Understanding the key terms in industrial communications

Communications enable the exchange of data between devices on an industrial control network. Without communications, controllers are unable to see the values coming from field devices and operators are unable to see the values coming from controllers. Here are some of the key terms we will use when talking about industrial networking and communications:

• Media: Communication requires wires, cables, or fiber-optic connections between devices. The connections used for communication are called the media, which differ from one another by properties such as the maximum distance for each connection and the maximum data transmission speed.
• Node: Each device on the network is called a node.
• Node address: Each node will typically have a unique identifier called a node address.
• Network: When multiple devices are connected under a common device address space, we call this a network.
• Topology: The physical structure of the network is called the topology.
• Bridge: This device creates a connection between two separate networks.
• Router: This device can forward data between two or more networks.
• Hub: This device can channel data between multiple nodes within a single network.

• Switch: This device can channel data from one node to another. Unlike a hub, a switch will intelligently route only the data destined for a device and is not prone to data loss due to network packet collisions.
• Segment: This device divides a localized section of the network with bridges, routers, or switches.
• Protocol: The language used to communicate data between each node across a network is called a protocol.

The following diagram illustrates a typical Rockwell automation Industrial Control Systems (ICS) network:

In networks that use a coaxial cable as media, such as ControlNet, you will find the following terminology:

• Tap: This device is small and T-shaped and connects a trunk-line cable (the top of the T) to a drop-line cable (the bottom of the T).
• Terminating resistor: This device is a 75 Ohm resistor that can be connected to a trunk-line connection, which absorbs energy and prevents electrical signal reflection.
• Trunk line: This device is the main cable that connects to the taps and drop line cables. A trunk line will have a terminating resistor at either side and one or more drop lines attached to it.

• Drop line: This device is a cable connection from a tap that connects down to a node.
• Repeater: This device is a two-port device that connects two trunk lines together and boosts the signal. It is helpful for connecting segments across longer distances or boosting the signal for a long network segment.

The following diagram illustrates a typical coaxial cable network:

Now that we have covered some of the basic terms, let's look at the various communication solutions used by Rockwell Automation.