Protocol Series: What is Modbus?

bg triangle top left

What is a protocol?

We’re talking about a communications protocol, which is a common ‘language’ which allows connected devices to ‘talk’ to and understand each other. An industrial control system will generally use a common protocol or language between devices such as a PLC, a HMI touch screen, a variable speed drive or a SCADA system.

What is Modbus protocol?

Modbus is a common network communications protocol used in industrial control systems. There are two common variants of Modbus: Modbus TCP and Modbus RTU. Modbus TCP is designed for use over an Ethernet network, while Modbus RTU works over a serial (RS232/RS485) connection. It was introduced in 1979 by Modicon, a PLC manufacturer who later became part of Schneider Electric.

Which devices support Modbus?

Modbus is a very common protocol, and most PLC and VSD manufacturers support either the TCP or RTU variants (or both) via onboard connectivity or separate communication cards. Schneider in particular offers broad support, while Allen-Bradley’s support varies: several MicroLogix models support it natively, while ControlLogix requires additional code blocks or third-party hardware. Siemens and Omron also offer Modbus support. Of course, there are also a plethora of converters or gateways available to convert from almost any protocol to Modbus.