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Gateways & Firmware Requirement

1. Manual Description

1.1 Manual Purpose

This manual guides field technicians to quickly configure the Robustel edge computing gateway to forward collected data to higher-level systems (such as SCADA, MES, or upper computer software) using the Northbound Modbus TCP Slave protocol.

1.2 Scope of Application

This manual is written for field implementation engineers. You do not need deep theoretical knowledge of the Modbus protocol. By following these step-by-step instructions, you can easily configure the gateway as a Modbus TCP Slave, map internal data tags to Modbus registers, and verify the data using third-party tools.

2. Basic Concepts & Network Topology

Gateway Role: From Collector to Server

In a typical IIoT project, imagine the gateway as a translator between your local sensors and the upper-level system. The data flow consists of two main stages:

First Stage (Southbound): The local field devices (such as a Modbus TCP Sensor) are connected to the gateway. The gateway actively collects data from these devices and stores the values internally as Tags.

Second Stage (Northbound): The gateway operates as a Modbus TCP Slave on the network. It takes those internal Tags, converts them into standard Modbus register addresses, and exposes them so that a higher-level system, such as SCADA, MES, or a Modbus testing tool like Modbus Slave, can continuously read the values.

Figure 2-1 Modbus TCP northbound network topology

3. Preparations

(Note: This document uses the LG3120 gateway and E2C Field software as an example. The configuration process for other supported gateway and software combinations is very similar.)

Before starting the configuration, please ensure the following preparations are met:

3.1 Hardware Device Requirements

3.2 Environment and Network Requirements

• LAN Connectivity: The gateway, the debugging PC, and the Modbus TCP device must be connected to the same Local Area Network (LAN) (e.g., connected to the same switch or the gateway's LAN ports).
• IP Allocation: Ensure all devices have valid IP addresses in the same subnet (e.g., 192.168.0.x) and there are no IP conflicts.
• Port Accessibility: Ensure the PC's firewall allows TCP traffic on port 502 (the standard Modbus TCP port).
• Modbus Testing Tool: Download and install Modbus Slave on your debugging PC. It is a free, industry-standard tool used to search and verify Modbus TCP Slave data.

4. Prerequisite: Southbound Data Collection

Before forwarding data via Modbus, the gateway must first collect data from the field devices. In this guide, we will use a Modbus TCP sensor simulator as an example to collect two tags: Temperature and Humidity.

4.1 Logging into E2C Field

1. Connect your PC to the gateway's LAN port.
2. Open a web browser and enter the gateway's IP address (the default is usually 192.168.0.1).
3. Log in with your credentials and click【Edge Computing】 in the gateway's Web GUI to enter the E2C Field interface.

Figure 4-1 Open E2C Field from the gateway Web UI

4.2 Adding a Modbus TCP Device

(Note: This section assumes that you already have a Modbus Slave device physically connected to the gateway via an Ethernet cable.)

1. On the left menu of E2C Field, go to【Data Collection】>【Wired Device】.
2. Click the【+Create】 button to add a new device.
3. Select【Modbus TCP】 as the Driver.
4. Fill in the basic device parameters (refer to your sensor's manual):

5. Click【Save】.

4.3 Creating Data Tags (Temperature & Humidity)

Now we need to read the specific registers from the Modbus device. Let's assume the client's Modbus Slave manual states that the Temperature data is stored in Holding Register 40001 and Humidity is stored in Holding Register 40002.

1. Click on the newly created device (Sensor1) to enter its configuration page.
2. In the Tag List section, click【Add】 to create two tags. Fill in the following parameters:

3. Click Save at the bottom, and then click Publish at the top right to start data collection.
4. Tip: You can check the Status to verify if the Temperature and Humidity tags aresuccessfully read values before proceeding to the Modbus configuration.

Figure 4-2 Create Modbus TCP data tags

Figure 4-3 Verify Modbus TCP data tag status

5. Northbound Configuration: Modbus TCP Slave

This is the core step where we expose our internal tags to the Modbus network.

5.1 Creating the Modbus TCP Slave

1. Go to【Data Forwarding】.
2. Click the 【+ Create】 button at the top, select 【Modbus TCP Slave】, and click 【Save】.
3. You will enter the Modbus TCP Configuration page. Enable Modbus TCP Configuration and configure the Basic Settings:

4. Click 【Submit】 to save the basic settings.

Figure 5-1 Configure the Modbus TCP slave

5.2 Mapping Tags to Modbus Registers (Crucial Step)

After completing the basic settings, you need to add a slave and map the tags created during data collection to specific register addresses under this slave.

5.2.1 Mapping Strategy Guide (Read before Configuration)

Before you start mapping, please review the following planning principles:

• Address Continuity: To minimize reads from your SCADA, map tags that are polled together to consecutive register addresses. For example: Temperature → 40001, Humidity → 40002.
• Data Type Footprint: An int16 consumes 1 register. A float32 consumes 2 consecutive registers (e.g., 40001-40002). A string consumes multiple registers by byte. Plan your address space to avoid overlaps.
• Function Code Selection: Most SCADA systems use 03 Holding Register for analog values. 04 Input Register is read-only and suitable for device status. 01/02 are for boolean values.
• Naming Convention: Use meaningful names for slaves and mapping tables (e.g., Slave_SCADA_Room1) for easier maintenance.

5.2.2 Adding a Slave

1. In the "Modbus TCP Slave List" section, click 【Add Slave】.
2. Configure the following parameters:

Figure 5-2 Add a Modbus TCP slave mapping table

5.2.3 Adding Mappings

After adding the slave, you need to add tag mappings for this slave. The upper-level system (e.g., Modbus Slave) does not understand internal tag names like "Temperature"; it only recognizes standard Modbus register addresses (e.g., 40001).The system supports two methods: Add Mapping (single) and Batch Add Mapping (bulk creation).

Method 1: Add Mapping (Single)

Use this method to map tags one by one.

1. In the row of the newly created slave, click the 【Add】 button on the right.
2. Fill in the mapping parameters according to the table below, then click 【Save】 :

Figure 5-3 Add a single Modbus register mapping

Method 2: Batch Add Mapping (Bulk Creation)

Use this method to map multiple tags from the same device at once, saving significant configuration time.

1. In the row of the newly created slave, click the 【Batch Add Mapping】 button.
2. In the pop-up window, configure the following:

3. After selecting the device, the table below will automatically display all tags from that device that match the selected function code. Check the boxes next to the tags you want to map (multiple selection allowed).
4. The system will automatically calculate the Mapped Address for each selected tag based on the starting address and the data type size. For example, if you select 3 tags starting from address 1, the mapped addresses will display as 40001 ~ 40004 (the exact range depends on the number and data types of the selected tags).
5. Click 【Save】 to complete the batch mapping.

Figure 5-4 Batch add Modbus register mappings

Final Steps

1. Repeat steps 5.2.1 and 5.2.2 to create all required slaves and complete the mapping configuration for all tags you need to forward (e.g., Humidity).
2. All configurations take effect immediately upon saving; no need to click the 【Publish】 button.

6. Verifying Configuration with Modbus Slave

Now, let's test if our gateway is successfully working as a Modbus TCP Slave.

1. Open the Modbus Slave software on your PC.
2. Click Connection > Connect on the menu bar.
3. In the pop-up window, configure the following parameters:

• Connection Type: Select Modbus TCP/IP
• IP Address: Enter the gateway's IP address (e.g., 192.168.0.1)
• Port: Enter 502 (or the port number you configured in Basic Settings)
• Click OK

4. Click Setup > Slave Definition on the menu bar, and configure the following parameters:

• Slave ID: Enter the slave address you configured (e.g., 1)
• Function: Select 03 Holding Register (4x)
• Address: Enter 1 (corresponding to mapped address 40001)
• Quantity: Enter the number of registers to read (e.g., 1 for Temperature)
• Click OK

5. In the Modbus Slave main window, you should see real-time data displayed at addresses 40001. If the displayed values match the values collected from the southbound device, your Modbus TCP Slave data forwarding is successful.

Figure 6-1 Verify Modbus TCP slave data with Modbus Poll

6. Testing Write-Back Function (Only for Read & Write points).If a mapped point shows Read & Write in the Mapping Table, you can use Modbus Slave to write a value to the gateway, and the gateway will write this value back to the corresponding southbound device.

• In Modbus Slave , locate the register address you want to write to (e.g., 40001).
• Double-click the Value cell for that address.
• Enter a new value in the pop-up window and click OK.
• Observe whether the value changes. If it updates successfully, the write-back function is working. If the value does not change or reverts to the original value, check:
• Whether the point's R/W Permission is Read & Write (if Read Only, write-back is not supported).
• Whether the southbound device supports write operations.

7. Common Troubleshooting

Debugging Workflow (Check in this order)

1. Network Layer: From your SCADA/upper computer, run ping [gateway IP address] in CMD/Terminal.- Fails? Check cable connections, IP address configuration, and whether your firewall allows the Modbus TCP port you configured.

2. Service Layer: Use the Modbus Slave software to try connecting to the gateway.- Timeout? Check whether the Modbus TCP Slave Configuration switch on the gateway is enabled and whether the port number matches your configuration (default 502).

3. Data Layer: Connected but reads 0 or error values?

• Go back to the gateway Web UI → Data Collection. Check if the Tag you mapped has live, updating values.
• If the Tag is fine, check if the Slave ID in your configuration matches the one in Modbus Slave.
• Check the Mapped Address: The offset address you configured (e.g., 1) maps to Modbus address 40001. Does the address in Modbus Slave match?