LTE Modems Multi-Protocol Conversion: How to Achieve Compatibility with Protocols Such as Modbus and DL/T645?

In the complex scenarios of the Industrial Internet of Things (IIoT), communication protocols between devices are akin to languages spoken in different countries—Modbus serves as the "lingua franca" in the field of industrial automation, DL/T645 is the "standard dialect" in the power industry, while TCP/IP and MQTT act as the "digital bridges" in the IoT era. When an LTE modem needs to simultaneously connect to devices such as PLCs, sensors, and electricity meters, and upload data to the cloud, multi-protocol compatibility becomes the key determinant of the system's success or failure. This article will dissect, based on practical experience, how LTE modems achieve cross-protocol communication and share technical details and pitfall avoidance guides from real-world scenarios.

1. The Essence of Protocol Compatibility: From "Language Translation" to "Data Reorganization"

The communication protocols of industrial devices are essentially agreements on data formats and transmission rules. For example:

• Modbus RTU: Adopts a master-slave mode, defines operations through function codes (such as 0x03 for reading holding registers), transmits data in binary format, and uses CRC16 for verification.
• DL/T645-2007: A dedicated protocol for electricity meters, encodes the data field in BCD code, adds 33H to each byte for processing, has a fixed frame header of 0x68, and supports dual channels of infrared/RS485.
• MQTT: A lightweight IoT protocol that publishes and subscribes to data through topics (Topic), suitable for weak network environments.

The multi-protocol compatibility of LTE modems essentially involves constructing a "protocol conversion engine" between the hardware interface and the network layer. Taking USR-N720-C1 from USR IoT as an example, its internal logic can be divided into three layers:

• Physical Layer Adaptation: Connects to devices through RS485/RS232 interfaces and supports adaptive baud rates ranging from 1200 to 115200 bps.
• Protocol Parsing Layer: Identifies the Modbus frame header, the 0x68 start symbol of DL/T645, extracts the data field, and decrypts it (such as subtracting 33H in DL/T645 processing).
• Data Reorganization Layer: Converts the parsed data into a unified format (such as JSON) and uploads it to the cloud via TCP/IP or MQTT.

Key Challenges: The timing, verification, and data encoding methods vary significantly among different protocols. For example, Modbus RTU requires strict master-slave timing, while DL/T645 allows electricity meters to actively report data; MQTT requires a clear definition of topic structures, whereas Modbus directly operates on register addresses. LTE modems must ensure that data is not lost or out of order during concurrent multi-protocol operations through state machine management and asynchronous processing mechanisms.

2. Practical Cases: How LTE Modems "Handle" All Protocols from Water Quality Monitoring to Electricity Meter Reading?

Case 1: Modbus + MQTT Linkage in a Sewage Treatment Plant

A sewage treatment plant needs to monitor parameters such as the pH, dissolved oxygen, and COD (Chemical Oxygen Demand) at the water inlet and trigger alarms through 4G LTE modems. The system architecture is as follows:

• Device Layer: pH sensor (Modbus RTU), dissolved oxygen sensor (Modbus TCP), COD sensor (custom protocol).
• LTE Modem Layer: USR-G780s 4G LTE modems, connected to sensors via RS485, supporting dual protocols of Modbus RTU/TCP.
• Cloud Layer: Alibaba Cloud IoT platform, receiving data via MQTT.

Technical Details:

• Protocol Conversion: The LTE modem converts the binary data from Modbus RTU into JSON format, for example:

• Threshold Alarm: The cloud defines rules. When the COD exceeds 500 mg/L, it issues a control instruction via MQTT:

The LTE modem parses the instruction and closes the water inlet valve through relay output.

Pitfall Avoidance Guide:

• Timing Issues: The polling interval of Modbus RTU must be longer than the device's response time; otherwise, frames will be lost.
• Data Precision: The format of floating-point numbers must be unified during transmission (such as IEEE 754) to avoid parsing errors in the cloud.
• Network Jitter: The QoS level of MQTT should be selected according to the scenario (QoS 0 is suitable for high real-time requirements but allows packet loss, while QoS 1 is suitable for critical data).

Case 2: DL/T645 + TCP/IP Integration in an Electricity Meter Reading System

A power grid company needs to remotely read data from 100,000 smart electricity meters and upload the data simultaneously to a marketing system and a metering center. The system architecture is as follows:

• Device Layer: Smart electricity meters (supporting DL/T645-2007).
• LTE Modem Layer: USR-N720-C1, connected to electricity meters via RS485, supporting DL/T645 to TCP/IP conversion.
• Cloud Layer: Marketing system (Oracle database), metering center (Hadoop big data platform).

Technical Details:

• Protocol Conversion: The LTE modem converts the BCD code data from DL/T645 into decimal format, for example:
  • Original electricity meter data: 68 AA AA AA AA AA AA 68 11 04 33 33 33 33 33 34 16 (reading the total forward active energy).
  • After parsing: Total forward active energy = 1234.56 kWh.
  • Converted JSON:

• Multi-Center Push: The LTE modem sends data simultaneously to the marketing system (port 8080) and the metering center (port 9090) via TCP/IP to ensure data consistency.

Pitfall Avoidance Guide:

• Data Encryption: Since DL/T645 does not define an encryption mechanism, the SSL/TLS function of the LTE modem should be used to ensure transmission security.
• Clock Synchronization: The clocks of the electricity meters and the LTE modem need to be calibrated regularly to avoid errors in data timestamps.
• Large-Scale Deployment: The LTE modem needs to support dynamic IP allocation and domain name resolution to avoid the high cost of fixed IPs.

3. Selection Guide: How to Choose an LTE Modem That is "Compatible with Multiple Protocols"?

3.1 Protocol Compatibility: Covering Mainstream Industrial and IoT Protocols

• Industrial Protocols: Modbus RTU/TCP, DL/T645, OPC UA, Profinet.
• IoT Protocols: MQTT, CoAP, HTTP/HTTPS.
• Special Protocols: Support custom JSON templates to adapt to private protocols.

Case: USR-N720-C1 supports Modbus RTU/TCP, DL/T645, and custom JSON templates, allowing direct connection to PLCs, electricity meters, and sensors without the need for a protocol conversion gateway.

3.2 Data Security: Full-Link Protection from Transmission to Storage

• Transmission Encryption: Supports SSL/TLS 1.2/1.3, as well as one-way/two-way certificate authentication.
• Storage Encryption: Adopts AES-256 encryption when caching data to a TF card.
• Access Control: Supports IP whitelisting and MAC address binding.

Case: A chemical park uses an LTE modem that supports SSL/TLS to prevent the tampering of toxic gas concentration data, meeting the requirements of Class III of the Cybersecurity Classification Protection 2.0.

3.3 Reliability: Adapting to Harsh Industrial Environments

• Hardware Design: Uses industrial-grade components, operates within a temperature range of -40°C to 85°C, and has an EMC anti-interference rating of ≥ Grade 4.
• Software Mechanisms: Features automatic reconnection after disconnection, data caching and resumption of transmission, and heartbeat packet keep-alive.
• Redundancy Design: Supports dual SIM cards, dual 4G modules, and dual power inputs.

Case: A cotton planting base in Xinjiang uses a rail-mounted LTE modem that supports solar power supply and has operated stably for 30 days in an environment of -30°C.

3.4 Usability: Reducing Development and Operation & Maintenance Costs

• Configuration Tools: Supports remote configuration via Web/APP/WeChat mini-programs, eliminating the need for on-site debugging.
• Log Management: Records events such as device online/offline status, data transmission and reception, and abnormal alarms.
• OTA Upgrades: Supports remote firmware updates to fix vulnerabilities without recalling devices.

Case: A steel plant remotely fixed a bug in the blast furnace temperature monitoring program through the OTA function of the LTE modem, avoiding downtime losses.

4. Future Trends: Three Major Directions for Protocol Compatibility of LTE Modems

• 5G + Edge Computing Convergence: LTE modems will integrate edge computing capabilities to complete data cleaning, aggregation, and preliminary analysis locally, reducing the load on the cloud. For example, USR-N720-C1 already supports local calculation of electricity consumption from Modbus data and triggers overload alarms.
• AI-Empowered Protocol Parsing: Machine learning will be used to automatically identify unknown protocol formats, reducing manual configuration costs. For example, the Qianfan large model can train LTE modems to recognize the communication characteristics of non-standard devices.
• Open-Source Protocol Ecosystem: Open-source implementations of protocols such as Modbus and MQTT (such as FreeModbus and Eclipse Paho) will lower the development threshold for LTE modems and promote the standardization of protocol compatibility.

Conclusion: Protocol Compatibility is the "Infrastructure" of the Industrial IoT

In the "language maze" of the industrial IoT, the multi-protocol compatibility capability of LTE modems is like a translator proficient in multiple languages—it not only needs to accurately understand the grammar of each protocol but also find a balance between real-time performance, reliability, and security. From Modbus alarms in sewage treatment plants to DL/T645 integration in electricity meter reading, and then to the future convergence of 5G and edge computing, the protocol compatibility technology of LTE modems is constantly breaking boundaries and paving the way for the large-scale implementation of the industrial IoT.

For technical practitioners, understanding the essence of protocol compatibility is not just about mastering a few lines of code or configuration parameters but about building a systems thinking approach that spans different fields and devices. As the mission of USR IoT states: "Connect value, value connection"—when LTE modems successfully break through protocol barriers, the flow of value among devices, data, and applications naturally occurs.