Ethernet "Switches" to Industrial

Now that Ethernet has permeated the plant floor, end users face the challenge to adapt Ethernet for industrial applications. Commercial Ethernet offers many advantages, but there also are key issues that must be addressed for successful industrial implementations. Ethernet routers and switches that address these issues will enjoy wide acceptance and strong growth.

Most industrial Ethernet customers don't want to use the star/tree topology that switched Ethernet provides, and many consider installations would be much more efficient with a bus/"daisy-chain" topology.

Network and user devices form the foundation for Ethernet technology. The network device is responsible for switching packets between the user devices. Communication may be single links, or multiple links trunked together for aggregated bandwidth and redundancy, but, in either case, the network is still based on a star topology.

Bus-type networks like ControlNet, DeviceNet, or Profibus allow installation of just one communication cable. Each device is then connected to this main cable via a short tap/trunk line. This is very flexible and cost efficient compared to a star topology.

A common example is a manufacturing line 200 ft. long with 50 different nodes. This installation might have a control cabinet at one end of the line with the main controller and an Ethernet port.

With Ethernet, this installation would need switches to get acceptable data update times between the PLC and each remote unit. According to Dahlstrom, an Ethernet network with switches would offer performance similar to fast fieldbus networks such as Profibus.

The problem is that switched Ethernet requires a star topology with attendant wiring requirements. "Our industrial customers tell us that some of their switched Ethernet installations resemble old hard-wired networks where each device had multiwire cable connecting it to the I/O rack of the PLC," observes Dahlstrom.

To address the topology issue, the Profibus organizations are working with the next generation of ProfiNet Ethernet networks to allow Ethernet bus/"daisy-chain" topologies for device connection on the factory floor.

This will be accomplished by integrating a four-port Ethernet switch into each end device. ProfiNet's intention is to integrate a low-cost four-port switch at each node. This would enable a combination of star and daisy chain/bus-type installations.

This solution is becoming feasible because of the low and ever-declining cost of Ethernet hardware. "Siemens is currently developing new silicon for this application. We expect it to reach the market in 2005, further lowering costs," adds Dahlstrom.

Although most Ethernet switches in new industrial installations are 100 Mbit, the judicious use of 1 Gbit switches can greatly improve network performance. Brute force in terms of more speed is good, but adding intelligence to switches also can improve network performance.

Availability is addressed through multicast control and redundancy. "Ethernet production networks can be very multicast-intensive because devices are sending constant streams of information into the network that other devices occasionally read. Intelligent switching filters this information from devices that did not request it, allowing them to perform better while still accessing needed data," says Larry O'Connell, product manager of the desktop switching business unit at Cisco.

Redundancy is implemented by Cisco through a rapid-spanning tree protocol designed to bring a network back on line after a link failure in less than a second.

Quality of service (QoS) is the concept of prioritizing data and allocating bandwidth for these data. "QoS provides the ability to ensure determinism by assigning a policy that prioritizes production data. A switched Ethernet network and QoS assure determinism in manufacturing environments even during periods of high network congestion," states Connell.