What is Modbus?

Modbus, initially developed in 1979, is one of the oldest, most widely used industrial communications protocols. Traditionally, Modbus was/is implemented using RS-232, RS-422, or RS-485 serial connections. There are several different Modbus protocols, but far and away the most common is Modbus Remote Terminal Unit (RTU). 

Modbus RTU makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods. 

Like every industrial protocol, Modbus has some major benefits and some significant drawbacks. To start with the positives, Modbus is very easy to implement. It is the perfect solution for very simple applications. Further, as it is open source, and has been around since 1979, there are innumerable pages of documentation, use cases, and best practices available. Finally, implementing Modbus is incredibly cost-effective. 

Modbus also has some drawbacks. First, RS-232, RS-422, or RS-485 serial connections have significantly slower transfer rates than Ethernet connections. Modbus supports a limited set of data types, primarily focused on numerical values and discrete inputs/outputs. Handling more complex data structures, such as strings or arrays, may require additional encoding or custom implementation. Finally, as Modbus is a basic protocol, it lacks advanced features like built-in diagnostics, redundancy mechanisms, or extensive error handling. Modbus is still an excellent choice for simple or small-scale automation tasks. 

For more information on Modbus and other industrial communications protocols, read our white paper guide to industrial communication protocols.