Preface

In the near future, we need to make a MODBUS/RTU to MODBUS/TCP gateway. First, you need to understand the MODBUS communication protocol.
This article is used as a summary in the learning process, if there are errors, please advise.

Introduction to MODBUS protocol

The MODBUS protocol is the RS485 bus-based communication protocol first introduced by the American Modicon company (now acquired), with RS232/RS422/RS485 interface standards. Modbus has become an industry standard for communication protocols in the industrial field, and is now a common connection method between industrial electronic devices.
MODBUS protocol is aMaster-slave Serial asynchronous Half duplex letter of agreement.
Master-slave Means that there is a master and multiple slaves (maximum 240) in the communication equipment. The master can communicate with the slaves in both directions, and can communicate with a single slave, or all slaves can communicate (broadcast) at the same time. No response is required at this time . The slave station can only communicate with the master station, and the slave stations cannot communicate with each other.
If the slave station wants to communicate with the slave station, it needs to be sent to the master station first, and then the master station sends it to the slave station.
Serial asynchronous
Asynchronous serial communication refers to serial data transmission in which both parties use a character (including specific additional bits) as the data transmission unit and the interval between the sender's transmission of characters is not necessarily, and has the characteristics of irregular data segment transmission.
Half duplex
Half-duplex means that both parties in communication can only send or receive at a certain time, just like making a call, when one party is talking, the other party does not talk.

MODBUS communication format

MODBUS uses asynchronous serial communication, which is an agreement for single character transmission. The baud rate, check method, number of stop bits and other data contained in it can be set by yourself, as long as the communication parties are consistent.
The serial port in the single-chip microcomputer also uses this method, but it is TTL level, while MODBUS uses the RS232/RS422/RS485 interface standard, and the communication with the single-chip microcomputer requires level conversion.

Information frame format

The information frame format is a message, which includes the definition of the frame structure, frame sequence, and function codes and data. These contents are user-defined, and other sequences and formats have strict requirements. All adopting MODBUS communication protocol must comply with these conventions.
The basic format of one frame of MODBUS information is:Meter header + function code + data area + check code + stop code
The header is different according to different communication methods, but the header form of each communication method is the same, and the number of bytes is also fixed.
The function code is also prescribed, and the symbol and the function it represents are obeyed by everyone.
The length of the data area is different due to the different function codes, but the length of the data areaformatIt is fixed.
The format of check code and stop code also has corresponding fixed format.
The format of the function code and the data area is fixed in all communication modes, and the different communication formats of other parts have their own standards and need to follow the requirements of the communication format.