January 2013
By Bill Lydon, Editor
Industrial Ethernet networks and protocols have become a workhorse of automation systems. But how extensively should it be used? It has certainly done a great deal to improve automation systems and provide connectivity to information systems. Ethernet is not ideal in all applications; consider the good, bad and the ugly:
The good thing about of using Ethernet is that it is based on commercial off the shelf technology (COTS) implemented with widely adopted standards for hardware, software, core protocols, cabling, and connectors.
Interfacing to enterprise systems is much easier when automation systems use the same Internet Protocol (IP) structure used by information systems. This has been a big plus for the industry.
Ethernet provides a big pipe for efficiently communicating a great deal of information between controllers, MES, and enterprise systems to support modern manufacturing systems.
The spoke and hub wiring of Ethernet is fine for controllers but creates a great deal of cost for sensor and actuator connections. I was recently attended a trade show that displayed motor control centers with contactors and starters, each with an Ethernet connection. Each motor control center cabinets included as industrial Ethernet switch, creating more costs and complexity. People have been using multi-drop networks such as Modbus, DeviceNet, and Profibus in motor control centers, which are simple to wire and have significantly fewer connections. Fewer connections increase reliability since typically connections are the most unreliable part of automation systems.
In addition, switches and routers further reduce reliability since they are another point of failure. Multi-drop wiring for 10 devices requires one cable, a master termination and a termination at each device totaling 11 terminations. The same installation with Ethernet requires 10 cables and 20 cable terminations (even if the end devices incorporate a 2 port switch) plus another cable and 2 terminations to connect a controller plus 11 ports on a switch or router. In addition, the router or switch needs to be configured unless a simple Ethernet hub is being used. But use of a simple hub can impact network performance.
The requirement for switches and routers is dramatically increased when using Ethernet to connect end devices. Switches and routers add complexity and introduce reliability issues. They require configuration management and maintenance and contribute to more costs. In addition, they will likely need to be replaced before the end devices connected to them.
As the saying goes, give a kid a hammer and everything is a nail. I think some of this is happening with an enthusiasm for applying Ethernet everywhere.
Having devices with a built-in 2 port router is one solution that makes sense for motor drives when they need to move a great deal of data.
There are some proprietary multi-drop Ethernet approaches. Ideally, if those architectures can be made an industry standard, then driving IP to the edge would be much simpler.
