Modbus TCP Server

Note: Modbus TCP Server is only supported on TKG (gateway running TKL) and TKE (edge server) deployments. The cloud platform does not support this feature.

ThinkLink's built-in Modbus TCP Server exposes device telemetry data and attributes as Modbus registers, allowing SCADA, HMI, and other industrial systems to read live IoT data over the standard Modbus TCP protocol. Bidirectional control is also supported: when a third-party system writes to a register, ThinkLink can automatically trigger a bound RPC to send a command back to the downstream IoT device.

Data Flow

ThinkLink device reports telemetry / attribute change
        ↓
Modbus registers auto-update
        ↑ (read registers)
SCADA / HMI / PLC (Modbus TCP Client)
        ↓ (write registers)
ThinkLink triggers bound RPC → sends command to IoT device

Configuration Steps

Step 1: Start the Modbus TCP Server

1. Confirm that the system administrator allows ModbusTcpServer in System Config → Organization Service Permissions.
2. Open System Management → Server Configuration → Modbus TCP Server in the current organization.
3. Configure the listening port (typically 502), click Submit, then Restart.
4. Refresh and check the running status and any error message.

Since 2.00.077, disabling global Modbus permission stops the service and prevents point-table access, configuration changes, manual starts, and automatic recovery. Configuration and mappings are retained. After allowing it again, click Restart manually. Saving configuration does not change the actual running status, and stopping the service preserves saved settings.

Each running instance provides Modbus service for one organization. Switching the service organization replaces the previous instance; different ports do not provide concurrent service for multiple organizations in the same process.

Step 2: Enable Modbus Mapping on a Device

Open the target device's configuration in Device Management:

1. Switch to the Basic Info tab
2. Set the modbus_tcp switch to TRUE
3. Save the device configuration

Only devices with modbus_tcp enabled will have their data mapped into the register address space.

Step 3: Configure Register Mappings

Go to Maintenance → Modbus TCP and open the register list. Add mapping entries for each enabled device:

Field	Description
Device EUI	Select the target device whose data you want to map
Register type	coils (1-bit read/write) / discrete_inputs (1-bit read-only) / holding_registers (16-bit read/write) / input_registers (16-bit read-only)
Register address	Hexadecimal address; ThinkLink automatically detects conflicts within the same register type
Data source	Which device data source to map: telemetry_data / shared_attrs / server_attrs
Field path (name_path)	The specific field name; supports dot-notation for nested paths, e.g. temperature, sensor.value
Value type	boolean / string / number
RPC binding	Optional. When bound, a Modbus write to this register automatically triggers the specified RPC to send a command to the device

Read / Write Behavior

Reading (Client reads from ThinkLink)

• ThinkLink returns the current value of the mapped device field
• Values update automatically after each device uplink — no additional polling configuration needed

Writing (Client writes → ThinkLink → device)

• A write is only actionable if the register has an RPC binding
• ThinkLink receives the write, then calls the bound RPC using the written value as the parameter
• The RPC dispatches the command to the device via LoRaWAN or MQTT

Typical use: a SCADA system writes a holding_register to change a device's sampling interval. ThinkLink triggers the "Set Sampling Interval" RPC and pushes it down to the device.

Export / Import Register List

The Modbus register list (address mapping table) can be exported to Excel for delivery to third-party system integrators, and can also be imported from Excel for bulk configuration.

1. Click "Export" on the register list page to download the full mapping
2. Modify as needed, then click "Import" to batch-write back (existing entries with the same ID are overwritten)

Before deployment, the register list is typically handed to BMS/SCADA integrators so they can configure their Modbus Client point addresses. The exported file includes object type, address, name, description, data type, unit, and read/write permissions.

Related Pages

• Configure BACnet fields in a thing model → BACnet
• Enable modbus_tcp on a device → Device Management