by PLC include ladder diagram, Boolean mnemonic, function diagram, function module, and statement description programming language. The diversity of programming methods makes programming simple and expands its application scope. The operation is very flexible and convenient, and it is very easy to monitor and control variables. Siemens PLC S7-300 series PLC installation and precautions: 1、 The auxiliary power supply has low power and can only drive low-power equipment (such as photoelectric sensors); 2、 Generally, PLCs have a certain number of occupied points (i.e. empty address wiring terminals), do not connect the wires; 3、 PLC has the issue of I/O response delay, especially in fast response equipment, which should be paid attention to. 4、 There are relay type and transistor type outputs (suitable for high-speed output), and the output can directly carry light loads (LED indicator lights, etc.); 5、 The input/disconnection time should be greater than the PLC scanning time; 6、 There is no protection in the PLC output circuit, so protective devices such as fuses should be used in series in external circuits to prevent damage to the PLC caused by load short circuits; 7、 Do not connect the AC power cord to the input terminal to avoid burning out the PLC; 8、 The grounding terminal should be independently grounded and not connected in series with other equipment grounding terminals. The cutting surface of the grounding wire should not be less than 2mm2; 9、 The input and output signal lines should be routed separately as much as possible, and should not be in the same pipeline or bundled together with the power line to avoid interference signals and misoperation; The signal transmission line adopts shielded wire and the shielded wire is grounded; To ensure signal reliability, the input and output lines are generally controlled within 20 meters; Expansion cables are susceptible to noise and electrical interference, and should be kept away from power lines, high-voltage equipment, etc.