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.”