1 Introduction An important component of ship automation is the main engine remote control system. Currently, there are various technical solutions for main engine remote control systems. This paper adopts a PLC industrial control network to realize the functions of the main engine remote control system, which has the characteristics of good economic performance, simple hardware circuit structure, and safe and reliable operation. Based on the design of the main engine remote control system using a multi-PLC control network, this paper discusses the overall structure and communication scheme of the entire PLC network, and realizes the automatic control of the main engine's start-stop function and the design of the safety system through the communication network. [b]2 Overall Structure Design and Communication Implementation of the Main Engine Remote Control PLC Network[/b] 2.1 Overall Structure Design of the Main Engine Remote Control PLC Network The PLC control network is used for the control of the main engine remote control system, including two S7-200 PLCs. One is used for the main engine start-stop and speed regulation control, and is installed in the engine room; the other is used to complete the task of the electronic speed governor and is installed in the engine room. A computer is set up as a monitoring platform to monitor important signals of the entire system. The main equipment of the entire network is: two S7-200 PLCs, one microcomputer, network connectors, PC/PPI cables, and RS-485 cables. Based on the overall communication design concept, our overall structure diagram is shown in Figure 1. As shown, the ground bus from the master PLC communication port communicates with the slave PLC and computer via a network connector and a PC/PPI cable. The master and slave stations communicate via the PPI protocol through an RS-485 bus, while the master station and computer terminal communicate via a free port through a PC/PPI cable. The network connector from the master PLC communication port is for isolation to prevent damage to the computer's RS-232 port. The ground wire from the network connector and RS-485 signals A and B communicate with the slave station by comparing high and low levels. Simultaneously, a 5-pin connector from the PC/PPI cable is converted into 3 lines via RS-485 and RS-232 for receiving, transmitting, and ground communication with the computer. The pin assignments for the S7-200 PLC communication ports are shown in the attached table. After completing the system design according to the above scheme, the actual communication performance meets the design requirements; the slave PLC and computer terminal can generally obtain information from the master station within 20ms. 3. Design of the Main Unit Remote Control System 3.1 Design of the Start-Stop Section The author chose the most common B&M low-speed engine for the design process. The start-stop control mainly includes five parts: start control, slow-speed control, stop control, and repeated start control. Taking forward start as an example, the system must ensure the on/off state of the forward start solenoid valve in two situations: start-up from a stopped position and normal reverse start-up. It must also determine the repeated start status. The forward start procedure is shown in Figure 5. Slow-speed control considers two situations: the main unit stopping time exceeding 30 minutes and power restoration after a power outage. Slow-speed control can be cancelled after the main unit has completed one revolution or in the event of an emergency operation signal. Stop control considers four situations: stop command, fault stop signal, vehicle command inconsistent with the direction of operation, and during the operation of the start solenoid valve. Repeated start control is further divided into four subroutines: start time monitoring subroutine, reversing time monitoring subroutine, start interval delay subroutine, and repeated start count subroutine. The above program flowcharts are not detailed here due to space limitations. 3.2 Design of the Speed ​​Control Section This design uses the EM235 analog input module to read the train telegraph command and speed feedback. The PLC master station first reads the analog signal, and through three shifts, obtains a 12-bit digital signal. Then, the input signal is digitally filtered through a timer interrupt program. The filtered data is compared with the normal signal voltage range determined by the external circuit to determine whether there is a broken wire fault at the three terminals of the train telegraph potentiometer. After ensuring the signal is normal, various limiting processes are performed. The limiting process mainly includes critical speed limiting, acceleration limiting, and load program limiting. The result of the limiting process is finally sent to the slave PLC electronic speed controller through the PPI communication protocol to control the speed of the master station. The speed limiting processing program flowchart is shown in 6. Another task of this design is to read and process the speed feedback value. After processing, various operating states of the main engine are obtained, including the reversing speed for forward and reverse rotation, the ignition speed, and the acceleration transition point. This provides the main engine status information needed for start-stop control and allows the monitoring platform to monitor in real time, intuitively reflecting the current status of the main engine and displaying the entire speed change process. 4. Conclusion This paper, based on the requirements for main engine remote control systems during the maintenance and modification of small and medium-sized ships, proposes a design and implementation scheme for a main engine remote control system with relatively simple and easy-to-implement hardware circuitry, fewer lines, easy programming, and comprehensive functions. The system uses two S7-200 PLCs and one computer to form the basic framework of the main engine remote control system. Information transmission between the various controllers and the computer is achieved using a serial communication bus. This scheme has been designed and debugged in a laboratory environment. Editor: He Shiping