Enhancing Bidirectional Modbus TCP \(\leftrightarrow\) RTU Gateway Performance: A UDP Mechanism and Markov Chain Approach  
[PERSON]  
1 Shandong Key Laboratory of Space Debris Monitoring and Low-Orbit Satellite Networking,  
Qinghai Zhang  
1 Shandong Key Laboratory of Space Debris Monitoring and Low-Orbit Satellite Networking,  
Qingjian Zhao  
1 Shandong Key Laboratory of Space Debris Monitoring and Low-Orbit Satellite Networking,  
up to 247 slave devices at once, surpassing most communication protocols, thus highlighting its high practical value. Therefore, developing a high-performance Modbus protocol conversion gateway is not only a technical challenge but also a practical necessity in the industrial field. The Modbus protocol, developed by Modicon as a master-slave communication architecture [4], was initially designed to provide a lightweight solution for control data transmission between controllers and sensors via serial links. Modbus dominated industrial distributed control systems (DCSs) from the 1990s to the early 2000s. Although the industrial sector has gradually been migrating toward service-oriented protocols such as Open Platform Communications Unified Architecture (OPC UA) and Message Queuing Telemetry Transport (MQTT), Modbus continues to play a pivotal role at the field level, particularly for the large installed base of legacy PLCs and remote I/O devices that remain in operation [5]. Modbus RTU and Modbus ASCII--two serial-based variants of the protocol--enable efficient communication between sensor/actuator-level devices and supervisory control layers [6]. Schneider Electric later extended the protocol's applicability through Modbus TCP, which supports heterogeneous network media [7], thereby enabling client-server communication across diverse bus systems and networked industrial devices. The Modbus protocol exhibits inherent limitations in practical implementations. Modbus RTU (serial-based) and Modbus TCP/UDP (Ethernet-based) demonstrate divergent performance characteristics [8]. Specifically, Modbus RTU suffers from lower data rates, constrained transmission distances, and complex cabling compared to its TCP/UDP counterparts. Conversely, Modbus TCP/UDP introduces packet loss vulnerabilities and reduced reliability relative to RTU, alongside challenges in cost and backward compatibility. Consequently, hybrid architectures integrating serial and Ethernet networks are often adopted [9]. While Modbus TCP enhances reliability through congestion control algorithms [10], this comes at the expense of increased latency--particularly pronounced under suboptimal network conditions where TCP latency significantly exceeds that of connectionless UDP [11; 12; 13]. However, as the demand for industrial automation continues to grow, higher demands are being placed on the real-time reliability of industrial networks [14; 15]. The minimal protocol overhead of UDP eliminates handshake-induced delays [16], making it attractive for time-sensitive applications [17]. Yet its lack of built-in reliability mechanisms compromises communication integrity [18]. Furthermore, the protocol incompatibility between serial Modbus and Ethernet-based variants necessitates gateway-mediated protocol conversion to bridge these heterogeneous networks. The existing conversion methods include using visual tools such as Node-RED and hardware protocol conversion gateways to achieve interoperability of heterogeneous data. Node-RED is a programming tool that aggregates hardware devices, Application Programming Interface (API), and online services, enabling the exchange of heterogeneous data through logical orchestration. It plays a significant role in cross-platform human-machine interfaces (HMIs) and has been widely used in IoT systems [19]. However, its reliance on software stack results in higher latency compared to hardware protocol conversion gateways, leading to lower real-time performance. In some scenarios, this may fail to meet the high real-time requirements.