Modbus Technology Solutions

What is Modbus?

Modbus is one of the oldest and most widely used communication protocols in automation. Developed by Modicon in 1979 for controllers, it is now open and licence-free. A client (formerly master) polls values from a server (formerly slave), which holds its data in four register types: coils and discrete inputs for bits, holding and input registers for 16-bit values. Modbus RTU runs serially over RS-485, Modbus TCP over Ethernet on port 502.

How Modbus is used in the IIoT

Modbus is the brownfield protocol par excellence. Energy meters, frequency drives, inverters, HVAC and refrigeration, compressors, temperature controllers and many older PLCs speak Modbus, often for decades. For IIoT projects this means: instead of installing new sensors, a gateway reads the existing registers and provides the values via MQTT or OPC UA. Typical applications are energy monitoring, supervising auxiliary equipment and connecting machines that lack a modern interface.

Modbus RTU or Modbus TCP?

Both use the same data model but differ in transport. Modbus RTU runs over an RS-485 two-wire line, which can connect up to 32 devices without repeaters and, at low baud rates, cable lengths of up to about 1,200 metres. Modbus TCP uses standard Ethernet and is faster and easier to network. Gateways translate between the two. Compared with OPC UA, Modbus has no semantics at all: only the datasheet tells you which value sits in which register.

How Modbus data gets into the IIoT

In practice, a gateway or edge device sits on the bus or network as a Modbus client. It polls the registers of connected devices cyclically, for example every second or every minute, converts the raw values with the appropriate scaling into physical quantities and passes them on with name and unit via MQTT or OPC UA. Serial devices on RS-485 and devices with Modbus TCP can be combined in the same gateway. Write access to registers is possible but should be enabled deliberately.

What to consider when choosing

Most of the work lies in register mapping. You need the manufacturer's register table for every device and must set data types, scaling and byte order correctly, or implausible values result. Gateways with ready-made device templates save a lot of time here. Because Modbus neither encrypts nor authenticates, it belongs in an isolated network segment and never directly on the internet. With RS-485, clean cabling, termination resistors and unique device addresses determine stable operation.