Abstract: Starting from the goal of optimizing the highway tunnel monitoring and control system, and considering the special environment of highway tunnels and the characteristics of numerous and dispersed electromechanical facilities, this paper fully utilizes the advantages of PLCs, such as good anti-interference ability, high degree of mechatronics integration, and powerful communication and networking functions. The PLC is used as the field area controller. The characteristics, hardware configuration principles, communication settings and implementation methods of the Controller Link network are discussed. A redundant PLC ring network with optical fiber as the transmission medium is built, realizing a monitoring and control system with large data transmission capacity, high communication speed, long transmission distance, and high reliability. Keywords: Computer control; Tunnel monitoring and control; PLC; Network; Communication Introduction Tunnels are special sections of highways. Their tubular structure and enclosed environment easily induce accidents such as rear-end collisions, congestion, and fires. To ensure safe vehicle travel and create a comfortable driving environment in tunnels, traffic engineering facilities such as ventilation, lighting, monitoring, and fire protection are installed. These facilities are relatively evenly distributed throughout the tunnel. To control these facilities and accurately and promptly transmit information such as traffic flow parameters, vehicle operating environmental indicators, and the operating status of field equipment collected by various detection devices deployed in the tunnel to the monitoring center, a fast, safe, and reliable tunnel monitoring and control system is needed. With the development of computer technology, the performance and functions of PLCs have been greatly improved. In particular, the high reliability, high anti-interference capability, and high mechatronics integration of PLCs make them more suitable for application in special environments such as highway tunnels. Furthermore, their communication and network functions have been greatly enhanced, facilitating easy linking between PLCs and computers, between PLCs, and between PLCs and other field devices. Furthermore, PLC systems can significantly reduce wiring, facilitate installation, and simplify system maintenance. Therefore, tunnel monitoring and control systems based on PLC networks are gaining widespread popularity and application. 1. Network Structure 1.1 System Scheme The tunnel monitoring and control system employs multiple host computers. The entire network adopts a two-layer network structure. The first layer is the management layer (i.e., the information layer), using an Ethernet LAN to handle management, decision-making, and control tasks. The second layer is the control layer, using a PLC fiber optic ring network (Controller Link) to implement field data acquisition, data transmission, and equipment control. The field PLC network is connected to the LAN via an Ethernet module from the main PLC, enabling Ethernet communication. This control system achieves field control by area controllers, and signal transmission is fully digitalized, with communication networks connecting from the bottom to the top layer. The system structure is fully decentralized, with field controllers directly controlling the equipment. The upper-layer communication network uses Ethernet communication, facilitating data sharing. This network structure reflects the advantages of the FCS control system. When only one point on the ring network fails, the data can be transmitted normally and the system can work normally. When there are multiple points of failure, the field PLC can work independently and wait for the system to recover before accepting instructions and exchanging data, which greatly enhances the reliability of the control system. The system structure is shown in Figure 1. 1.2 Controller Link Network Features The Controller Link network is a network used in the FA field for large-capacity data exchange between PLCs and between computers and PLCs. The computer can monitor the PLCs on the network through the Controller Link support software. There are two types of CLK support cards for personal computers: one is the cable type 3G8F5-CLK21-E, and the other is the optical fiber type 3G8F5-CLK11-E. The Controller Link network supports data linking, data sharing and information communication. The data link area can be freely set to form a data link system. (1) The communication medium can be twisted pair or optical fiber. The cable type Controller Link network uses twisted pair connection. Twisted pair is easier to handle and maintain than coaxial cable or optical fiber, reducing network costs. The communication distance can reach 1km. The fiber optic Controller Link network uses fiber optic connection to obtain more large-capacity data links and longer communication distances, with a communication distance of up to 20km. (2) High-speed data communication The communication baud rate of the controller network can reach 2Mbps, which is much higher than the 128Kbps of the PC Link network. (3) Large-capacity data links Each PLC can send up to 1K words, and the total number of bytes sent and received can reach 8K. Data links can be set automatically or manually according to actual needs, which is very flexible. (4) Information communication Information communication is achieved by executing the communication instructions SEND, RECV, and CMND in the user program. Network communication can be easily achieved by executing the communication instructions. (5) Flexible network interconnection The Controller Link network can be configured as a single level, that is, all PLCs are installed with only one CLK unit and connected by cables. The maximum number of nodes in a single level is 32. If a PLC is installed with two or more CLK units, and they are connected to the CLK units of other PLCs to form their own subnets, a multi-level system is formed. A PLC can be equipped with both Controller Link and Ethernet units, and seamless communication within a three-level network can be achieved using communication commands. (6) Improved error handling Because the error log contains the time and details of the error occurrence, it makes it possible to quickly handle errors. When a token node fails, another node will automatically become the token node, preventing one node's failure from affecting other nodes in the network and ensuring the reliability of the system. 1.3 Hardware configuration The correct selection of PLC plays an important role in ensuring the technical and economic performance indicators of the entire control system. The selection of PLC includes the selection of model, capacity, I/O modules, power supply, etc. In order to ensure the stability of communication between the PLC and the host computer, a CS1D series PLC controller is set up in the central control room to connect with the area controllers mentioned above, so as to receive information and issue commands from the central control room. The CS1D series PLC uses a faster CPU and can be connected to the host computer (with dual Ethernet cards) through two Ethernet modules CS1W-ETN21D, which greatly enhances the real-time performance and stability of the system communication. Because the area controller selected for the tunnel monitoring and control system needs to support Ethernet and remote communication functions, and the control system needs to be stable and reliable, the area controller must have a certain storage capacity. The selection of the field PLC should be based on the control performance requirements of the controlled object, as well as the type and characteristics of the PLC's input and output quantities, to determine the PLC model and hardware configuration. For integrated PLCs, the models of the basic unit and expansion unit should be determined; for modular PLCs, the model of the frame (or base plate) and the required module models and quantities should be determined. The CS1 series PLC can complete the control of the tunnel field equipment, saving space in the area control box and reducing investment in control equipment; therefore, the CS1 series PLC is selected. 2. Communication Setup and Implementation 2.1 Communication Setup Taking Xuegongling Tunnel as an example, the system has one main control PLC and eight field PLCs (area controllers). Stations #2, #3, #4, #6, #7, and #8 are mainly used for tunnel traffic control and environmental monitoring, while stations #1 and #5 are mainly used for tunnel ventilation and lighting control. The Controller Link network communication setup is completed in three steps: (1) Connect the network consisting of Controller Link modules CS1-CLK52-V1 using GI type optical cables. (2) Set the node number and unit number according to the station number by turning the UNIT No and NODE No knobs on the CS1-CLK52-V1 panel. Create a routing table for each station with Ethernet as network 2 and Controller Link network as network 1 using C-NET software and download the routing table content to the corresponding control station. (3) Data link table setup. There are two ways to set the data link table: automatic mode and manual mode. The setting mode can be selected using a handheld programmer or CX-P software in the DM area of ​​the CPU unit of the starting node PLC. ① Automatic mode can be used to establish a simple data link, where all receiving nodes share the same data as the sending node. Zone 1 is selected from the IR, CIO and LR areas, and Zone 2 is selected from the data storage areas DM and EM. Each node is not allowed to receive or send only a part of the data, and all nodes can be specified to join or not join the data link. ② Manually setting data links distinguishes several situations, including: (a) the order of sending and receiving nodes is free; (b) some nodes can only send data without receiving it; (c) some nodes can only receive data without sending it; (d) a node can only receive a specified number of words starting from the start of the area; (e) a node can only receive a specified number of words starting from a specified word position, with the start word set to an offset from the start of the data transmission. Since all area controllers in the highway tunnel monitoring system need to communicate, this paper sets the data link table so that the receiving nodes share all the data of the sending nodes. The order of sending and receiving nodes adopts a free format. Each area controller sets the link area size to 800 words, and all transmissions start from D1000. 2.2 Communication Implementation (1) Starting the data link table to realize communication between PLCs By starting the data link table, the data sharing of the link area set above can be realized, thus achieving the purpose of communication. This method is simple and convenient to implement, but it is not flexible. The PLC cannot realize data sharing in other memory areas. There are three ways to start and stop the data link table: ① Using a programming device or user program, the start bit of the CS1 series PLC is the 0th bit in the start node word DM30000×CLK unit number. When the start bit is set from OFF to ON or is already ON when the power is turned on, the data link is started; when the start bit is changed from ON to OFF, the data link is stopped. ② Using Controller Link support software, on the host computer or host computer node, the Controller Link support software sends start/stop data link commands to the nodes in the data link. ③ Using the FINS command, the network communication instruction CMND is used to send RUN ("0401") / STOP ("0402") instructions from a Controller Link node (PLC or computer) to a node in a data link to start/stop the data link. (2) Network instruction communication FINS communication can be realized by sending network instructions SEND, RECV and CMND within the network. This communication method is flexible and can perform any operation on the target node PLC. Sending network commands involves first storing the command data to be sent in a given memory area, determining the local memory address and the target station memory address, and then communicating with the remote station via network commands. (3) Operation process: The operation steps of the tunnel monitoring and control system based on PLC network are as follows: First, the area controller diagnoses whether the controller is working normally based on the controller status, memory area status, and port status. Second, the area controller collects the input data of the local control input devices, processes it, and sends the result data to the data link sharing area. At the same time, it reads the data shared by other controllers to the data link area, judges the status of the entire tunnel by collecting all the data, and queries the host computer to see if it executes the corresponding program according to the judgment result. After confirmation, the area controller immediately executes the corresponding program. If the area controller does not receive the host computer command for a long time, it controls according to the prescribed plan according to the specific situation. The control status of the field equipment is changed according to the operation of the plan program, and the output is sent to the display device according to the logic truth table corresponding to the control status. After the PLC executes the external response and control program, the I/O refresh outputs the result to the field equipment. 3. Conclusion Using a PLC as the local controller and a PLC network to achieve tunnel monitoring and control integrates data acquisition and control, avoiding complex wiring and reducing the probability of errors. This improves the reliability and safety of the entire system. The innovation of this paper lies in applying PLC and its network to a highway tunnel monitoring and control system. On the one hand, it fully utilizes the PLC's advantages of good anti-interference capabilities, high degree of mechatronics integration, and powerful communication and networking functions, broadening the application scope of PLCs. On the other hand, it meets the requirements of the harsh environment inside the tunnel, including temperature, humidity, noise, dust, vibration, and high-frequency interference from vehicle ignition, ensuring the stability and reliability of the monitoring and control system.