Origins
Modbus is a serial communication protocol developed in 1979 by Modicon. Initially designed for use in programmable logic controllers (PLCs), Modbus has become one of the most widely used protocols in industrial automation due to its simplicity and reliability.
The key to its success is that it is a very simple protocol, totally open, free and can be implemented on any microcontroller. We started marketing the first modbus equipment in 1998, and we are currently associated with modbus.org.
One of the distinctive features of modbus is its master-slave communication architecture. In this system, a central device called master controls the network and manages communication with one or more slave devices. The master sends requests to the slaves, which respond with the required data.
It is an architecture that facilitates centralized control and is ideal for many industrial applications. Proof of this is the number of modbus devices that are currently marketed, from water meters, network analyzers, refrigeration production machines, boilers, …
Modbus and RS-485
The Modbus protocol is usually implemented over the RS-485 communication standard, which is widely used in industrial environments due to its robustness. RS-485 is a serial transmission standard that allows the connection of multiple devices on a single data bus, originally supporting 32 devices on the same network. Currently there are transceivers that support up to 400 devices.
Subsequently, Modbus TCP and Modbus RTU over ethernet protocols were added. These allow higher communication speed and lower latencies.
RS-485 Features and Benefits
- Long distance capability: RS-485 allows guaranteed communication over distances of up to 1200 meters (4000 feet) without the need for repeaters, making it ideal for industrial plants and applications where devices are distributed over large areas.
- Resistance to electromagnetic interference (EMI): Thanks to its differential transmission design, RS-485 is highly resistant to electromagnetic interference, ensuring reliable communication even in noisy industrial environments.
- Support for multidrop networks: RS-485 allows multiple devices to be connected on the same network, which is essential for distributed control systems where multiple devices must communicate with a central controller.
- Transmission speed: RS-485 supports data transmission rates of up to 10 Mbps over short distances, making it suitable for applications requiring fast information transfer. Although it is standard to use speeds of 9600, 19200, 57600 and 115200. The 9600 and 19200 baud rates are supported by all equipment.
As an anecdote we mounted some 25 years ago some equipment for groundwater monitoring at the airport of Palma de Mallorca with a RS485 network without repeaters with 15Km of cable, running at 9600 baud without problems. We are aware that we exceed specifications.
Advantages of Modbus
- Simplicity and ease of use: Modbus has a simple structure and is easy to implement, making it accessible to engineers and technicians without the need for advanced communications skills.
- Interoperability: As an open protocol, Modbus is compatible with a wide variety of devices from different manufacturers, facilitating the integration of heterogeneous equipment in the same control network.
- Flexibility: Modbus can be implemented over different communication media, such as serial cables (Modbus RTU/ASCII over RS-485) and TCP/IP networks (Modbus TCP and Modbus RTU over Ethernet), adapting to various communication needs in industrial environments.
- Efficient data transmission: The Modbus protocol is lightweight, meaning it requires few resources to transmit data, ideal for applications where speed and efficiency are essential.
- Widespread industry adoption: Thanks to its reliability and robustness, Modbus has become a de facto standard in industrial automation, with technical support, documentation and development tools widely available.
