In industrial automation, distributed input/output (I/O) systems are commonly used to acquire sensor data and control actuators in real time. These systems consist of multiple I/O modules, each with its own input and output channels, that are spread throughout the industrial facility. To communicate with each other, these modules use a variety of communication protocols that are optimized for the particular requirements of industrial automation.
Distributed I/O and controller technologies are used in industrial automation to distribute the input/output (I/O) processing and control functions across multiple devices, instead of utilizing a central controller for everything. When this approach is used, we find that it allows for greater flexibility, scalability and efficiency in industrial automation control systems.
The basics of distributed I/O and controllers can be broken down into three main components.
1. I/O devices: These are devices that are located in the field near the sensors and actuators. They receive signals from the sensors and provide signals to the actuators. They can be analog or digital, and they can be wired or wireless.
2. Controllers: These are devices that process the signals received from the I/O devices and make decisions based on them. They can be centralized or distributed, and they can be programmed using various programming languages and software tools.
3. Communication networks: These are the networks that connect the I/O devices and controllers. They can be wired or wireless, and they can use various protocols and technologies, such as Ethernet, Profinet, Profibus or Modbus.
Together, these three components form the basis of distributed I/O and controllers. By distributing the I/O processing and control functions across multiple devices, industrial automation systems can achieve the flexibility, scalability and efficiency needed, while also reducing wiring costs and improving reliability.
When we consider distributed I/O systems, what we see most typically are systems that communicate with the controller using one communication protocol or a combination of communication protocols. The communication protocol chosen establishes a standardized method for data exchange between the I/O devices and the controller.
One common protocol used for distributed I/O systems is the industrial Ethernet protocol. In this protocol, the I/O devices and the controller are connected to a local area network (LAN) using Ethernet cables. The I/O devices and the controller communicate with each other using the transmission control protocol/Internet protocol (TCP/IP) suite of communication protocols.
TCP/IP protocols are used for reliable and error-checked transmission of data over the Internet and other networks, and we find these protocols utilized and essential in many applications.
TCP is a connection-oriented protocol that provides reliable, ordered and error-checked delivery of data between applications. IP, on the other hand, is a connectionless protocol that provides the addressing and routing mechanisms needed for packet delivery.
TCP/IP protocols tend to be utilized where there is a constant need for interchange of information between the application and the controller. These protocols typically are concerned with the state of things. In other words, they will be checking for the condition requirement of the process. If we’re filling a bottle, there would be a sensor to check for the presence of the bottle, and then that information would be communicated to the controller, which would then open the valve to fill the bottle and close the valve once the condition or the state is met, per the requirements.
Another common protocol used for distributed I/O systems is the DeviceNet protocol. In this protocol, the I/O devices and the controller are connected to a network using a two-wire cable. The I/O devices and the controller communicate with each other using a messaging protocol called the common industrial protocol (CIP).
While DeviceNet protocols are more reliable than TCP/IP protocol, they are not concerned with state. These operate more like an on/off switch relying on programming and timing to operate accurately. In some circumstances, this is preferred where reliability is paramount and feedback from the controller is not required.
In both cases, the controller sends requests to the I/O devices and receives responses from them. The I/O devices send data back to the controller, such as the status of sensors or the state of output devices. The controller uses this data to make decisions and control the process or equipment being monitored by the distributed I/O system.
Overall, the choice of communication protocol for a distributed I/O system will depend on factors such as the required data transfer rate, the number of I/O modules, the complexity of the control system and the specific needs of the industrial process. Engineers will want to consider the multiple factors of their particular applications when they are deciding which I/O communication protocol to utilize.
