What is Modbus, and How It Works
1.Application Scenarios of Modbus
Modbus is an open protocol, which means manufacturers can embed it into their devices without paying patent fees. It has become the standard communication protocol in the industrial industry, and is now the most common way to connect industrial electronic equipment, widely used in various industries of many manufacturers.
Modbus is usually used to transmit signals from instruments and control equipment to main controllers or data acquisition systems. For example, a system that measures temperature and humidity and transmits the measurement results to a computer.
Modbus is also commonly used to connect monitoring computers with Remote Terminal Units (RTUs), which is applied in Supervisory Control and Data Acquisition (SCADA) systems.
Modbus protocol has versions running on serial lines (Modbus RTU and Modbus ASCII) and Ethernet (Modbus TCP).
1.1 Working Principle of Modbus
Modbus transmits data between devices through serial lines. The simplest configuration is a single serial cable connecting the serial ports of two devices: client and server.
Data is sent as a series of bits (0s and 1s), and each bit is transmitted by voltage: zero is sent with positive voltage, while one is sent with negative voltage. These bits are transmitted at a very fast speed, and the typical transmission rate is 9600 baud rate (bits per second).
2.Different Versions of Modbus Protocol
2.1 Modbus RTU
Modbus RTU (Remote Terminal Unit) is the binary implementation of Modbus protocol. It is generally used for serial communication and is well-known for its compact data representation, which makes it efficient and fast. Such high efficiency greatly simplifies its implementation in hardware devices. Communication under this protocol uses physical interfaces such as RS-485 or RS-232.
Modbus RTU over TCP is a hybrid of Modbus RTU and Modbus TCP protocols. Although Ethernet is the physical medium, data transmission follows the RTU format. The obvious advantage of this matching mode is that it can migrate the original RTU-based systems to Ethernet with minimal hardware upgrades. Therefore, designers often choose this scheme when expanding the control scope to reuse as much on-site equipment and structures as possible.
2.2 Modbus ASCII
Modbus ASCII (American Standard Code for Information Interchange) is the ASCII implementation of Modbus protocol. Its efficiency is lower than Modbus RTU, but it uses human-readable characters, so it is easier to operate and debug.
2.3 Modbus TCP/IP
Modbus TCP/IP is a version of Modbus protocol running on TCP/IP networks, which supports long-distance communication across different networks.
2.4 Modbus UDP
Modbus UDP (User Datagram Protocol) is a Modbus protocol version transmitted via UDP transport protocol. It is less reliable than Modbus TCP/IP because it cannot guarantee packet delivery or correct transmission sequence, but it has faster speed and occupies less bandwidth.
2.5 Modbus Plus
Modbus Plus (MB+ or Modbus+) is a proprietary variant of Modbus protocol launched by Schneider Electric. It is a point-to-point communication protocol, which can provide higher speed and more deterministic data transmission compared with standard Modbus.
3.Communication Media Used by Modbus
3.1RS-232
3.2RS-422
3.3Ethernet
4.Master and Slave in Modbus
4.1Definition of Master
The master is the device that initiates Modbus communication. It controls the whole communication process, sends requests to slaves, and processes response data returned by slaves. One master can connect multiple slaves, and each slave must have a unique address.
4.2 Definition of Slave
The slave receives and responds to requests from the master. The slave cannot actively initiate communication; it only replies when the master sends a request. The slave address is used to uniquely identify its position in the communication network.
4.3 Feature Difference Between Master and Slave
The master usually has strong computing and control capability, and can initiate and manage communication with multiple slaves at the same time. The slave has simpler functions, which only responds to master requests and feeds back data.
4.4 Role Switching of Master and Slave
The roles of master and slave can be switched. For example, a device can act as a master to send requests to multiple slaves in one communication process; in another communication process, the same device can act as a slave to receive requests from other masters.
5.What is Baud Rate?
5.1Definition of Baud Rate
Baud rate, also known as bit rate, refers to the number of bits transmitted per second in serial communication. It represents the data volume transmitted per unit time, and the unit is "Baud" or bps (bits per second).
Standard baud rates supported by Modbus:
300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, 57600, 115200 bps
5.2Function of Baud Rate
Baud rate is a core parameter of serial communication, which determines data transmission speed and communication efficiency. Specifically, the higher the baud rate, the faster the data transmission speed and the higher the communication efficiency. However, the selection of baud rate is not fixed, and it needs to be adjusted according to specific application scenarios and hardware devices.
5.3Rules for Selecting Appropriate Baud Rate
1.Confirm communication requirements
Clarify whether high-speed mass data transmission or low-speed small data transmission is required, and determine the range of baud rate according to actual needs.
2.Consider hardware limitations
Different hardware devices have maximum baud rate limits; the selected baud rate cannot exceed the upper limit supported by the hardware.
3.Consider transmission distance
Longer transmission cables need lower baud rate to ensure reliable data transmission.
4.Consider bit error rate
High baud rate will increase the probability of transmission errors due to fast transmission speed, so select a moderate baud rate to reduce error rate.
5.Test and verify
After narrowing down the optional baud rate range, conduct actual communication tests, compare the communication effect and transmission speed of different baud rates, and select the optimal parameter.
5.4Summary
Baud rate is the most basic and important parameter of serial communication, which determines transmission speed and communication reliability. Reasonable selection of baud rate needs to comprehensively consider communication demand, hardware limit, transmission distance and bit error rate, and confirm the optimal value through actual tests to realize stable and efficient serial communication.
6.Maximum Number of Slaves Connected to One Modbus Master
The maximum number of slaves that can be connected to a master varies with different communication media, as shown in the table below:
7. Modbus Register Classification and Address Allocation
Register is a core concept in Modbus protocol, and all data is stored in registers. The concept of register originally comes from PLC, and with the wide application of Modbus, it has been generalized — it not only refers to physical hardware registers, but also can represent a piece of memory space.
Modbus registers are divided into four categories according to data type and read-write attributes. The address space of the four categories can be continuous or discontinuous, which is defined by device developers.
Address Allocation Note
The accessible address range is different for different devices; the above table only shows the standard definition of each register type.
8. Differences Between Modbus ASCII and Modbus RTU
1.Encoding & Check Mode
Modbus RTU: Adopt binary encoding, use CRC error detection.
Modbus ASCII: Low transmission efficiency, adopt LRC error detection with weaker performance.
2.Frame Framing Mode
ASCII mode uses special ASCII characters as start and end marks of messages; RTU uses silent interval of 3.5 character time to distinguish message frames.
3.Mutual Compatibility
These two modes are incompatible. Devices configured as ASCII mode cannot communicate with devices running RTU mode.
4.Data Volume Overhead
Modbus ASCII messages need twice the bytes to transmit the same data content as Modbus RTU messages, with lower transmission efficiency.