Jonas Berge, a director with Emerson Process Management in Singapore, pointed out, \“One should be careful when saying Ethernet is replacing fieldbus because there are two distinct and very different levels of fieldbus: H1 fieldbus and H2 fieldbus. H2 fieldbus is used between the control system and underlying package unit PLC, remote I/O, drives, motor starters, MCC and wireless gateways and includes protocols like Modbus/RTU, Profibus DP and DeviceNet, etc. On the other hand, H1 fieldbus is very different and is used for sensors and actuators, where the dominant connection technology is hardwired analog 4-20mA and discrete on/off signals. I see a little bit of Ethernet used in magnetic and Coriolis flow meters as well as electric actuator/motor-operated valves. However, Ethernet is struggling to take the place of 4-20mA and on/off signals. For digital communication with intelligent pressure, temperature, level and interface level transmitters, vortex flow meters, as well as pH, conductivity and amperometric analyzers, control valve positioners, and intelligent on/off valves I do not see any Ethernet.\“ Judging by Berge\’s comments and his affiliation with Emerson Process Management, it\’s clear that he is very familiar with Foundation Fieldbus. But are his assertions essentially correct? To help clarify his statement and get further input on this topic, I asked representatives from several fieldbus and Ethernet protocol organizations to comment. Here\’s what they had to say. \“I would generally agree with the comments from the reader about Ethernet protocols not being competitive at the sensor level if the term \’Internet-based protocol\‘ is substituted for \’Ethernet protocol,\’\“ says Joey Stubbs of EtherCAT Technology Group. \“There is a huge difference between the typical Internet-based protocol examples that the reader references, and real Level 1-capable Ethernet fieldbus protocols that are based on the Ethernet physical layer, but don\’t have the overhead of Internet protocols, such as EtherCAT.\“ Also agreeing that Berge\’s statements are essentially correct is Carl Henning of PI North America (Profibus/Profinet). \“I agree with him for the most part,\“ Henning says, but \“factory fieldbuses are another story. Although Profibus DP and Profinet are complementary, Profinet can do whatever Profibus DP does – and better.\“ To explain his point, Henning provides the following example: If you start with a Profinet backbone you can \“connect PACs, PLCs or DCSs, then I/O, vision systems, drives, etc., to the Profinet backbone. If you need a process fieldbus because you need power over the bus or intrinsic safety, you can use Profibus PA or Foundation Fieldbus and proxy it onto Profinet. WirelessHART can proxy onto Profinet as well. If you need to connect to smart sensors and actuators, there are IO-Link masters that connect to Profinet. And, if it\’s not a greenfield plant, and there are legacy networks like DeviceNet, Profibus DP, HART or others, you can proxy them onto Profinet, too.\“ I also spoke with Larry O\’Brien of the Fieldbus Foundation. Like the other respondents, O\’Brien agrees with Berge\’s core points, though he did add, \“We do not use the term \’H2\‘ any more. Our high-speed network is called HSE, and is based on standard Ethernet and standard IP.\“ O\’Brien acknowledges that the discussion of Ethernet replacing fieldbus has been ongoing for some time now and that he has even addressed it in his Fieldbus Foundation blog. The problem with this discussion, O\’Brien says, is that people tend to latch onto the term Ethernet and view it as a panacea for everyone\’s network requirements. \“Yes, many plant networks today, even Foundation fieldbus HSE, are based on Ethernet,\“ he says. \“So the discussion is not one of fieldbus vs. Ethernet. It is rather a discussion of Ethernet and standard IP-based networks in automation vs. other networks.\“
David Greenfield is Director of Content for Automation World. He has been covering industrial technologies, ranging from software and hardware to embedded systems, for more than 20 years. Prior to joining Automation World in June 2011, David was Editorial Director of UBM Electronics# Design News magazine, which covers system and product design engineering. He moved to UBM after serving as Editorial Director of Control Engineering at Reed Business Information (a division of Reed Elsevier Inc.), where he also worked on Manufacturing Business Technology as publisher, several years earlier. In addition, he has held editorial positions at Putman Media and Lionheart Publishing.
