What is Modbus?
Modbus has its roots from the end of the 70s. During the development of the protocol the two developing companies did not agree, which led to a variant to Modbus called Jbus. The main difference is that the register address has a displacement of 1, which even today can be a source of confusion.
In the early days all the communication was by RS232 but in not too long RS485 was supported, which meant multi-drop, longer cables and higher speed. Later support was added for Modbus over TCP, which meant that an extra block was added in the message. The most important part in this is the so called transactionsid which makes the traffic over TCP more secure from a communication standpoint.
The prediction was that Modbus was going to die out a few years ago since it is an old protocol. However, it seems Modbus has had a renaissance and is being used in a variety of situations. Modbus is one of the few protocols that is big across all continents.