Ethernet vs. fieldbus: the right network for the right application

Time-sensitive networking has pushed Ethernet, but fieldbus still accounts for more than half of industrial uses.

By Mike Bacidore, editor in chief

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Industrial Ethernet continues to grow, but don’t count out fieldbus networks just yet. The upgrade from 4-20 mA analog signals to fieldbus had a significant impact on industry, but Ethernet networks are positioned to support time-sensitive networking (TSN).

Ethernet’s steady march over the decades has found its way into protocols such as Profinet, PowerLink, EtherCAT and EtherNet/IP. Still, it accounts for less than 40% of market share, according to 2016 research from HMS Industrial Networks.

The fieldbus foothold remains strong at 58% of the market with an annual growth rate of 7%, thanks largely to Profibus’ dominance with 17% market share. Ethernet’s 20% annual growth, however, has continued its rapid expansion. But what are the major differences between Ethernet networks and fieldbus networks? Which protocols run where? And ultimately how do they affect the Industrial Internet of Things (IIoT) and TSN?

 

What’s the difference?

The major differentiation has focused on deterministic hard real-time performance for manufacturing and machine-building applications in discrete automation, explains Tom Burke, president of OPC Foundation. “Safety and security have been very important, and by definition the industrial fieldbus device networks have done a superior job building deterministic reliable real-time solutions,” he says. “The associations that represent the different fieldbus networks guarantee interoperability as their members who develop products for the respective technology certify their implementations of the standards and often use certified chipsets.”

The beauty of commercial off-the-shelf Ethernet allows any vendor to develop devices that can interact and communicate over the Ethernet, continues Burke. “The world is going to change with the deployment of TSN, given the focus on deterministic high-speed 50 GB real-time operation,” he says. “Clearly, you're going to see all the fieldbus networks scrambling to address and support the TSN networks for their respective networking technology. I expect things like the safety protocol and reliability protocols of the respective organizations to be migrated to the TSN technology, so there still will be somewhat of a differentiation between the industrial networking alternatives and the commercial off-the-shelf TSN Ethernet solution.”

The majority of the industrial fieldbus networks claim connectivity of standard Ethernet devices on the network with the same deterministic operation, says Burke. “But the reality is the majority of the manufacturing operations always segregate the networks, having their standard Ethernet devices running on commercial off-the-shelf Ethernet and then having their fieldbus networking devices running on the fieldbus network, sometimes having a simple bridge or gateway between the two network connections,” he explains.

“One significant difference between fieldbus networks and Ethernet networks is speed, as in most cases Ethernet networks will be faster despite the additional overhead,” explains Ian Verhappen, senior project manager, automation, CIMA+. “Part of the reason for this is that fieldbus systems were designed for the typically noisy plant environment, and as a result speed needed to be slower and of course a suitable physical media found, especially for those protocols in which data and power are in the same single cable.”

Many end devices don’t support Ethernet, says Verhappen. “They don’t need the bandwidth and do need the ability to have power and signal in the same cable,” he explains. “I foresee a need for fieldbuses as a critical element of IIoT and beyond. Unfortunately, PoE doesn’t have the same voltage and power levels as field systems but certainly is useful to remotely power a switch or IP-based device such as a camera or a small controller on a process skid.”

Ethernet still has the distance challenge from the hub. “In many facilities, the processing facilities are greater than this distance from even the distributed I/O or interface room,” says Verhappen. “This means having to build an infrastructure to within reasonable limits of the process, which isn’t difficult but is another step that fieldbus systems can avoid.” Both systems have the advantage for modular systems. “Each module can be fully tested independent of the full facility and then connected to the balance of plant by a single communication cable—Ethernet or fieldbus multiconductor—connection.”

 

Design and use differences

The most obvious difference between fieldbus networks and Ethernet networks is the design intent, explains Talon Petty, marketing and business development manager, FieldComm Group. Most process applications don’t require high-speed communications. “Ethernet is a collision-based network, meaning it uses high speed to get data through in a timely manner,” he says. “If data-packet collisions occur, it just simply retries until it goes through. Because of the high bandwidth, collisions are typically not a concern for applications such as office Wi-Fi. In process, these collisions, depending on the severity, could have a major impact. The same is true with discrete systems such as assembly lines. These networks just need to repeat the same action over and over. In process, it is a constantly varying data network. Temperatures have to go up and down, and valves must fluctuate flow rates. It’s, by definition, a process. These processes also don’t require the high-speed capabilities of Ethernet. We’re bytes of data at a time, and we’re talking about processes that can have tolerances for updates in seconds and tens of seconds, even minutes for tank farms. Updates in the millisecond range are not necessary.”

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  • I’m so glad to see an article that finally gets it, that fieldbus and Ethernet are complementary, not competing. Plants need both because they have mutually exclusive characteristics. It is just like USB and Ethernet on your laptop. You need both. I would like to remind all that there are two classes of fieldbus: H1 and H2. H1 fieldbus refers to the bus at the lowest levels, connecting sensors and actuators in the field, taking the place of 4-20 mA and on-off signals. H2 fieldbus refers to a higher level bus between higher level devices like PLC and motor drives etc. With this in mind, I have the following input: I agree that (H1) fieldbus is taking the place of 4-20 mA and on-off signals, but there are still plants being built based on 4-20 mA and on-off signals for the sensors and actuators. More work is required to evangelize digital transformation at the sensor/actuator level. I agree with Tom that fieldbus networks are deterministic, reliable, and guarantee interoperability. Moreover, I’d like to add you can now also get interchangeability – which is much harder to achieve but is required at the sensor/actuator network level. Some fieldbusues like FOUNDATION fieldbus, already are time-synchronized. This is one of the reasons why FF is the best choice for process control sensors and actuators. Motion control buses also are time synchronized. I agree with Ian that fieldbus has a distance advantage. This is another reason why H1 fieldbuses were designed with moderate speed is because with lower speeds comes long distance. Process plants are huge, sensors and actuators are sprawled out. H1 fieldbus can stretch 1.9 km which is sufficient, but the faster Ethernet is limited to 100 m which is insufficient. I agree with Ian and Kathrin that fieldbus has an advantage providing power over the same two wires for simplicity I personally agree with Ian that H1 fieldbus (and wireless sensor networks) are a must for IIoT. You can’t run a pair of wires for each signal for tens of thousands of additional sensors. IIoT needs digitally networked sensors. We must set the expectations right; INDUSTRIAL Ethernet equipment which is industrially hardened, hazardous area certified is much more expensive than the Ethernet gear in the consumer electronics and IT markets Note that we can’t standardize on a single technology because we don’t get Ethernet in pressure and temperature transmitters, or in control valves and on-off valves. These sensor/actuator level devices use H1 fieldbus. Ethernet and H1 fieldbus complement each other. http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf Remember that there are many application protocols for industrial Ethernet: PROFINET, EtherNet/IP, Modbus/TCP, FF-HSE, HART-IP, and EtherCAT etc. A tool developed for PROFINET does not work for EtherNet/IP. In other words, although there is a whole suite of tools developed for Ethernet, not all can be used for the particular Ethernet application protocol you use in your plant. It might still be more than for fieldbus, but not a whole lot more. Many users ask why is it that from time to time they need to update the firmware in IP-based devices to tackle vulnerabilities like Heartblead, but they never have to do it for bus devices? When it comes to integrating devices to the DCS, the IT security guys in the plant still prefer vendors to connect using Modbus rather than Ethernet. It remains to be seen if the Ethernet version of devices will actually be lower cost. It would be interesting to make a price comparison between the Ethernet version vs. e.g. the Modbus version of some products. It should also be noted that today many devices come with both H2 fieldbus and Ethernet built in. Wireless gateways, for instance come with both Modbus/RTU (RS485) and Modbus/TCP (Ethernet). I agree with John that Ethernet provides a common physical layer for certain devices. However, it doesn’t do anything for sensors and actuator like pressure transmitters and control valves. Therefore we still need H1 fieldbus and in the future we need APL. APL could also be a good solution for networking small appliances at home in preparation for IIoT. I agree with John that H2 fieldbus might be a bottleneck at the higher level so Ethernet might be needed. However, the same is not true for H1 fieldbus at the sensor/actuator level. Only a few sensors and actuators sit on each bus so speed need not be as high. H1 fieldbus is not the bottleneck. The first step is to use H1 fieldbus instead of 4-20 mA and on-off signals. H1 fieldbus at the lowest level feed the Ethernet networks above. I agree with Bill and Doug that Ethernet has advantages over H2 fieldbus, but it is important to stress this is over H2. Ethernet cannot be used in place of H1 fieldbus in pressure transmitters and control valves. We still need H1 fieldbus to take the place of 4-20 mA and on-off signals at the sensor/actuator level. It is interesting to note that modern laptops no longer have an Ethernet port I agree with Michael that organizing data into application profiles and with an object oriented structure is absolutely essential for a communication protocol to enable easy use of the data. FOUNDATION fieldbus also does this extremely well. At the Ethernet level FF-HSE also takes advantage of this. https://www.linkedin.com/pulse/rich-data-poor-what-tells-software-does-jonas-berge It should be noted that FOUNDATION fieldbus also supports discrete signal devices like on-off valves which are commonly used in the process industries. That is, FF takes the place of both 4-20 mA and on-of signals in process plants. At the Ethernet level FF-HSE also takes advantage of this. Devices will probably end up having to support multiple Ethernet application protocols to interface with any control system they may be connected to. For instance, WirelessHART gateways today already support many protocols for this reason. There is ongoing work driven by the European NAMUR user organization to marry Ethernet with fieldbus, combining the higher speed and multi-protocol capability of Ethernet with the distance power and intrinsic safety of fieldbus - an “Etherbus” if you will. Officially referred to as the Advanced Physical Layer (APL): http://automation2.com/2015/06/29/report-from-achema-2015/

    Reply

  • I’m so glad to see an article that finally gets it, that fieldbus and Ethernet are complementary, not competing. Plants need both because they have mutually exclusive characteristics. It is just like USB and Ethernet on your laptop. You need both. I would like to remind all that there are two classes of fieldbus: H1 and H2. H1 fieldbus refers to the bus at the lowest levels, connecting sensors and actuators in the field, taking the place of 4-20 mA and on-off signals. H2 fieldbus refers to a higher level bus between higher level devices like PLC and motor drives etc. With this in mind, I have the following input: I agree that (H1) fieldbus is taking the place of 4-20 mA and on-off signals, but there are still plants being built based on 4-20 mA and on-off signals for the sensors and actuators. More work is required to evangelize digital transformation at the sensor/actuator level. I agree with Tom that fieldbus networks are deterministic, reliable, and guarantee interoperability. Moreover, I’d like to add you can now also get interchangeability – which is much harder to achieve but is required at the sensor/actuator network level. Some fieldbusues like FOUNDATION fieldbus, already are time-synchronized. This is one of the reasons why FF is the best choice for process control sensors and actuators. Motion control buses also are time synchronized. I agree with Ian that fieldbus has a distance advantage. This is another reason why H1 fieldbuses were designed with moderate speed is because with lower speeds comes long distance. Process plants are huge, sensors and actuators are sprawled out. H1 fieldbus can stretch 1.9 km which is sufficient, but the faster Ethernet is limited to 100 m which is insufficient. I agree with Ian and Kathrin that fieldbus has an advantage providing power over the same two wires for simplicity I personally agree with Ian that H1 fieldbus (and wireless sensor networks) are a must for IIoT. You can’t run a pair of wires for each signal for tens of thousands of additional sensors. IIoT needs digitally networked sensors. We must set the expectations right; INDUSTRIAL Ethernet equipment which is industrially hardened, hazardous area certified is much more expensive than the Ethernet gear in the consumer electronics and IT markets Note that we can’t standardize on a single technology because we don’t get Ethernet in pressure and temperature transmitters, or in control valves and on-off valves. These sensor/actuator level devices use H1 fieldbus. Ethernet and H1 fieldbus complement each other. http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf Remember that there are many application protocols for industrial Ethernet: PROFINET, EtherNet/IP, Modbus/TCP, FF-HSE, HART-IP, and EtherCAT etc. A tool developed for PROFINET does not work for EtherNet/IP. In other words, although there is a whole suite of tools developed for Ethernet, not all can be used for the particular Ethernet application protocol you use in your plant. It might still be more than for fieldbus, but not a whole lot more. Many users ask why is it that from time to time they need to update the firmware in IP-based devices to tackle vulnerabilities like Heartblead, but they never have to do it for bus devices? When it comes to integrating devices to the DCS, the IT security guys in the plant still prefer vendors to connect using Modbus rather than Ethernet. It remains to be seen if the Ethernet version of devices will actually be lower cost. It would be interesting to make a price comparison between the Ethernet version vs. e.g. the Modbus version of some products. It should also be noted that today many devices come with both H2 fieldbus and Ethernet built in. Wireless gateways, for instance come with both Modbus/RTU (RS485) and Modbus/TCP (Ethernet). I agree with John that Ethernet provides a common physical layer for certain devices. However, it doesn’t do anything for sensors and actuator like pressure transmitters and control valves. Therefore we still need H1 fieldbus and in the future we need APL. APL could also be a good solution for networking small appliances at home in preparation for IIoT. I agree with John that H2 fieldbus might be a bottleneck at the higher level so Ethernet might be needed. However, the same is not true for H1 fieldbus at the sensor/actuator level. Only a few sensors and actuators sit on each bus so speed need not be as high. H1 fieldbus is not the bottleneck. The first step is to use H1 fieldbus instead of 4-20 mA and on-off signals. H1 fieldbus at the lowest level feed the Ethernet networks above. I agree with Bill and Doug that Ethernet has advantages over H2 fieldbus, but it is important to stress this is over H2. Ethernet cannot be used in place of H1 fieldbus in pressure transmitters and control valves. We still need H1 fieldbus to take the place of 4-20 mA and on-off signals at the sensor/actuator level. It is interesting to note that modern laptops no longer have an Ethernet port I agree with Michael that organizing data into application profiles and with an object oriented structure is absolutely essential for a communication protocol to enable easy use of the data. FOUNDATION fieldbus also does this extremely well. At the Ethernet level FF-HSE also takes advantage of this. https://www.linkedin.com/pulse/rich-data-poor-what-tells-software-does-jonas-berge It should be noted that FOUNDATION fieldbus also supports discrete signal devices like on-off valves which are commonly used in the process industries. That is, FF takes the place of both 4-20 mA and on-of signals in process plants. At the Ethernet level FF-HSE also takes advantage of this. Devices will probably end up having to support multiple Ethernet application protocols to interface with any control system they may be connected to. For instance, WirelessHART gateways today already support many protocols for this reason. There is ongoing work driven by the European NAMUR user organization to marry Ethernet with fieldbus, combining the higher speed and multi-protocol capability of Ethernet with the distance power and intrinsic safety of fieldbus - an “Etherbus” if you will. Officially referred to as the Advanced Physical Layer (APL): http://automation2.com/2015/06/29/report-from-achema-2015/

    Reply

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