An article to understand S7-200 series PLC serial communication protocol

S7-200 series PLC features a: strong features

1.S7-200 has 6 kinds of CPU module, can expand 7 expansion modules at most, expand to 256 digital I\O or 45 analog I\O, have 24KB program storage space and 10KB user data storage space at most.

2. Integrate six high speed counters with 13 operating modes and two high speed pulse generators/pulse width modulators. The maximum counting frequency of the high-speed counter of the CPU224XP is 200kHZ, and the highest frequency of the high-speed output is 100kHZ.

II: Advanced Program Structure

The program structure of the S7-200 is simple and clear. In the programming software, the main program, subroutines, and interrupt programs are stored in separate pages. Use local variables in each block. It is easy to quickly migrate programs to other projects. Subroutines use input and output parameters as software interfaces to implement structured programming. The S7-200's instructions are powerful and easy to master.

Three: Flexible and convenient memory structure

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.

Four: powerful, easy-to-use programming software

The programming software STEP7-Micro\WIN can use many languages ​​including Chinese. There are ladder diagrams, statement tables and function block diagram programming languages, as well as SIMATIC, IEC61131-3 two programming modes.

Five: Wizard Functions to Simplify Complex Programming Tasks

Programming and application such as PID control, network communication, high-speed input, high-speed output, position control, data recording, recipes, and text displays are difficult points in PLC program design. Programming them using common methods is both tedious and error-prone.

VI: Powerful Communication Function

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, and can be used with other S7-200, S7 without additional hardware. -300\S7-400PLC, frequency converter and computer communication. The S7-200 can use communication protocols such as PPI, MPI, Modbus RTU Slave, Modbus RTU Master, and USS, as well as Freeport communication mode.

Basic hardware composition of S7-200 series PLC

The S7-200 series PLCs are available in 4 different basic units and 6 models of expansion units. The system consists of a basic unit, an expansion unit, a programmer, a memory card, a writer, a text display, and the like.

1. Basic unit

The S7-200 series PLC can provide 4 different basic models of 8 kinds of CPUs for selection and use. The distribution of input and output points is shown in Table 4-11:

2. Extension unit

S7-200 series PLC mainly has 6 types of expansion units, which itself has no CPU, and can only be used in connection with the basic unit. It is used to expand the number of I/O points. The S7-200 series PLC expansion unit models and the distribution of input and output points are listed. 4-12 shows.

3. Compiler

When the PLC is officially running, no programmer is required. The programmer is mainly used to compile, store and manage user programs, and send user programs to 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 inexpensive. It is a very good on-site programming and monitoring tool, but its display function is poor. It can only be input by the instruction table, and it is not convenient to use. The intelligent programmer uses a computer to perform programming operations. The dedicated programming software is loaded into the computer. The ladder diagram language can be directly programmed to realize on-line monitoring. It is very intuitive and powerful. The dedicated programming software for the S7-200 series PLC is STEP7. -Micro/WIN.

4. Program memory card

In order to ensure the safety of the program and important parameters, the small PLC is generally equipped with an external EEPROM cartridge interface. Through this interface, the contents of the cartridge can be written to the PLC, and the programs and important parameters in the PLC can be transferred to the external EEPROM cartridge. As a backup. The program memory card EEPROM has two kinds of 6ES 7291-8GC00-0XA0 and 6ES 7291-8GD00-0XA0, and the program capacity is 8K and 16K respectively.

5. Writer

The function of the writer is to realize the program transfer between the PLC and the EPROM. The program in the RAM area in the PLC is solidified to the program memory card through the writer, or the program in the program memory card in the PLC is transmitted through the writer. Go to RAM area.

6. Text display

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 the input/output amount. The display of text information can display up to 80 messages with the selection/confirmation method, and the status of each message can be up to 4 variables. Process parameters can be displayed on the display and can be modified at any time. Each of the 8 programmable function keys on the TD200 panel is assigned a memory bit. These function keys enable parameter setting and diagnosis when the system is started up and tested.

S7-200 series PLC serial communication protocol

In the industrial automation control program, an automation control project usually consists of a number of control devices, such as lower PLCs, frequency converters, industrial control instruments, and other equipment. These devices can communicate with the master PLC through serial communication in the control system. Man-machine interface or industrial computer realizes data communication and realize centralized management of decentralized control functions. At present, PLC communication functions are applied more and more in projects. Below we use serial communication between S7-200 series PLC and Delta inverter to realize the control function through the man-machine interface.

Let's take a look at the communication protocols supported by the S7-200 series PLCs. See the table below:

Serial port pin definition of S7-200 series PLC

Among the communication protocols of the S7-200 series PLCs, the most used one is the Modbus protocol. Now Modbus is the world's most popular protocol in the industrial field. This protocol supports legacy RS-232, RS-422, RS-485, and Ethernet devices. Many industrial devices, including PLCs, DCS, smart meters, etc., are using the Modbus protocol as their communication standard. With it, the control equipment produced by different manufacturers can be connected into an industrial network for centralized monitoring.

Modbus has the following features:

(1) Standard, open, users can use the Modbus protocol free of charge, with no need to pay licensing fees, and will not infringe intellectual property rights. Currently, there are more than 400 manufacturers that support Modbus and more than 600 products that support Modbus.

(2) Modbus can support a variety of electrical interfaces, such as RS-232, RS-485, etc., and can also be transmitted on various media, such as twisted pair, optical fiber, and wireless.

(3) Modbus frame format is simple, compact and easy to understand. Users are easy to use, and vendor development is simple.

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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