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Version History
Updated | Document Version | Description |
2026-06-18 | V1.0 | Initial version |
Gateways & Firmware Requirement
Software | E2C Factory | E2C Field | E2C Facility |
Gateways supporting | EG3120; EG5120; EG5200 | LG3120 | LG5120 |
Firmware Requirement (RobustOS Pro Version) | EG3120 (≥ROS Pro:2.4.103); EG5120 (≥ROS Pro:2.4.105); EG5200 (≥ROS Pro:2.3.108) | LG3120 (=ROS Pro:2.4.111) | TBD |
1. Manual Description
1.1 Manual Purpose
This manual guides technicians to quickly configure the Robustel edge computing gateway to collect data from Southbound Modbus devices (Modbus TCP or Modbus RTU). It also covers how to forward the collected data to Northbound systems using MQTT or an OPC UA.
1.2 Scope of Application
This manual is written for field implementation engineers and technicians. You do not need deep theoretical knowledge of the Modbus protocol. By following these step-by-step instructions, you can easily connect devices, read tags, and forward data.
2. Basic Concepts & Network Topology
2.1 Gateway Role in the Network
In the Modbus network, the Robustel Gateway acts as the data forwarding hub. Its job is to actively initiate requests, read data from the underlying Modbus devices (such as PLCs, VFDs, smart meters, or temperature controllers), and then translate this data into IIoT protocols (like MQTT/OPC UA) for Northbound systems.
2.2 Connection Topology
- Modbus TCP: Devices are connected via standard Ethernet cables. The gateway and the Modbus devices are typically in the same local network subnet. If they are not in the same subnet (e.g., cross-network segment via a router), ensure network routing is properly configured and the port number (default 502) is not blocked by firewalls.
Figure 2-1 Modbus TCP connection topology
- Modbus RTU: Devices are daisy-chained via an RS485 serial cable. You must connect the "A+" and "B-" wires correctly, ensure all devices share the same baud rate, data bits, parity, and stop bits, and add a 120Ω termination resistor at both ends of the bus.
Figure 2-2 Modbus RTU connection topology
3. Preparations
3.1 Hardware Device Requirements
Device Type | Specific Requirements |
Gateway | Robustel Gateway with E2C Field / E2C Factory / E2C Facility software installed. In this document, we use LG3120 with E2C Field for an example. |
Modbus TCP Device | PLC, meter, sensor, etc. supporting Modbus TCP protocol (IP address, port number, and slave address must be known). |
Modbus RTU Device | VFD, temperature controller, smart meter, etc. supporting Modbus RTU protocol (serial parameters and slave address must be known). |
Wiring | Standard Ethernet cable (for TCP) or 2-wire RS485 cable (for RTU). |
PC | A laptop connected to the gateway's LAN port for web configuration. |
3.2 Environment and Network Requirements
- The PC and the Gateway must be in the same network segment. (Default Gateway IP: 192.168.0.1)
- It is highly recommended that you have the Register Map (Point List) provided by the Modbus device manufacturer. This list tells you the specific meanings, addresses, function codes, and data types of the tags. If you do not have it, you may need trial and error to determine the correct register configuration.
4. Southbound Configuration: Modbus TCP
Chapter Goal: Connect and collect data from a Modbus TCP device. You will complete 3 tasks in sequence:
- Configure the gateway's LAN IP to ensure network connectivity with the device
- Add a Modbus TCP device in the system
- Import or manually add tags
After completion, you should see real-time device data on the gateway interface.
If your device uses an RS485 serial connection, skip to Chapter 5.
4.1 Physical Layer: Configuring LAN IP
Purpose of this step: To establish basic network communication. The gateway's LAN IP must be configured so it can successfully "ping" and communicate with the target Modbus TCP device over Ethernet.
- Log in to the gateway's Web GUI (Default IP: 192.168.0.1, User: admin, Password: admin).
- Go to Network > LAN.
- Edit the LAN1 interface and fill in the parameters according to the table below:
Parameter Name | Configuration Description | Recommended / Example Value |
IPv4 Address and Subnet Mask | Set the gateway's IP to be in the same subnet as the end Modbus device | e.g., 192.168.0.1/24 (if device is 192.168.0.100) |
- Click Submit and Save & Apply to apply the network configuration.
Figure 4-1 Configure the gateway LAN IP
4.2 Adding a Modbus TCP Device
Purpose of this step: To define the target device in the Data Collection engine so the gateway knows exactly which IP address and port to send data requests to, and how to identify itself for slave communication.
First, click Edge Computing in the gateway's Web GUI to enter the E2C Field software. Then follow these steps to add a Modbus TCP device.
- Go to Data Collection > Wired Device.
- Click the + Add Device button to add a new device.
- Configure the following parameters in the popup window:
Parameter Name | Configuration Description | Recommended / Example Value |
Name | Create a custom name for easy identification on the platform | Packaging-Machine-03 |
Driver (Protocol) | Select the protocol type for the Southbound device | Modbus TCP |
IP Address | The IP address of the target PLC/Device | 192.168.0.3 |
Port Number | TCP communication port | Keep default 502 unless specified otherwise |
Slave Address | The Modbus Slave ID of the device | 1 |
Polling Cycle | Frequency of data requests (1~999999Seconds) | 10 |
Description | Additional info for user reference. | Machine 03 in Workshop A. |
Enable High-speed Collection Mode | Supports millisecond-level data acquisition. Once published, the system automatically generates high-speed collection groups. Note: High-speed tags and their information cannot be modified after generation; only regular tags can be maintained. To change high-speed settings, create a new device. | Enabled |
High-speed Polling Cycle | Must be selected when high-speed mode is enabled. Supported intervals: 50ms, 100ms, 200ms, 500ms. | 100ms |
Package Count | Must be selected when high-speed mode is enabled. Defines how many polling cycles occur before data is reported as a batch. Supported values: 10, 20, 30. | 30 |
Connection Timeout | Limit for establishing a TCP connection (ms). | 10000 |
Collection Timeout | Limit for the device to respond to requests (ms). | 1000 |
Communication Interval | Minimum time between two requests (ms). | 10 |
Enable Batch Collection | Enabled by default for efficiency. | Enabled |
Max Batch Count | Max tags per batch request (Default 64). | 64 |
Byte Order Settings | Sequence of data bytes (Default ABCD). | ABCD |
- Click Save to create the device card.
Figure 4-2 Add a Modbus TCP device
4.3 Adding tags
Purpose of this step: To define which specific register values (tags) need to be read from the added Modbus device.
Method 1: Batch Import (Recommended)
If you have the register map provided by the device manufacturer, you can quickly create tags via batch import.
- In the Data Collection list, click on the device name you just created to enter its tag list.
- Click the Import button and download the template file.
- Fill in the tag information according to the template format:
Figure 4-3 Open batch import for Modbus TCP tags
Figure 4-4 Select a Modbus TCP point table
Figure 4-5 Check the imported Modbus TCP tags
- Upload the completed template; the system will automatically create the tags.
Figure 4-6 Verify imported Modbus TCP tags
Method 2: Manually Add
If batch import is unavailable, click Add and configure:
Parameter Name | Configuration Description | Recommended / Example Value |
Collection Type | Select High-Performance or Normal | e.g., Normal |
Name | Tag name, a readable name for the tag | Inlet_Pressure |
Tag Type | Select Device Tag or Virtual Tag. For reading data from device, select Device Tag | Device Tag |
Function Code | Select the appropriate Modbus Function Code (01-04) | 03 Holding Register |
Address | Enter the Modbus address (Decimal). Ensure it falls within the range of the selected register type (e.g., 40001 corresponds to Address 1) | 1 |
Data Type | Must match the device's data format | short (int16) |
R/W Permission | Select Read Only or Read & Write | Read Only |
Encoding Format | Appears only when the Data Type is set to string. Specifies the character encoding used to interpret the raw bytes of a string tag, ensuring the gateway decodes the data correctly as readable text. Common options: ASCII, UTF-8, UTF-16LE, UTF-16BE, etc. | UTF-8 |
Decimal Places | Number of decimal places (for Float/Double) | 2 |
Length | Appears only when the Data Type is set to string or raw-data. Specifies the length (in bytes) of the string or raw data to be read. | 1 |
Negation | Appears only when the Data Type is set to bool. When enabled, performs a logical NOT operation on the read boolean value. Default is "False" (no negation). | False |
Unit | Engineering unit | Mpa |
Associated Group | Assign the tag to a specific data group. | - |
Description | Brief explanation of the tag's purpose. | Total production count. |
Data Operation | Mathematical processing (Scaling, Offset, etc.). | None |
High/Low Byte Arrangement | Appears when the Data Type is a multi-byte numeric type (e.g., short, ushort, int, uint, float, double). Defines the byte order of multi-byte data within Modbus registers. Options: ABCD, BADC, CDAB, DCBA. Default is ABCD if not specified. | ABCD |
Figure 4-7 Manually add a Modbus TCP tag
- Repeat for all tags. Once finished, click Publish in the top right corner to apply the configuration.
- Check the Latest Value column. If live numbers appear and update periodically, the configuration is fully successful.
- If you see a red status or no data, refer to Chapter 8 Troubleshooting for diagnosis.
Figure 4-8 Verify Modbus TCP data collection
5. Southbound Configuration: Modbus RTU
If your field device uses a 2-wire RS485 serial connection, follow this chapter.
First, click Edge Computing in the gateway's Web GUI to enter the E2C Field software. Then follow these steps to add a Modbus RTU device.
5.1 Physical Layer: Configuring the Serial Port
Purpose of this step: To align the gateway's serial port hardware settings with the field device. In RS485 communications, all devices on the bus must speak at the exact same speed and format, otherwise, no data can be exchanged.
Warning: Ensure the RS485 wiring is correct (A+ to A+, B- to B-). If communication fails later, try swapping the A and B wires.
- Log in to the gateway's Web GUI.
- Go to Interface > Serial Port, set the Serial Port Type to RS485.
Figure 5-1 Configure the RS485 serial port
- Go to Edge Computing > System Settings > Serial Port, find the serial port connected to your device (e.g., COM1) and click to configure.
- Configure according to the table below:
Parameter Name | Configuration Description | Recommended / Example Value |
Baud Rate | The communication speed. Must exactly match the end device | Usually 38400 or 9600 |
Data Bit | The length of the data payload | 8 |
Check Bit | No Parity / Even Parity / Odd Parity | No Parity |
Stop Bits | The end-of-byte signal length | 1 |
Parity | Error checking method | None |
- Click Submit to apply the serial configuration.
Figure 5-2 Configure serial port settings
5.2 Adding a Modbus RTU Device
Purpose of this step: To tell the Data Collection engine which serial port to listen on and what the RTU slave address of the target device is.
- Go to Data Collection > Wired Device.
- Click the + Add Device button to add a new device.
- Configure the following parameters in the popup window:
Parameter Name | Configuration Description | Recommended / Example Value |
Name | Create a custom name for easy identification on the platform | Senosr-IO-01 |
Driver (Protocol) | Select the protocol type for the serial device | Modbus RTU |
Communication Channel | Select the serial port that connects to the device | /dev/ttyCOM1 |
Slave Address | The Modbus Slave Address of the device (Range: 1-247) | 1 |
Polling Cycle | Frequency of data acquisition (Unit: Seconds) | 10 |
Description | Additional info for user reference. | Machine 03 in Workshop A. |
High-speed Data Collection Config | Enable millisecond-level high-speed acquisition. Disabled by default. When enabled, sub-options appear: High-speed Poll Interval, Package Count.Note: Groups are auto-generated and tags cannot be modified after publishing. | Enabled |
High-speed Polling Cycle | Appears only when high-speed mode is enabled. Polling cycle in milliseconds. Options: 50ms, 100ms, 200ms, 500ms. | 100ms |
Package Count | Appears only when high-speed mode is enabled. Number of polls before data is reported as a batch. Options: 10, 20, 30. | 10 |
Connection Timeout | The maximum time (in milliseconds) the gateway waits for a response after sending a request to the device. If no valid response is received within this period, the gateway considers the communication attempt failed. Default 10000, range 2-60000. This parameter also applies to Modbus RTU serial communication, defining the maximum wait time for a response from the serial device. | 10000 |
Communication Interval | Minimum idle time between two communication requests (milliseconds). Default 10, range 0-1000. | 10 |
Enable Batch Collection | Enable batch reading (reading multiple consecutive registers in one request). Enabled by default for efficiency. | Enabled |
Max Batch Count | Appears only when batch collection is enabled. Maximum number of registers per batch request. Range 2-125, default 64. | 64 |
High/Low Byte Arrangement | Device-level default byte order applicable to all tags under this device (can be overridden per tag if supported). Recommended to set consistently for multi-byte tags. Options: ABCD, BADC, CDAB, DCBA. Default ABCD. | ABCD |
Serial Parameters | Displays Baud Rate, Data Bit, etc., based on the selected channel for verification. | - |
Note: High/Low Byte Arrangement is the device‑level default; individual tags may override it (if supported by the firmware).
- Click Save to create the device card.
Figure 5-3 Add a Modbus RTU device
5.3 Adding tags
Purpose of this step: Let the software know which point data to collect.
Method 1: Batch Import (Recommended)
Operating steps are the same as Modbus TCP, refer to Section 4.3 Method 1.
Method 2: Manually Add
If batch import is unavailable, click Manually Add and configure:
Parameter Name | Configuration Description | Recommended / Example Value |
Name | Tag name, a readable name for the tag | e.g., Temperature |
Collection Type | Select High-Performance or Normal | e.g., Normal |
Tag Type | Select Device Tag or Virtual Tag. For reading data from device, select Device Tag | e.g., Device Tag |
Function Code | Select based on register type (e.g., 03 Holding Register) | e.g., 03 |
Address | Decimal Modbus address (e.g., 40001 corresponds to Address 1) | e.g., 1 |
Data Type | Must match the device (e.g., short, float) | e.g., short (int16) |
R/W Permission | Read Only, Read/Write, or Restricted by FC | e.g., Read Only |
Unit | Physical unit (e.g., Mpa, ℃, Hz) | e.g., Mpa |
Figure 5-4 Import Modbus RTU tags
- Repeat for all tags. Once finished, click Publish in the top right corner to apply the configuration.
- Check the Latest Value column. If live numbers appear and update periodically, the configuration is fully successful.
Figure 5-5 Manually add a Modbus RTU tag
6. Device Status and Data Verification
Once the configuration is complete, it is crucial to verify that the gateway is successfully communicating with the field devices and collecting accurate data.
6.1 Checking Device Connection Status
Purpose of this step: To confirm if the gateway has successfully established a connection with the Southbound Modbus device.
- Go to Edge Computing > Data Collection > Wired Device.
- Locate the configured device card in the list.
- Check the indicator on the top left corner of the device card:
- Green: The device connection is normal.
- Red: The device connection failed.
Figure 6-1 Check device connection status
6.2 Viewing Data Collection Status and Latest Values
Purpose of this step: To verify that the configured tags are successfully returning valid readings from the field device.
- Observe the Status, Latest Value, and Update Time columns:
Column Name | Description |
Status | Shows the collection status of each tag. A green icon indicates the tag is collected normally, while a red icon indicates collection failure |
Latest Value | Displays the real-time data collected from the field device (e.g., a temperature reading of 23.5) |
Update Time | Shows the timestamp of the last successful update for this tag |
Figure 6-2 View data collection status and latest values
6.3 Modifying Data Values (Write Operation Verification)
Purpose of this step: To send control commands back to the field device (e.g., turning on a relay or setting a new temperature setpoint).
- Ensure the tag has Read/Write permissions (e.g., Holding Register with Function Code 03, and R/W Permission set to Read & Write). Points with Read Only permission (like Input Registers with Function Code 04) cannot be modified.
- In the tag list, click the Edit icon (usually a pencil symbol) next to the point you want to modify.
- A Modify Tag Value dialog box will appear. Enter the new desired value and click Save or Confirm.
- Observe the Latest Value column. It should update to reflect the new value if the write operation was successful.
Figure 6-3 Modify a data value
7. Northbound Configuration: Two Core Scenarios
Once the gateway successfully reads data from Southbound Modbus devices, you can forward this data to Northbound systems.
7.1 Creating Data Groups (Preparation for Data to Cloud)
Purpose of this step: To organize the collected Modbus tags into logical groups before sending them to the cloud or application systems. Grouping allows you to control exactly which tags are packed and uploaded together in a single message.
- Go to the Data to Cloud > Groups page.
- Click the + button to add a new group, and set the report type (Period, Change or Change & Period).
- In the tag selection area, select the Modbus devices and the specific tags (tags) you want to include in this group.
- Click Save to create the group.
Note: You will use this created Group in the following MQTT transmission settings to define the upload content.
Figure 7-1 Create a data group
7.2 Scenario A: Forwarding to MQTT Broker (Data to Cloud)
This scenario is used to push data to cloud platforms (like AWS, Azure, or private IoT platforms).
- Go to Data to Cloud, click Create Cloud Service, and choose MQTT.
- Configure the MQTT connection:
Parameter | Description | Example |
Server Address / Port | Enter your cloud server IP and Port (usually 1883/8883) | 192.168.1.100:1883 |
Client ID | Set a unique ID for this connection | Gateway_001 |
Username / Password | Fill in if required by your Broker | - |
- Go to the Message Management section and click Add to add a publish rule:
Parameter | Description | Example |
Topic | Define the topic where data will be published | /factory/line1/temperature |
Data Group | Select the group you created | Group_01 |
- Click Save and ensure the connection status turns to Connected.
- Use an MQTT client tool to subscribe to the corresponding Topic and verify data reception.
Figure 7-2 Configure the MQTT cloud service
Figure 7-3 Configure MQTT message management
Figure 7-4 Verify MQTT data forwarding
7.3 Scenario B: Forwarding to OPC UA Server (Data to SCADA)
This scenario is used to provide data to local factory SCADA or BMS systems. The gateway acts as an OPC UA Server, waiting for the SCADA to connect to it.
- Go to Data to Cloud > OPC UA.
- Click Add to add an OPC UA service.
- Enable the OPCUA Server Configuration and set the Port (default 4840).
- Set Maximum Connections, and choose Authentication Mode (Anonymous or User). If User Mode is selected, you need to set a username and a password.
- Click Submit and Save.
- In your SCADA software, use the URL opc.tcp://[Gateway_LAN_IP]:4840 to discover the gateway and browse the Modbus tags.
Figure 7-5 Configure the OPC UA server
Figure 7-6 Verify OPC UA data forwarding
8. Troubleshooting & FAQ
8.1 Modbus TCP Common Issues
Symptom | Possible Cause | Solution |
Device status persistently offline | Gateway and device in different subnets | Ensure gateway IP is in the same subnet as the target, or properly configure cross-segment access via router |
Wrong target IP or port | Verify the Target IP and Port (Default 502) | |
Firewall blocking the Modbus TCP port you configured | Check firewall rules on gateway and device side, open the Modbus TCP port you configured | |
Slave address mismatch | Confirm the actual slave address of the device; some devices default to non-1 | |
Connected but values are null or incorrect | Wrong function code | Check register map to confirm correct function code (01/02/03/04) |
Data type mismatch | Confirm the actual PLC data type (INT/UINT/FLOAT, etc.) | |
Wrong byte order | Switch Byte Order (ABCD/CDAB/BADC/DCBA) | |
Write operation failed | Function code does not support write | Ensure function code is 01/03, not read-only like 04 |
Value out of range | Confirm the write value is within the device allowed range | |
Data updates slowly or stutters | Polling cycle too long | Reduce the Polling Cycle value |
Batch read too large causing timeout | Reduce Max Batch Count, or disable Batch Read | |
High network latency | Check network quality, use wired connection |
8.2 Modbus RTU Common Issues
Symptom | Possible Cause | Solution |
Device status persistently offline | Serial parameters mismatch | Check device manual, ensure baud rate, data bits, parity, and stop bits exactly match |
Wrong slave address | Confirm the actual slave address of the device; some devices default to non-1 | |
Connected but values are null or incorrect | Wrong function code | Check register map to confirm correct function code (01/02/03/04) |
Data type mismatch | Confirm the actual device data type (INT/UINT/FLOAT, etc.) | |
Wrong byte order | Switch Byte Order (ABCD/CDAB/BADC/DCBA) | |
Write operation failed | Function code does not support write | Ensure function code is 01 (Coil) or 03 (Holding Register). Function codes 02 (Discrete Input) and 04 (Input Register) are read-only and cannot be written |
Value out of range | Confirm the write value is within the device allowed range | |
Data updates slowly or stutters | Polling cycle too long | Reduce the Polling Cycle value |
Batch read too small causing inefficiency | Increase Max Batch Count | |
Collection timeout too short | Increase Collection Timeout value | |
Some devices offline during multi-device polling | Slave address conflict | Disconnect and verify one by one, ensure unique addresses on bus |
Response timeout too short | Increase Collection Timeout value | |
Device does not support batch read | Disable Batch Collection, switch to single-point polling | |
Values jump or abnormally large | Wrong byte order | Switch Byte Order (ABCD/CDAB/BADC/DCBA) |
Data type error (signed/unsigned confusion) | Confirm the actual device data type | |
Address misalignment (double-word/float crossing register boundary) | Ensure addresses are contiguous and do not overlap with other points |
9. Core Parameter Quick Reference
For quick on-site configuration, refer to these industry-standard default values. Unless explicitly required by the site, do not change them:
Parameter Name | Protocol | Default/Recommended Value | Description |
Port Number | Modbus TCP | 502 (TCP) | Standard Modbus TCP port |
Slave Address | Modbus TCP/RTU | 1 | Range 1-247; 0 is broadcast address |
Baud Rate | Modbus RTU | 38400 or 9600 | Must exactly match the end device |
Data Bit | Modbus RTU | 8 | Usually fixed at 8 |
Check Bit | Modbus RTU | No Parity | Options: None/Even/Odd |
Stop Bits | Modbus RTU | 1 | Usually fixed at 1 |
High/Low Byte Arrangement | Modbus TCP/RTU | ABCD | Byte arrangement sequence for multi-byte numeric types (short/ushort/int/uint/float/double); if data is abnormal, try other arrangements (e.g., CDAB, BADC, DCBA) |
Polling Cycle | Modbus TCP/RTU | 10 seconds | Adjust according to data real-time requirements |
Enable Batch Collection | Modbus TCP/RTU | Enabled | Enabled by default for efficiency |
Max Batch Count | Modbus TCP | 64 | Max tags per request |
Max Batch Count | Modbus RTU | 64 | Range 2-125 |