Abstract: A PLC-based tunnel ventilation monitoring system is proposed. The system composition and functions, communication implementation, and ventilation control algorithm are introduced in detail. Keywords: PLC; PROFIBUS; ventilation control 1. Introduction Railway tunnels are mostly located in remote areas with harsh conditions. Therefore, it is necessary to improve their automation level and achieve less or no personnel on duty. PLC can not only perform complex logic operations, but also realize various sequential and PLD adjustment functions. At the same time, due to its high hardware reliability, simple programming, convenient use, easy wiring, good versatility, easy maintenance, and ability to be connected to control network systems, it is widely used in various fields. In addition to the above features, SIEMENS' S7300 series PLC can meet various automatic control needs, and also has good scalability, low price and powerful instruction functions [1,2]. 2. System Composition and Functions This system was developed to meet the ventilation monitoring requirements of a railway tunnel. Considering factors such as advanced technology, reliability, and economy, the system adopts a complete set of Total Integrated Automation (TIA) technology from Siemens (SIEMNES) of Germany. It uses S7 series programmable controllers to complete local monitoring data acquisition and automatic control, and connects to the host computer (industrial control computer) of the remote network center through the PROFIBUS bus. The system management mode adopts a combination of local unattended operation and remote monitoring management. One central control room can manage several tunnels, reducing the number of tunnel management modes [3]. For details, please click: PLC-based tunnel ventilation monitoring system