By definition, Modbus is a communication protocol that was developed by Gould-Modicon in 1979. It enables simple, reliable and fast data transmission between automation and field devices. Originally designed for programmable logic controllers, Modbus has since developed into an industry standard that is used in a wide range of applications. The architecture is based on a master-slave or client-server model, which enables flexible and manufacturer-independent communication.
The serial Modbus protocol was developed to standardize and facilitate communication between programmable logic controllers (PLCs). Thanks to its open specification, it was able to spread quickly and was adopted by many manufacturers. Today, Modbus is an integral part of the IEC 61158 standard and is used internationally as the "Serial Communications Protocol".
Architecture and functionality
Modbus works on a master-slave architecture in which a master device, often a computer, controls the communication. The connected slave devices, such as control and measuring units, respond to requests from the master. This communication can take place either via Ethernet or serial connections. Each participant or server device in the network is assigned a unique IP address, with address 0 reserved for broadcast messages. This enables efficient and coordinated data transmission in the network.
Master-slave architecture
In a Modbus network, requests are sent from the master to at least one slave. The slave only receives their responses. A slave cannot send data without first receiving a request from the master. This hierarchical structure enables clear control and management of data communication.
Addressing and communication
Each bus device requires a unique IP address that controls communication in the network. Address 0 is reserved for broadcast messages that can be sent to all slaves. Communication can take place both via Ethernet and via serial connections, which makes Modbus extremely flexible.
TCP
Modbus TCP/IP uses the TCP/IP protocol to transfer data via Ethernet. This enables easy handling and installation. The message structure includes a transaction number, a protocol identifier, the number of bytes to follow, the address, the function code and the actual data.
Thanks to the use of Ethernet, long distances and high data rates can be handled easily, making the TCP version of the communication protocol ideal for industrial applications. Modbus TCP also requires no additional calculation of control bytes, which further simplifies implementation and operation.
RTU
The version with Modbus RTU Remote transmits binary data via serial interfaces such as EIA-232 and EIA-485. With serial standards such as RS-232, messages begin and end with waiting times; the transmission speed determines their length. Here too, the address of the receiver and the function code are defined in 8 bits.
A cyclical redundancy check (CRC) ensures data integrity. Serial communication allows a line length of up to 1200 meters with EIA-485, which makes RTU particularly suitable for applications where the devices are far apart (e.g. air traffic control).
ASCII
ASCII codes are used for data transmission. This operating mode is human readable, but slower in data transmission. The message begins with a colon and ends with an LRC command and the characters CRLF. Modbus ASCII is useful in some specialized applications due to its readability and ease of implementation, but is less commonly used than Modbus TCP or Modbus RTU.
Message types and registers
In addition to these operating modes, Modbus supports various message types. Commonly used register types include Coils (switching states), Discrete Inputs (digital inputs), Input Registers (analog inputs) and Holding Registers (analog outputs). These register types enable the versatile use of Modbus in various applications, from simple switch queries to complex data acquisition.
Modbus is widely used in numerous industrial sectors due to its flexibility. These include industrial automation, building automation, the energy industry and process control. Its standardized and standardized structure enables fast and reliable data transmission between devices from different manufacturers. The independent data structure ensures that Modbus is compatible with a wide range of devices, making it a preferred protocol in industry.
Industrial automation
In industrial automation, Modbus is often used to control and monitor production processes. It enables communication between programmable logic controllers, sensors and actuators, thus ensuring smooth production processes.
Building automation
Modbus is also used in building automation, for example to control heating, ventilation and air conditioning systems. Thanks to standardized communication, systems from different manufacturers can be easily integrated.
Energy management and process controls
In the energy industry, Modbus is used to monitor and control energy distribution and generation. Modbus is also frequently used in process control, for example in water treatment plants, due to its reliability and flexibility.
The continuous further development of Modbus enables integration into modern IoT (Internet of Things) systems. The combination with other communication protocols expands the areas of application and improves functionality.
However, there are also challenges, such as security aspects and compatibility issues. Protection against cyber attacks and authentication are essential to ensure the integrity of the data. In addition, compatibility between different implementations must be ensured to enable smooth communication.
The integration of Modbus into IoT systems opens up new possibilities for remote monitoring and control of devices and processes. This enables more efficient use of resources and more precise control.
Security aspects are of great importance as Modbus is used in critical industrial applications. Measures such as the use of VPNs, firewalls and secure authentication mechanisms help to minimize security risks.
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