Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. Modbus is popular in industrial environments because it is openly published and royalty-free. It was developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions – other than the datagram (packet) size – on the format of the data to be transmitted. Modbus uses the RS485 or Ethernet as its wiring type. Modbus supports communication to and from multiple devices connected to the same cable or Ethernet network. For example, a device that measures temperature and a different device to measure humidity, both of which communicate the measurements to a computer.
Modbus is often used to connect a plant/system supervisory computer with a remote terminal unit (RTU) in Supervisory Control and Data Acquisition (SCADA) systems in the electric power industry. Many of the data types are named from industrial control of factory devices, such as Ladder logic because of its use in driving relays: A single physical output is called a coil, and a single physical input is called a discrete input or a contact.
However, Modbus protocol has some critical advantages that become critical with the latest technology jumps, especially the IoT technologies which provide some solutions that give it an advantage on the Modbus protocol.
Disadvantages of Modbus include:
- Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a master station (once again Ethernet TCP/IP proving the exception).
- Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types is limited to those understood by PLCs at the time. Large binary objects are not supported.
- No standard method for a node to find the description of a data object, i.e. finding a register value represents a temperature between 30◦ and 175◦.
- Since Modbus is a master/slave protocol, there is no way for a field device to “report by exception” (except over Ethernet TCP/IP, called open-mbus)
- No security against unauthorized commands or interception of data
- Transmissions must be contiguous which limits the types of remote communications devices to those that can buffer data to avoid gaps in the transmission.
- A great amount of configuration and programming required
- The protocol is not common in the SIMATIC family
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