There are multiple nodes connected to the DCS communication network, typically divided into two main categories. The first type is directly linked to the production process through I/O boards and is known as the controller. Controllers can be further categorized into data acquisition controllers and loop controllers based on their functions. These two types can also be combined into a single node. The second category consists of human-machine interface (HMI) nodes that connect to operators. These HMIs collect data from the controller via the communication network and are divided into operation stations, engineer workstations, historical trend stations, and dynamic data servers.
Operation stations serve as the primary equipment for factory operations, allowing operators to read data from the controller and send commands back, enabling two-way communication. Engineer workstations play a crucial role in configuring the system before it goes live. They load algorithms, set parameters, and ensure the system operates smoothly. This configuration process differs from traditional programming; it’s more about setting up the system according to specific requirements.
The controller contains various control algorithms, such as PID, arithmetic operations, trigonometric functions, matrix operations, advanced algorithms like the Smith predictor, and even interfaces for BASIC and C languages. If the built-in mapping function in the operation station doesn’t meet the needs, the engineering station can create custom dynamic flow charts and upload them to the operation station.
To upgrade the controller and operator station, a reverse engineering station can be used to extract the current configuration without modifying it. This configuration is then downloaded to the upgraded hardware. Additionally, the reverse engineering station helps identify faults by analyzing the controller's setup.
The dynamic data server is another critical HMI node, acting as a bridge between the DCS and the MIS system. It ensures data flows only upward and is designed to handle large volumes of information. A historical trend station can either function independently or be integrated with the dynamic data server, depending on the system's needs.
When a DCS system is first deployed, network congestion often occurs due to incomplete configurations. However, this issue tends to resolve itself over time as the system stabilizes. Recently, with the need for real-time data from DCS to MIS systems, network congestion has become more frequent, especially when the dynamic data server is active. This leads to crashes in various HMI nodes, which depend on several factors including data volume and network structure.
Each node connected to the DCS network usually has a network interface. Data is sent from the controller to the interface, and HMIs retrieve data using the network protocol. The physical network structure can be a ring or a bus, with buses often logically forming a ring. Star networks are mainly used in small systems with fewer than 100 I/O points. The most common protocol is broadcast, where all nodes receive the data, and only those needing it process it. Another method involves querying data from other nodes, but if no node has the required data, repeated queries may cause congestion.
Operation station crashes have been a recurring issue since the introduction of DCS in the 1970s. The main software involved includes the operating system, monitoring software, and controller drivers. In early systems, these were often developed internally, leading to compatibility issues. Pressing certain keys could trigger crashes. Later, some systems reused existing software, which led to more frequent problems due to limited testing and integration.
Dynamic data servers were not always part of the initial setup. Many users added them later as needed. Over time, maintenance personnel change, and configurations evolve, sometimes without proper validation. When the dynamic data server connects, it reads all data points, many of which may be invalid, causing network overload and crashes. Using a reverse engineering station to compare configurations and remove invalid points can help reduce congestion and improve stability.
Another important consideration is ensuring all software versions are compatible when accessing the dynamic data server. Inconsistent versions can disrupt data transmission. To address network congestion, an exception reporting method can be implemented. This approach transmits data only when changes occur, reducing traffic. To prevent unnoticed failures, reports are sent periodically, even if no changes occur. Adjusting the exception report thresholds can further optimize data transfer and reduce network load.
Modern operation stations often run on the NT operating system with widely used monitoring software like FIX or INTOUCH. Due to their broad adoption, these systems experience fewer issues. Their open architecture improves reliability, reduces crashes, and lowers maintenance costs. Spare parts are no longer restricted to DCS manufacturers, marking a significant advancement in DCS technology.
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