ModBus is one of the most widely used protocols in the world. It is an open communication standard. The protocol was first published in 1979. It defines messages format and transmission modes within network. Protocol is based on Master/Slave architecture and used within industrial automation systems to establish communication between electronic devices.
Despite the fact that ModBus has been around for a long time, it is still successfully used in many entities. Protocol is extremely popular and new products are developed because of advantages of specification. The main benefits of protocol are compatibility with different devices, unification of commands for exchange and reliability of data transmission.
ModBus RTU is a serial protocol, running on either the RS-232 or RS-485 physical interfaces. The latter is used more frequently because of its capability to connect multiple devices into one daisy chain. In this case the clear Master/Slave architecture is created. It must be kept in mind that slaves can be multiple but master is only one. To ensure stable communication experts recommend that no more than 32 devices should be connected to daisy chain. Operation principle of ModBus RTU is very simple. Master sends a message to a slave with request of data. In response slave sends a packet with data and stores data in the register requested.
Due to specification of ModBus TCP, all devices are capable to freely exchange of data. It has no limit to number of masters. The second difference is change of format of packet. The packet has a header to allow routing. To address the request master must know IP-address of specific device.
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