Industrial control systems are deeply rooted in classical control theory. As computer technology becomes more widely used in industrial settings, the need for effective feedback control has become increasingly important. This involves ensuring that control commands reach the actuators efficiently and that the resulting control effects are accurately returned to the system through sensors. In complex industrial environments, fieldbus technology plays a crucial role by enabling reliable, fast, and stable communication. This ensures that the industrial control system can access timely and accurate field data, which is then processed efficiently to issue precise execution commands.
**1. The Basic Model of Communication**
Communication refers to the transfer of information between two points, such as in telephone, radio, or television systems. Similarly, industrial communication systems require transmitters, receivers, and communication links. These links can include twisted pair cables, coaxial cables, optical fibers, radio signals, and microwave transmissions. The transmitter modulates the data, while the receiver demodulates it to reconstruct the original information. The rules governing this process are known as protocols.
**2. Special Requirements for Industrial Communication**
Unlike general communication systems, industrial communication must meet unique demands due to the harsh and variable nature of the industrial environment. It typically deals with process data, state variables, and maintenance information. These data types have specific time-related features such as real-time performance, frequency, jitter, response time, synchronization, and time-space consistency.
**3. How Does the Fieldbus Meet the Requirements of Industrial Communication?**
The OSI model provides a framework for the physical layer, which is often confused with physical media like cables, connectors, and wireless hardware. The physical layer defines not only the interface requirements but also encoding methods, verification techniques, bandwidth, signal types, and transmission characteristics. The data link layer handles the creation, transmission, and reception of data packets, often divided into LLC (Logical Link Control) and MAC (Media Access Control) layers.
The application layer defines how data is exchanged and specifies device information, states, and parameters. Fieldbus communication data is generally categorized into identification data and general messages. Identification data refers to the transfer of process variables between controllers, sensors, and actuators, while general messages are used for system configuration and maintenance tasks.
**3.1 Replacing the 4–20 mA Wiring Scheme**
Previously, industrial communication relied on analog signals, such as the well-known 4–20 mA technology. However, fieldbus technology introduced digital communication, offering advantages like noise resistance, reduced cabling, and enhanced diagnostic capabilities. As a result, fieldbus is gradually replacing traditional 4–20 mA systems.
**3.2 RS-232 and RS-485 Electrical Standards**
Although some people may refer to RS-232 and RS-485 as buses, they are actually interface standards. They define electrical characteristics, mechanical designs, and pin functions. The main difference lies in how logic levels are represented: RS-232 uses voltage levels, while RS-485 uses voltage differences. While RS-232 supports point-to-point communication, RS-485 enables bus-type networking, making it a common basis for many fieldbuses.
**3.3 Manchester Coding, Encoding, and Symbol Concepts**
Cable and connector pin assignments depend on the type of cable and network architecture. Signal encoding determines how 0s and 1s are transmitted. Most fieldbuses use Manchester coding, where each symbol contains a transition to encode data. Understanding symbols, bits, baud rate, and bit rate is essential for grasping how data is transmitted.
**3.4 Communication Media**
Fieldbus typically uses wired connections, such as twisted pairs, coaxial cables, and fiber optics. With the advancement of wireless technology and evolving industrial wiring needs, wireless communication is becoming increasingly common in fieldbus systems.
**3.5 Communication Models**
There are two main communication models: client/server and producer/consumer (publisher/subscriber). The client/server model is ideal for transferring state data, while the publisher/subscriber model is better suited for event-driven data, such as notifications and status updates.
**3.6 Communication Mechanisms**
Fieldbus communication mechanisms include periodic, response-based, and event-triggered approaches. Periodic communication is commonly used in control systems, where data is transmitted at regular intervals. Response mechanisms ensure message integrity, while triggering mechanisms allow for efficient data exchange based on events.
**3.7 Error Control and Flow Control**
Error detection and recovery mechanisms are usually handled at the application level. Some fieldbus systems provide both sender-side and receiver-side error management. Flow control helps regulate data transmission rates to prevent congestion and ensure smooth operation.
Network Switches
A Network switch is a type of network device used to connect multiple devices in a local area network (LAN). It can receive and forward data packets through physical or wireless interfaces, sending data packets from the source device to the target device. Network switches can identify and forward packets based on MAC addresses so that data can be accurately transmitted to the target device.
A network switch usually has multiple ports, each of which can connect to a device, such as a computer, server, printer, etc. When a device sends a packet, the switch forwards the packet to the corresponding port according to the MAC address of the target device, thus achieving communication between devices.
The network switch can also divide the virtual local area network (VLAN) into multiple logical subnets, so that different devices can communicate independently on the same physical network, improving network security and performance.
In short, a network switch is a network device used to connect and forward packets, which can realize communication and network management between devices. It is an important part of building local area networks and enterprise networks.
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