MODBUS TCP

Industrial Communication: Modbus TCP Protocol

Modbus is one of the most established and widely used protocols in industrial automation. Originally developed by Modicon in 1979, the protocol has evolved into Modbus TCP by migrating to Ethernet infrastructure. Today, it is an indispensable pillar of IIoT (Industrial Internet of Things) systems.

1. What is Modbus TCP?

Modbus TCP is a variant of the Modbus family that operates over the TCP/IP protocol stack. By utilizing the Ethernet physical layer instead of traditional serial lines (RS-485/RS-232), it enables high-speed data exchange over local area networks (LAN) or the global internet.

Client/Server Architecture: The device requesting data is the "Client" (e.g., a SCADA system), and the device providing data is the "Server" (e.g., a PLC).
Standard Port 502: Communication standardly occurs over TCP port 502.
High Speed: Operating at Ethernet speeds (10/100/1000 Mbps) allows for the polling of thousands of registers in milliseconds.

2. Packet Structure: The MBAP Header

The primary difference between Modbus TCP and its serial counterpart (RTU) is the MBAP Header (Modbus Application Protocol). This header replaces the checksum (CRC) found in serial packets, as the underlying Ethernet layer handles its own error checking.

Transaction Identifier (2 Bytes): Matches requests with corresponding responses.
Protocol Identifier (2 Bytes): Always 0, representing the Modbus protocol.
Length (2 Bytes): Indicates the byte count for the remainder of the message.
Unit Identifier (1 Byte): Identifies a specific slave device when communicating through a serial-to-Ethernet gateway.

3. Data Types and Register Structure

Regardless of the physical medium, Modbus organizes data into four primary tables:

Data Type	Access	Description
Coils (0x)	Read/Write	1-bit digital outputs (e.g., Motor Start/Stop)
Discrete Inputs (1x)	Read Only	1-bit digital inputs (e.g., Limit switches)
Input Registers (3x)	Read Only	16-bit analog inputs (e.g., Temperature sensor)
Holding Registers (4x)	Read/Write	16-bit configuration data (e.g., PID setpoints)

4. The Hubbox Advantage: Integration & Security

Hubbox Connect devices act as a bridge, transforming Modbus TCP data into secure, cloud-ready information.

Protocol Translation: Effortlessly convert Modbus TCP data into modern MQTT or OPC UA streams for ERP and Cloud integration.
Long Distance Access: Bridge devices across different continents using fiber optics or wireless bridges managed through the Hubbox interface.

Critical Security Layer

Security Warning: Modbus TCP lacks native encryption and authentication. Exposing port 502 directly to the internet is a severe security risk. Hubbox Connect solves this by encapsulating your Modbus traffic inside an encrypted VPN tunnel, ensuring that only authorized users can access the industrial network.

For Modbus TCP register mapping guides and Node-RED integration flows: www.hubbox.io