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By Dan Hebert, PE, Senior Technical Editor
Industrial Ethernet is the unquestioned digital network leader in manufacturing, but alternative networks continue to thrive in its shadow. Industrial Ethernet, heretofore referred to as just Ethernet, has overcome many limitations in recent years, but barriers to widespread application in some manufacturing applications remain.
One of the most obvious barriers is an installed base of non-compatible automation hardware.
“We will continue to use ControlNet, DeviceNet and DH+/RIO for connectivity to legacy equipment that is installed all over our plant,” says Dave Jones, automation engineer/IT administrator at Ash Grove Cement in Leamington, Utah.
“Ethernet might eventually supplant these other networks, but I still see others in our position. Just look at how long the DCS has hung around. Ethernet adoption still is hindered by the fact that the process control world doesn’t move nearly as fast as the IT world—thank goodness.”
ADCO Manufacturing in Sanger, Calif., makes packaging, cartoning and box-making equipment (Figure 1). In addition to Ethernet they use DeviceNet, Data Highway+ and, infrequently, ControlNet. “Besides being deterministic, these non-Ethernet networks provide backward compatibility with many other devices and integrate into our mostly PLC control structure more easily,” notes Kevin Gilpin, electrical engineering manager at ADCO.
Figure 1: This ADCO Manufacturing Intermittent Auto Loader collates and loads ice cream sandwiches into a carton. Its Ethernet network links the PLC, the touch screen and the VFD.
One reason for the large market share of non-Ethernet networks is that customers have machines that were developed many years ago and are still assembled the same way today, says Stephan Stricker, product manager at B&R Industrial Automation (www.br-automation.com). “As long as these machines work well, there is little pressure for the machine builder to change the network,” says Stricker. “In retrofit applications, many old controllers can’t be upgraded to Ethernet, but the newer controllers can support Ethernet as well as older technologies,” adds Stricker.
If installed base were the only issue holding back Ethernet, then we’d expect Ethernet to eventually triumph as end users upgrade over the years. But proponents of alternative networks point to other Ethernet problems, chief among them topology.
Dealing with stars can be difficult, whether it’s the movie industry or industrial networks. “The biggest problem with Ethernet is the star topology. If I have ten devices, I need at least a 12-port switch, and that’s expensive,” explains ADCO’s Gilpin. “If each Ethernet-enabled device had a minimal switch built in, it would be simple to construct a ring topology network, and Ethernet adoption would progress at a more rapid rate.”
Others agree with Gilpin’s assessment. “Ethernet cables typically need to come back to a central location rather than daisy chain down the length of a system,” says Jonathan Van Sweden, electrical engineering supervisor at Möllers North America in Grand Rapids, Mich. Möllers makes packaging/material handling equipment, primarily bag palletizers and stretch-hooders. “Some manufacturers are starting to integrate unmanaged switches into their Ethernet-enabled devices, but this adds additional hardware costs.”
Suppliers agree. “Ethernet offers only the limited star topology unless each device is equipped with an integrated switch,” notes Helge Hornis, PhD and manager of the intelligent systems group at Pepperl+Fuchs. “While it’s doable, it clearly drives the cost up.”
A network issue related to star topology is power over Ethernet (PoE). “While PoE is available, the limitations are significant, and it’s currently not possible to use it to power a large number of field devices,” explains Hornis. “In practical terms, this means that each field device must be powered separately, and that creates a complex solution.”
Yet another topology-related issue is distance limitation. “While Ethernet is limited to 100 m cable runs, CAN-based buses support cable lengths up to 5,000 m without any additional hardware,” notes Stricker.
Using Ethernet to transmit small amounts of data “is like using an 18-wheeler for a trip to the grocery store,” observes Tom Edwards, senior technical advisor at Opto 22. “It’ll work, but it’s not the most efficient way to go. We like to say that nothing is too complex for Ethernet, but some things are too simple. Simple serial-based industrial approaches such as RS-232, RS-422 and RS-485 likely will always have their place, as will the protocols commonly used to communicate over these networks such as Profibus and BACnet.”
An experienced end user has his take on Ethernet overkill. “Ethernet sends out 1,500 words at a time in one packet,” explains Gary Crenshaw, electrical engineer at Beam Global Spirits & Wine in Clermont, Ky. “Even though overall transmission rates are fast, for moving small amounts of data like discrete I/O in a PLC, Ethernet is so inefficient that it’s actually slower than some competing networks.” So, just like computers use a different protocol for communications on the motherboard, it appears that certain parts of the plant aren’t always best suited for Ethernet.
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