A text to understand the S7-200 series PLC serial communication protocol
1.S7-200 has 6 kinds of CPU modules, which can expand up to 7 expansion modules, and expand to 256 digital I\O or 45 analog I\O, with up to 24KB of program storage space and 10KB of user data storage space. 2. Integrated 6 high-speed counters with 13 operating modes and two-point high-speed pulse generator\pulse width modulator. The high-speed counter of the CPU224XP has a maximum count frequency of 200kHZ and a high-speed output with a maximum frequency of 100kHZ. The program structure of the S7-200 is simple and clear. In the programming software, the main program, subprograms and interrupt programs are stored in separate pages. Use local variables in each block. Easy to port programs to other projects. The subroutine uses the input and output parameters as a software interface to implement structured programming. The S7-200's commands are powerful and easy to master. The input (I), output (Q), bit memory (M), sequence control relay (S), variable memory (V) and local variable (L) of the S7-200 can be (bit), byte, word and Double word reading and writing. The programming software STEP7-Micro\WIN can use multiple languages ​​including Chinese. There are ladder diagrams, statement tables and function block diagram programming languages, as well as SIMATIC and IEC61131-3 programming modes. Programming and application of PID control, network communication, high-speed input, high-speed output, position control, data recording, recipe and text display are the difficulties in PLC programming. Programming them in a common way is both cumbersome and error-prone. The CPU module of the S7-200S has one or two standard rs-485/'target='_blank'>RS-485 ports, which can be used for programming or communication. It can be combined with other S7-200 and S7 without adding hardware. -300\S7-400PLC, inverter and computer communication. The S7-200 can use PPI, MPI, ModbusRTU slaves, ModbusRTU master and USS communication protocols, as well as free port communication mode. The S7-200 series PLC is available in 4 different basic units and 6 types of expansion units. The system configuration includes a basic unit, an expansion unit, a programmer, a memory card, a writer, a text display, and the like. S7-200 series PLC can provide 8 kinds of CPUs with 4 different basic models to choose from. The distribution of input and output points is shown in Table 4-11: S7-200 series PLC mainly has 6 kinds of expansion units. It has no CPU itself and can only be connected with the basic unit. It is used to expand the number of I/O points. The S7-200 series PLC expansion unit model and the number of input and output points are distributed as shown in the table. 4-12. The PLC does not require a programmer when it is officially running. The programmer is mainly used for programming, storing and managing user programs, and sends the user program into the PLC. During the debugging process, monitoring and fault detection are performed. S7-200 series PLC can use a variety of programmers, generally can be divided into simple and intelligent. The simple programmer is compact, simple and practical, and low in price. It is a good on-site programming and monitoring tool, but the display function is poor. It can only be input by the instruction list, which is not convenient enough. The intelligent programmer uses a computer to perform programming operations, and loads special programming software into the computer. It can be directly programmed in ladder language to realize on-line monitoring. It is very intuitive and powerful. The special programming software for S7-200 series PLC is STEP7. -Micro/WIN. In order to ensure the safety of the program and important parameters, the general small PLC has an external EEPROM card box interface, through which the contents of the card box can be written into the PLC, and the programs and important parameters in the PLC can be transferred to the external EEPROM card box. As a backup. The program memory card EEPROM has 6ES 7291-8GC00-0XA0 and 6ES 7291-8GD00-0XA0, and the program capacity is 8K and 16K steps respectively. The function of the writer is to realize the program transfer between the PLC and the EPROM. The program in the RAM area of ​​the PLC is solidified into the program memory card by the writer, or the program in the program memory card of the PLC is transmitted through the writer. Go to the RAM area. The text display TD200 is not only a display device for displaying system information, but also can be used as a control unit to modify a certain amount of values ​​or directly set an input/output amount. The display of text information can display up to 80 pieces of information with a selection/confirmation method, and the status of each item can be up to 4 variables. Process parameters can be displayed on the display and can be modified at any time. The eight programmable function keys on the TD200 panel are each assigned a memory bit that can be parameterized and diagnosed when the system is started and tested. In the industrial automation control program, an automation control project usually consists of several control devices, such as the lower PLC, inverter, industrial control instrument, and other equipment. These devices can communicate with the main control PLC through serial communication in the control system. The man-machine interface or industrial computer realizes data communication and realizes the function of centralized management and decentralized control. At present, the communication function of PLC is more and more applied in the project. Below we use the serial communication mode between S7-200 series PLC and Delta inverter to realize the control function through man-machine interface. Let's take a look at the communication protocols supported by the S7-200 series PLC, as shown in the following table: Among the communication protocols of the S7-200 series PLC, the most used one is the Modbus protocol. Modbus is now the most popular protocol in the industry. This protocol supports legacy RS-232, RS-422, RS-485, and Ethernet devices. Many industrial equipment, including PLCs, DCS, smart meters, etc., use the Modbus protocol as the communication standard between them. With it, control equipment produced by different manufacturers can be connected into an industrial network for centralized monitoring. Modbus has the following features: (1) Standard and open, users can use the Modbus protocol freely and confidently, without paying license fees or infringing intellectual property rights. At present, there are more than 400 manufacturers supporting Modbus and more than 600 products supporting Modbus. (2) Modbus can support a variety of electrical interfaces, such as RS-232, RS-485, etc., can also be transmitted on a variety of media, such as twisted pair, fiber, wireless and so on. (3) Modbus's frame format is simple, compact and easy to understand. Users are easy to use and simple to develop. Based on the openness and popularity of the Modbus communication protocol, this example also uses the Modbus communication protocol for communication.
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