Modbus, DNP3 and HART

Modbus

Modbus is a serial communication protocol developed by Gould-Modicon systems (now Schneider Electric) in 1979 for integrating and using it with PLCs (Programmable Logic Controller). Modbus has become an industry standard for connecting various industrial devices.

Modbus is a master-slave communication protocol and can support up to 247 slaves for connecting and communications with supervisory computers with a remote terminal unit (RTU) in SCADA systems. The device supplying the information is the Modbus slave, while the Modbus master is the device requesting the information.

Why Modbus? A few of the reasons for widespread use of Modbus are:

1. Developed specifically for industrial purpose
2. Open source protocol
3. Easy to develop, deploy and maintain.

Modbus variants

Modbus has many variants available for serial communication. They are:

1. Modbus RTU
2. Modbus over TCP/IP
3. Modbus over UDP
4. Modbus ASCII
5. Modbus Plus (MB+)
6. Pemex Modbus
7. Enron Modbus

The most widely and commonly used Modbus variant is Modbus RTU. Modbus RTU uses RS-485 or RS-232, and all communication in Modbus RTU happens over UART (Universal Asynchronous Receiver Transmitter). One bit is transferred at a time and it uses a baud rate from 1200–115200 bits per second.

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Modbus security issues

Since Modbus was developed in 1979, security concerns were not taken into consideration. Due to this, many security issues exist in Modbus. These include:

1. All the messages in Modbus are communicated without authentication
2. There is no encryption present; all the communication can be easily read and understood
3. Lack of broadcast suppression
4. Lack of message checksum for checking errors and garbled messages

DNP3

DNP3 stands for Distributed Network Protocol. It was developed by GE Harris in 1993 and is widely used in the U.S. and Canada. Like Modbus, DNP3 is also an open-source serial communication protocol, which is one of the key reasons for its wide (Read more...)