Technical Details of Fieldbus

Fieldbus is a specialized data communication network used in industrial automation and control systems. Serving as a critical communication bridge between field devices and control systems, Fieldbus plays an essential role in modern industrial automation. This article delves into the technical details of Fieldbus, including its network architecture, communication principles, data frame format, protocol stack, and device types, to help readers better understand and apply this crucial technology.

Fieldbus Network Architecture and Topology

Network Architecture

Fieldbus networks typically adopt a distributed control system architecture, differing from traditional centralized control systems. Each field device (such as sensors and actuators) has a built-in microprocessor capable of directly processing data and executing control tasks. Controllers communicate with these field devices through the Fieldbus network, exchanging data and commands.

Topology

Fieldbus networks support various topologies, including bus, star, ring, and tree structures. The bus topology is the most common, where all devices are connected via a single backbone cable forming a communication bus. The star topology connects each device to a central hub through individual cables, while the ring topology connects all devices in a closed loop to enhance network reliability and redundancy.

Fieldbus Communication Principles

Fieldbus networks use a time-division access control mechanism to ensure orderly data exchange among devices. Here are some key communication principles:

Master-Slave Communication

In a Fieldbus network, there is usually one master device (master station) and multiple slave devices (slave stations). The master device manages the network, initiates communication requests, and the slave devices respond to the master’s requests to transmit data.

Periodic and Aperiodic Communication

Fieldbus supports two types of communication: periodic and aperiodic. Periodic communication is used for regularly exchanging control and status data, ensuring the system’s real-time performance and stability. Aperiodic communication is used for transmitting diagnostic information, configuration parameters, and other data that do not require frequent updates.

Fieldbus Data Frame Format

Fieldbus data frames are the basic units of data transmission in the network. A typical Fieldbus data frame includes the following parts:

1. Start Delimiter: Marks the beginning of the data frame.

2. Address Field: Specifies the address of the target device.

3. Control Field: Contains frame type and control information.

4. Data Field: Contains the actual data being transmitted.

5. Checksum Field: Used for error detection during data transmission.

6. End Delimiter: Marks the end of the data frame.

Different Fieldbus protocols may have variations in the data frame format, but the basic structure remains similar.

Fieldbus Protocol Stack and OSI Model Correspondence

The Fieldbus protocol stack closely aligns with the OSI model, consisting of multiple layers, each responsible for different functions. Here is the correspondence between the Fieldbus protocol stack and the OSI model:

1. Physical Layer: Corresponds to the OSI model’s first layer, responsible for physical connections and signal transmission, including cables, connectors, and signal encoding.

2. Data Link Layer: Corresponds to the OSI model’s second layer, responsible for assembling data frames, address recognition, and error detection.

3. Network Layer: Present in some advanced Fieldbus protocols, responsible for routing and data forwarding.

4. Transport Layer: Ensures reliable data transmission, providing error recovery and flow control mechanisms.

5. Application Layer: Corresponds to the OSI model’s seventh layer, responsible for data exchange and communication control between applications.

Fieldbus Device Types and Their Functions

Fieldbus networks include a variety of devices, each playing specific roles and having unique functions. Here are some common Fieldbus device types:

Master Devices (Master Stations)

Master devices are typically controllers or PLCs that manage network communication, schedule data transmission, send control commands, and receive feedback information.

Slave Devices (Slave Stations)

Slave devices include sensors, actuators, and other field devices responsible for data acquisition, executing control commands, and uploading data to the master device.

Gateways

Gateways connect different types of networks, performing protocol conversion. For example, they link Fieldbus networks with Ethernet or other industrial networks to enable data exchange between different systems.

Repeaters

Repeaters extend the physical range of the Fieldbus network, enhance signal strength, and ensure data transmission over long distances.