The data acquisition system is designed to collect live signals and transfer them into the control computer. However, it typically lacks a loop control function block within the computer itself, meaning it's mainly used for displaying information through a human-machine interface (HMI). In essence, this system is quite straightforward in its design. From a hardware perspective, the structure can be divided into one or two levels, including I/O boards, controllers, and HMI components. For simpler applications, the system might only consist of basic I/O boards and an HMI. However, certain industries have unique requirements. For instance, in power plants, the boiler control system often operates separately from the data acquisition system (DAS). While boiler control may use a DCS (Distributed Control System), other parts rely on DAS. There are many manufacturers worldwide that produce these systems, and generally, the technology is well-established. Still, power plants may impose special conditions, such as the need for event sequence recording (SOE), which requires a fast interrupt function—usually within 2 milliseconds. These points are limited in number, around 100, and the resolution should be in the millisecond range. The main challenge lies in implementing SOE, which involves storing historical data and allowing users to view the timeline of events when needed. SOE functionality is often provided by specialized manufacturers, and some DCS systems tailored for power plants also include this feature. One of the most widely used data acquisition systems is the OPTO22 product from the United States. It features small modules that support both digital (DI, DO) and analog (AI, AO) inputs/outputs. There are 42 different modules available with varying voltage levels, giving users flexibility in selection. These modules are mounted on larger boards (such as B1 and B2 boards), where each board holds 8 analog channels or 16 digital channels. In addition to the modules, these boards also include serial communication ports. Once installed on a shelf inside a cabinet, the system becomes fully operational. The CRT-based HMI runs on the Windows platform and uses monitoring software like Modbus drivers. Previously, PAREGON was commonly used, but now more people prefer FIX or INTOUCH. The HMI can read I/O signals from the modules and display them on the screen. Over 20 plants in the U.S. produce similar products, and they are comparable in performance to the STD bus. However, due to the complexity involved in engineering work, STD bus products are gradually being phased out. In some purely data collection systems, OPTO products are frequently used because of their low maintenance and reliability. If closed-loop control is required, a controller like the LC4, which functions similarly to a PC, is added between the HMI and the I/O. This controller contains configurable function blocks that allow for various control strategies. Although the number of function blocks is relatively limited, OPTO22 systems are not typically used for large-scale projects with thousands of I/O points. The most commonly used module in OPTO for switching is the isolation module, which acts as a DCS input module, protecting the system from lightning strikes. Due to its simple manufacturing process, these modules are also popular among domestic users, offering a cost-effective alternative to imported versions. In the early 1990s, with the advancement of computer technology, it became feasible to use standard PCs for control tasks. PCs had sufficient computing power and memory to meet control requirements. This led to the concept of front-end and back-end operations, where DCS or PLC systems no longer needed a separate controller. Instead, the function code and ladder logic from DCS and PLC controllers were moved to the NT operating system. However, software migration proved difficult. As a result, microcomputers were directly connected to I/O boards. Since the control algorithms were already embedded in the PC, it served both as the HMI and the controller. Some referred to this setup as "soft DCS" or "soft PLC." American AB company developed a soft PLC, and each PC was connected via Ethernet. Despite this, the application has remained limited, with only smaller systems using soft DCS or soft PLC solutions.

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