Introduced in 1979, Modbus has been the de facto standard ever since. Modbus is an application layer messaging protocol. Placed at level 7 of the OSI model, it provides client/server communication between devices connected via different types of communication buses or communication media. Modbus is the most widely used ICS protocol, mainly because it's a proven and reliable protocol, simple to implement, and open to use without any royalties:
On the left-hand side in the preceding figure, we see Modbus communicating over serial (RS-232 or RS-485). The same application layer protocol is used for communicating over Ethernet, as shown on the right-hand side.
The Modbus protocol is built upon a request and reply model. It uses Function Code in combination with a data section. The Function Code specifies which service is requested for or responded to...
