To control a machine or process, many times multiple controllers or intelligent devices must work together to accomplish the task. In order to work together, these devices must communicate. In order to program a PLC, communications must take place – because the Programming device (computer) must communicate with the PLC CPU in order to transfer the configuration and control logic before the PLC can even begin to run. For these reasons, it is important for anyone in automation to have a basic understanding of PLC communications.
Types of Communications
There are three basic levels/categories of communications that we are usually concerned with in industrial control: serial communications, industrial communications networks, and industrial I/O networks.
The most basic form of communication is a direct, one way, connection between two devices where data is transmitted one bit at a time. We call this Serial Communications. When programming software on a PC connects directly to a PLC via an RS-232 port – serial communication is occurring.