Modbus is a serial communication protocol first introduced by Modicon in 1979 for use with programmable logic controllers (PLCs). Over time, it has evolved into the de facto standard for industrial communication and is now widely used to connect various electronic devices in industrial environments. The protocol supports multiple network types, including serial lines, Ethernet, and other IP-based networks.
The most common physical layer for Modbus communication is EIA-485, which allows for reliable data transfer over long distances. There are two main variants of the serial protocol: Modbus RTU and Modbus ASCII. While both follow the same serial communication approach, they differ in how data is represented and transmitted. Modbus RTU uses a compact binary format, making it efficient for high-speed communication, while Modbus ASCII offers a human-readable format that is easier to debug but less efficient in terms of data throughput.
RTU messages include a Cyclic Redundancy Check (CRC) for error detection, whereas ASCII messages use a Vertical Redundancy Check (VRC). Due to these differences, devices configured for RTU cannot communicate with those using ASCII and vice versa.
In this analysis, we'll explore the structure of a Modbus RTU message and how it is used for both reading from and writing to devices. Let’s begin by understanding the components of a Modbus RTU frame.
A typical Modbus RTU message consists of several parts: the slave address, function code, data field, and a CRC checksum. For example, if an MCU receives a message such as "01 06 00 01 00 17 98 04," it can be broken down as follows:
- **Slave Address**: 01
- **Function Code**: 06
- **Data Address**: 00 01
- **Data Value**: 00 17
- **CRC Checksum**: 98 04
If the MCU is configured to respond to this address, it will validate the CRC and then update the corresponding register, such as `HoldDataReg[1] = 0x0017`.
When the master reads data from the slave, the message might look like "01 03 00 01 00 01 D5 CA." This includes the slave address, function code (read), data address, number of registers to read, and the CRC. After validating the CRC, the slave will return the requested data, formatted as "01 03 02 00 17 F8 4A."
Understanding the structure of Modbus messages is essential for implementing communication between devices. Each message typically follows a similar format, consisting of the following elements:
- **Slave Number** (1 byte): Ranges from 0 to FFH, with 0 being a broadcast address.
- **Function Code** (1 byte): Indicates the type of operation (e.g., read, write).
- **Data Area** (n bytes): Contains the actual data being transferred.
- **CRC Checksum** (2 bytes): Ensures data integrity during transmission.
This framework applies to all Modbus RTU frames, whether they are sent from the master to the slave or vice versa. By mastering these components, developers can effectively implement Modbus communication in their systems.
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