Abstract This paper provides an introduction to Programmable Logic Controllers (PLCs), comparing the performance and functional characteristics of MOD ICON and QUAN TUM series PLCs in terms of input modules, host, output modules, programmers, programming software, and upper-level monitoring systems. The application of MOD ICON PLCs in power plant FSSSs (Software-Defined System Systems) is introduced from three aspects: technical specifications, system composition, and system working principle. Several points to note are also given. It is pointed out that PLCs are widely used in power plants, replacing cumbersome relay logic connections and directly accessing the distributed control system data highway through a gateway, offering advantages such as high speed, large capacity, and multiple functions. Keywords : Programmable Logic Controller; Thermal Power Plant; FSSS; Input/Output Module 1. Introduction to Programmable Logic Controllers A Programmable Logic Controller (PLC) is a process control device with a microprocessor as its core. It is mainly configured with digital input and output channels for executing sequential control functions. Small PLCs have 16–256 I/O points, medium-sized ones have 512–1024 points, and large ones have 2048–8192 points. PLCs are mainly used in production processes for time-sequential or logical sequence control, replacing complex relay control devices. They are primarily used by electrical technicians; therefore, the programming language used is a highly intuitive ladder diagram language, basically derived from relay circuit symbols. After the entire ladder diagram is input into a dedicated programmer, the programmer automatically processes it into an internal program executable by a microprocessor. After successful trial operation, it is stored in an operable programmable read-only memory (EPROM) and installed on the PLC for execution. During operation, the PLC continuously monitors the status of each contact, and based on its changes and predetermined timing and logical relationships, accordingly changes the internal relays or starts timers, ultimately outputting switching signals to control the actuator's operation. The program's execution cycle is determined by the program length and CPU instruction execution time. Output methods can include relays, thyristors, or transistors. Newer PLCs offer analog control blocks, whose input/output analog standards are the same as those of digital controllers. They also provide proportional, integral, and derivative (PID) control algorithms. The high reliability and continuously enhanced functionality of PLCs have led to their increasingly widespread application in the process control level of distributed control systems. [Download full text of MOD ICON PLC series application in thermal power plants]