Modbus RTU and Modbus TCP both belong to the Modbus protocol family, but they have fundamental differences in the transport layer and data encapsulation methods:
Modbus RTU: Based on serial communication (RS-485/RS-232), its frame structure is [Slave Address] [Function Code] [Data] [CRC Check]. It relies on physical layer timing and CRC to ensure data integrity, and is suitable for interconnection of on-site devices that require strong anti-interference performance for short-distance scenarios.
Modbus TCP: Encapsulated in the TCP/IP protocol stack, its frame structure consists of [MBAP Header] [PDU]. MBAP (Modbus Application Protocol Header) contains the Transaction Identifier, Protocol Identifier, Length, and Unit Identifier. It removes the CRC check mechanism and relies on TCP's reliable transmission capabilities.
The two protocols share exactly the same data model (coils, registers, etc.), but the transmission carrier has been upgraded from a "serial bus" to "Ethernet", and the communication paradigm has evolved from "point-to-point polling" to "networked concurrent access".
Industrial automation is undergoing IT/OT convergence and digital upgrading, which drives the iteration of protocols:
Networking Requirements: The traditional RS-485 bus can only support a maximum transmission distance of 1200 meters and up to 247 nodes, making it difficult to support cross-plant and remote monitoring scenarios.
Bandwidth and Efficiency: Ethernet (100Mbps+) far outperforms serial ports (115.2kbps), which can meet the demands of high-frequency data collection and concurrent access of multiple devices.
System Integration: SCADA, MES, and cloud platforms are all built on the IP network. Modbus TCP can be directly connected without additional protocol conversion gateways.
Case Reference: A pharmaceutical filling line experienced a parameter transmission delay of more than 20 minutes due to protocol heterogeneity between HMI (Modbus RTU) and PLC (Modbus TCP), leading to a 40% drop in daily production capacity.
Although Modbus TCP improves transmission efficiency, it has severe security vulnerabilities:
No Encryption: All data is transmitted in plaintext, and tools like Wireshark can directly capture register values and control commands.
No Authentication: Any device connected to the network can send function codes (such as the 06 Write Register command). In 2024, a petrochemical plant suffered an accidental valve opening incident caused by unauthorized access.
No Integrity Protection: TCP checksum cannot defend against man-in-the-middle tampering, and attackers can replay historical commands to cause system disorder.
Network Dependency: A single-point switch failure can paralyze the entire bus, as there is no redundant mechanism.
The national standard GB/T 41868-2022 clearly specifies that traditional Modbus TCP does not meet the industrial security level protection requirements.
| Protocol | Advantages | Applicable Scenarios |
| Modbus TLS | Superimposes TLS 1.2/1.3 encryption on the TCP layer, retains the original protocol structure, and supports two-way certificate authentication | Security reinforcement for legacy systems without replacing existing devices |
| OPC UA | Built-in encryption, identity authentication, data modeling, subscription mechanism, cross-platform support, object model and semantic description | New smart factories, cloud-edge collaboration, digital twins |
| MQTT over TLS | Lightweight publish-subscribe architecture, suitable for IoT scenarios with low bandwidth and high latency | Distributed sensor networks, edge node data reporting |
OPC UA has become the default recommended protocol for manufacturers including Siemens, Schneider Electric and ABB. Its "object model" can abstract a temperature sensor as ns=2;s=Boiler.Temperature to realize semantic integration, which far exceeds Modbus's "register address" mode.
Reference product link: PUSR Serial to Ethernet Converters Series
Network Planning
Assign a static IP (e.g. 192.168.1.100) to the PUSR serial to Ethernet converter, ensuring it is in the same network segment as the SCADA server. Leverage the TCP/IP or UDP/IP protocol capabilities supported by the device's 400MHz high-performance chip to guarantee transmission response speed. Open port TCP 502, and configure network access rules with the device's built-in SSL/TLS encryption mechanism to ensure data transmission security.
Serial Port Configuration
Connect the Modbus RTU device bus using the RS-485 interface supported by this product series. Set the baud rate (9600), data bits (8), stop bits (1), parity (None) and slave address (1~247), which must be fully consistent with the parameters of the original RTU devices. Rely on the device's built-in EMC Level 3 protection (anti-surge, anti-static, anti-EFT) and power reverse connection protection capabilities to avoid electromagnetic interference and wiring errors affecting communication stability.
Gateway Configuration
Log in to the built-in web management interface of the PUSR serial to Ethernet converter (http://192.168.1.100), and enable the Modbus TCP Server function under Modbus transparent transmission mode. Complete the device mapping configuration: specify the RTU slave address, and directly associate it with the corresponding starting TCP register address (e.g. 0x0000). You can enable the device's built-in edge computing function to pre-process Modbus data locally and reduce the operating pressure on the backend server; meanwhile, enable the MQTT protocol and automatic reconnection function, so that the connection can be restored automatically even after a temporary network outage, ensuring continuous data transmission.
Device Registration
Add a Modbus TCP device in the SCADA system, enter the pre-configured IP of the PUSR serial to Ethernet converter and port 502. Complete the register mapping configuration: for example, set the read operation of the 40001 (Holding Register) to correspond to address 0x0000 of the PUSR device, and the supporting virtual serial port software of this series can further simplify the configuration process.
Debugging and Verification
Use tools like ModScan32 or QModMaster to send read requests to the PUSR serial to Ethernet converter, to verify the transparency and integrity of data transmission from Modbus RTU to Modbus TCP. Check the indicators on the device panel: normal flashing of the RX/TX lights indicates unobstructed data interaction between the serial port side and the Ethernet side.
Experience Tips
The PUSR serial to Ethernet converter series has passed complete quality system certification. If you encounter parameter adaptation problems during the debugging phase, you can directly submit a work order to obtain global technical support from PUSR, and quickly locate problems with its application experience in more than 40 countries. If you are connecting to a Schneider M340 PLC, note that it uses "1-based addressing" by default. If the upper computer adopts the 0-based addressing rule, you need to offset the register address by 1 position before configuring the mapping, to avoid data reading misalignment.
The evolution from Modbus RTU to Modbus TCP is a microcosm of industrial communication transitioning from "local control" to "network intelligence". In an era where security requirements are increasingly strict, Modbus TLS serves as a transitional choice, while OPC UA is the foundation for future development. During deployment, prioritize selecting gateways with security certifications, and gradually migrate to semantic protocols to build a truly reliable industrial digital infrastructure.
