Technical characteristics of MODBUS bus control system - Database & Sql Blog Articles

MODBUS is an industrial fieldbus communication protocol that defines a structured information frame, allowing devices to communicate and exchange data. It operates independently of the physical layer, making it adaptable to various network types. The protocol establishes a master-slave communication model, where the master initiates requests, and the slaves respond accordingly. In a standard MODBUS network, only one master exists, while multiple slaves are connected via an asynchronous serial bus. Communication follows a request-response mechanism, ensuring controlled and reliable data exchange.

Due to its low development cost and ease of implementation, MODBUS has become a widely adopted standard in industrial automation. Many controllers, PLCs, and displays come equipped with MODBUS interfaces, enabling seamless integration across different manufacturers' devices. This makes it ideal for building centralized monitoring systems in industrial environments.

Originally designed for PLC communication, MODBUS uses 24 command codes to exchange data between the PLC and external devices. These commands support analog input/output (AI/AO) and digital input/output (DI/DO) operations. However, many MODBUS devices only use a subset of these commands and ignore the rest, depending on their specific application needs.

1. MODBUS Communication Format

The MODBUS protocol supports two transmission modes: RTU (Remote Terminal Unit) and ASCII. In RTU mode, each byte of data is sent as an 8-bit character, whereas in ASCII mode, it is represented by two ASCII characters. For example, the value "20" would be encoded as "00100000" in RTU and "00110010" + "00110000" in ASCII. This makes RTU approximately twice as efficient as ASCII when transmitting binary data. ASCII is typically used for text-based communication, while RTU is preferred for larger volumes of binary data.

In MODBUS communication, the master station sends a message containing the slave address, function code, data field (including data start address and content), and a CRC check code. The slave responds with the same structure, including any error codes if necessary. Each slave must have a unique address to ensure proper communication. The master sends requests, and only the addressed slave responds, preventing unnecessary traffic on the network.

MODBUS Function Codes

2. Implementation of CRC Check

In RTU mode, the last two bytes of each message contain a 16-bit CRC (Cyclic Redundancy Check) code. The sender calculates this code based on the address, function code, and data fields using a predefined algorithm. The receiver performs the same calculation and compares the result with the received CRC. If they match, the data is considered valid; otherwise, an error is reported.

The CRC-16 algorithm is commonly used in MODBUS RTU communication. It employs the polynomial X16 + X15 + X2 + 1, with a constant value of 0x8005. The CRC calculation process involves initializing a register, XORing it with incoming data, shifting bits, and applying the polynomial if needed. After processing all data, the final CRC value is appended to the message, with the low byte first and the high byte second.

3. Link Characteristics

MODBUS supports various physical layers, including RS-232, RS-422, and RS-485. While RS-232 is suitable for short-distance communication, RS-485 is often used for longer distances due to its robustness and ability to support multiple devices. In multi-point configurations, only RS-485 is used, allowing up to 31 slaves on a single bus. Communication is half-duplex, meaning only one device can transmit at a time, preventing collisions and ensuring orderly data exchange.

The RTU mode uses a frame format of 1 start bit, 8 data bits, 2 stop bits, and no parity. Data integrity is ensured through the CRC-16 check. The receiving device identifies the start of a frame based on the time gap between received characters. If no new data arrives within 3.5 character intervals, the current frame is discarded, and the next character is treated as the start of a new frame.