Detailed explanation of communication principle between [industrial control old horse] single chip microcomputer and Siemens S7-200

One 、 Hardware and communication principle analysis

Siemens S7-200 series PLC Have RS-485 Serial port , So we should make MCS51 SCM and S7-PLC To communicate , Several communication modes can be adopted . One of them is that you can use MCS-51 Serial port and MAX485 Chip connection , Then with S7-200 PLC Of RS-485 Communication through port , The hardware connection is shown in the figure 1 Shown .

S7-200 PLC It is the most abundant small serial communication mode PLC, Support multiple communication protocols , Such as point-to-point interface protocol （PPI agreement ）、 Multipoint interface protocol （MPI agreement ） and PROFIBUS Protocol and free communication protocol . Free communication protocol is also called user defined protocol , Use free port mode , User defined communication protocols can be implemented , Connect multiple intelligent devices , It is very convenient to use , It has achieved great success in the third-party project access .

In free port mode ,PLC The serial communication interface of is controlled by the user , Through ladder diagram program and assembly language with the single chip microcomputer , To use the completion interrupt 、 Character reception interrupt 、 Transmission completion interrupt, etc , The communication protocol is completely controlled by the user . At this time, the MCU is in the host state , The MCU sends handshake signal actively ,PLC After receiving the signal, you can passively feed back the information .

chart 1 MCS-51 SCM and S7-200 Hardware wiring diagram of

Two 、 Communication system design

2.1 Communication protocol design
The definition is based on experience and relevant references , Define the protocol structure and parameters .
（1） The baud rate of communication is 9.6kbps, No verification ,8 Data bits ,1 Programmable bits ,1 Bit start bit ,1 Bit stop bit .
（2） The format of defining the data flow structure of the communication protocol is the starting code 、 Command code 、 Element first address 、 Number of bytes 、 Data blocks 、BCC Check code and end code .
● Start code ： It means single chip microcomputer and PLC Start sending data , Is the first character of the data stream , tell PLC Communication started , It can be used 00H Express

● Command code ： Indicates that the single-chip microcomputer is right PLC Various operations of ：
40H： Read the target component I、Q、V、M、SM、L、T、C The data or state of ;
41H： Modify the target component I、Q、V、M、SM、L、T、C The data or state of ;
42H： Force the target unit to be ON;
43H： Force the target unit to be OFF;

● Element first address ： Express PLC Internal component type and register address （ But it cannot represent a bit address ）. The first two bytes indicate the register type , The last two bytes represent the register number .00 00（H）：I Register area 01 00（H）：Q Register area .02 00（H）：M Register area 08 00（H）：V Register area ;

● Number of bytes ： Starting from the first address of the component , Read or write PLC Number of data blocks of components ： Ready to read or write PLC The data or state of ;

● BCC Check code ： During transmission , The instruction may be subject to any interference to distort the original data signal , The instruction at this time is of course wrong , To detect errors in the transmission of instructions , The receiving party must further confirm the instruction , To prevent erroneous instructions from being executed , The simplest way is to use the check code .BCC The method of the check code is the string to be transmitted ASCII Codes are XOR and in bytes , And transmit the XOR and as part of the instruction ; similarly , After receiving the instruction, the receiving party , XOR and... The received string in the same way , And compare with the value sent by the sender , If the values are equal , It means that the received instruction is correct , The opposite is wrong

● End code ： The end character marks the end of the instruction , In this case, it is defined as FFH, Different PLC The slave station can define different end characters to receive the data for the PLC Instructions .

2.2 Implementation of communication program
（1） The realization of single chip computer program . MCU initializes in the main program , Adopt serial port working mode 3[2], The baud rate is 9.6kbps, Single chip microcomputer is used as the host , towards PLC Make a call , Read data or write data regularly , See figure for the procedure flow chart 2.

chart 2 Single chip computer communication program flow chart

（2）PLC Implementation of end program flow chart .PLC The end acts as a slave , Use ladder diagram or STL Programming , It is mainly to set the communication protocol first , Then process the collected data according to the protocol , And then send it to the host microcontroller , The specific program flow chart is shown in Figure 3 Shown .

chart 3 PLC End communication program flow chart

3、 ... and 、 Conclusion

In this paper, the single-chip microcomputer and PLC Serial communication method of , Successfully applied to multiple projects , The practice shows that this method is simple and reliable , The cost is low , And it is easy to expand other economical and practical functions , Such as A/D、D/A And so on , Good social and economic benefits have been achieved .