Using EtherNet/IP in Process Automation Instead of Fieldbus
EtherNet/IP takes advantage of Ethernet commercial technologies to surpass alternative solutions.
Syndication Source: Control Engineering
Since its invention in 1973, Ethernet has changed the world. It will continue to deliver the fastest data throughput, improve the architectures upon which it is delivered, evolve into varying electromechanical spectrums to meet the next industry trend, and penetrate down into the tiniest of microprocessors. Our world of process and factory automation is no exception to the ever-reaching technological advancements of this network.
Around 20 years ago, the process automation market had proprietary ways to meet the demands of remote I/O peer-to-peer communications. These approaches were successful and supportable, but users began to demand that their automation systems interface and share more data automatically with their front office systems over Ethernet.
Automation vendors began connecting their control systems via Ethernet, but there was no workable way to deploy device control requirements over a non-deterministic network infrastructure like Ethernet. As process users started to transition from traditional 4-20 mA analog devices and demanded digital device communications, fieldbus networks emerged to meet the demands that Ethernet couldn’t.
Today, Ethernet communication has overcome many of the disadvantages of previous years and established its presence in field device communications.
In factory automation, Ethernet-based networks are being used to connect robots, variable speed drives, and actuators to automation controllers. In the process control world, EtherNet/IP now connects flow meters, pressure instrumentation, and similar field devices to distributed control systems, programmable controllers, and hybrid programmable automation controllers (see Figure 1).
While there is no network panacea, EtherNet/IP has benefits that some fieldbus architectures cannot deliver. EtherNet/IP:
- Is easier to connect to a variety of host systems
- Can communicate with multiple hosts simultaneously
- Is instantly familiar to anyone with Ethernet experience
- Can use all available Ethernet tools and technologies
- Can use quality of service (QoS) to prioritize network traffic
- Can use simple network management protocol (SNMP) to monitor and manage the network
- Has more network topology options when switches are deployed
- Provides better support for wireless data transmission
- Provides better security through the use of standard Ethernet tools
- Offers economies of scale that promise future gains that are outpacing fieldbus.
This article explores these benefits.
Industrial Ethernet protocols
Within the Ethernet frame, one can place almost any application protocol. There is no one particular protocol that serves all the needs of industry. Instead, application protocols are like a tool chest, with users picking the ones that support the demands of their particular automation applications to provide the required performance, security, and safety.
The focus for this article is on EtherNet/IP, the industrial Ethernet protocol supported by the Open Device Vendor Association (ODVA). EtherNet/IP uses the standard Ethernet frame as defined by IEEE 802.3 and uses ODVA’s and ControlNet International’s Common Industrial Protocol (CIP) application protocol library of objects.
Figure 2 shows how the CIP application library can be deployed upon several different physical network architectures. This is a unique benefit to users because there are no physical application interfaces between the layers. This gives the CIP library almost seamless bridging and routing among different physical networks—both Ethernet-based and others, such as CAN-based networks.
Ethernet and EtherNet/IP
EtherNet/IP in the process industry is definitely a developing technology—unlike fieldbus, which has enjoyed 20 years of refinement. However, recent developments and technology breakthroughs are making EtherNet/IP a viable alternative to fieldbus.
Ethernet IEEE 802.3 can currently support data transmissions up to 10 Gbit/sec. Although EtherNet/IP-enabled devices deployed over the 802.3 standard currently support only 10/100 Mbit/sec transmission rates over copper and fiber, traffic through the network can still use the higher transmission rates if the network architecture supports it. And future variants of EtherNet/IP will advance along with Ethernet to support even higher transmission rates.
One advantage of EtherNet/IP is that it can support wireless transmission by using industry standard devices. When deploying EtherNet/IP over wireless, the user must consider how wireless system deployment creates latency in the EtherNet/IP message timing. Note that the same latency problems exist with wireless fieldbus, but without the advantages of the latest technological developments from the Ethernet wireless world.
Cabling distances depend on the 802.3 standard; i.e., 100 meters for device-to-device when deploying over copper and 2,000 meters when using fiber deployments. Power over Ethernet (PoE) is available so that power supplies may not be needed in the field. However, product availability varies by vendor.
Ethernet switches are also available for use in hazardous locations. Some switches use intrinsically safe PoE for connecting to field instruments in Zones 1 and 2. Unlike fieldbus, which can handle multiple devices in hazardous areas, one switch vendor recommends putting only one device on a single cable, which is becoming less of an expense as Ethernet switch prices rapidly decline. Again, product availability varies by vendor.
Typical Ethernet network topology is trunk-star. However, device manufactures are starting to embed micro Ethernet switches into their devices—allowing for linear and ring topologies—which reduce the need to create star network topologies. Redundancy can be achieved through the appropriate switch architecture and in some instances by adding a communication interface to allow a single fiber or copper port to be a node on a redundant ring infrastructure. In other words, it is possible to put multiple instruments and devices on the same cable and to provide redundancy when needed (see Figure 3).
Process instrument perspective
Looking at the EtherNet/IP protocol from the process instrument perspective, to whom and to what does an instrument have to report? The primary responsibility is to the automation or host system. Historically, this has involved the primary process variable. Secondary responsibility is instrument diagnostics, and last is instrument configuration data.
Each of the users or consumers of the data that the instrument produces has different tools and mechanisms to acquire the data. Each has its own unique requirements for the use of the data. Considering each of these areas—and how EtherNet/IP not only serves their unique requirements, but also creates commonality and convergence in the process—will help us understand how EtherNet/IP is not only a very capable fieldbus-type network, but also provides benefits beyond what typical-level fieldbuses deliver today and in the future.
Process variables: EtherNet/IP communicates process variables or I/O data back to the host system at a requested packet interval rate (RPI). This RPI is defined by the user. Typically, RPI is set based on application requirements. RPI rates for EtherNet/IP-enabled devices will vary based on the manufacturer of the device and the applications they serve.
Typical RPI times for process instruments, such as Coriolis and electromagnetic flow meters, on EtherNet/IP networks are from 5 msec to 10 sec. The device will communicate I/O data to the automation system at the RPI rate established when the device is configured in the system. This variability in selection of the RPI data rate enables the user to optimize the flow of I/O data through the process and optimize the data crunch through the microprocessors in the data chain without relying on the actual network bus rate or frame size specifications.
I/O data can also be provided simultaneously to multiple consumers (processors, devices, etc.) in the architecture. In addition to the primary process variable, multivariable devices, such as mass flow meters, can transmit multiple variables such as flow, volume, and temperature simultaneously, similar to traditional fieldbus architectures.
Configuration of what variables will be transmitted in the I/O data structure is typically determined by the manufacturer of the devices. Some manufacturers allow user configuration of the I/O data structure. Device vendors deploy device profiles that will interface with the automation system and define what these variables are.
If profiles are well defined, the process control engineer has very little work to do to get devices online and communicating data throughout the system. Typically, just verifying the actual device, revision of device, RPI, and the Ethernet address of the device is all that is required to get a device up and running.
See next page for more about diagnostics and additional graphics
Diagnostic data: Diagnostic data can be a very general term and is defined by the task that is being performed by the technician or operator requiring it. From the device perspective, the device can provide diagnostic data to the automation system, operations personnel, maintenance personnel, reliability personnel, and IT personnel, to name a few.
Some of this diagnostic data can be included in the I/O data structure. For example, diagnostic data for a Coriolis flow meter includes empty pipe detection, sensor drift, sensor error, electronics error, inhomogeneous mixture error, ambient and process temperature errors, and other information. Whatever data are considered critical can be included in the I/O data during configuration.
Devices also need to provide diagnostic data to technicians operating outside the control area and the automation system’s operator interface tools. One example is an electrical and instrumentation technician using device configuration software to reference the voltage delta between the measuring electrodes in an electromagnetic flow meter. With appropriate software, the technician can access the necessary data without interfering with process control operations.
Devices on EtherNet/IP can also be polled by a condition monitoring system to determine if there are any diagnostic messages that need to be sent to maintenance personnel as an alert. An industrial PC equipped with asset management, maintenance, condition monitoring, or HMI/SCADA software can access all the I/O and diagnostic information it needs directly from the devices via the Ethernet interface (see Figure 4). With fieldbus, the same software has to access the information from the process historian or database in a DCS—at considerable extra cost.
Most EtherNet/IP-enabled devices support SNMP. This enables IT technicians to monitor, troubleshoot, and administer network devices using standard network management tools. For example, suppose that IT is monitoring network traffic using an SNMP-enabled tool. The software tool reports that an EtherNet/IP device has exceeded its normalized packet transmission rate, and an e-mail alert is created and sent to a technician. The technician can then use the internal Web server of the device for troubleshooting.
This leverages the investments a company has made in its IT support infrastructure, and minimizes the burden on the process control engineer from having to also be an IT support engineer.
Fieldbus, on the other hand, requires detailed knowledge of the fieldbus architecture and cannot leverage a company’s IT infrastructure; the burden is still placed on the process control engineer to be a network expert. Fieldbus requires specialized training and knowledge, while EtherNet/IP is instantly familiar to process automation and other professionals who have worked with Ethernet.
EtherNet/IP has two main messaging connections: I/O data and explicit connections (see Figure 5). Explicit connections are messages that are not scheduled as with I/O data, but are delivered on demand. While the device is handling I/O data requests, it can simultaneously handle on-demand requests. Figure 5 illustrates the mechanism—UDP/TCP in the TCP/IP suite of Ethernet—to simultaneously deploy the I/O data and messaging data for the CIP library.
These examples demonstrate a few of the various requirements of device diagnostic data and the varied locations to which these data are sent. The ability of Ethernet to allow this simultaneous collection of data from the devices is a key benefit.
Compared to traditional fieldbuses, EtherNet/IP has minimal need to create additional configuration code in the host system. This reduces the footprint of the process configuration on the host. There is no need to have an additional software configuration package for the network or to add additional network interfaces, thus reducing hardware and software costs.
Some of these benefits are derived from the mere use of Ethernet and cannot be wholly attributed to the EtherNet/IP protocol. However, implanting these functions often makes fieldbus installations expensive, cumbersome, difficult to support, and sometimes unappealing. Deploying an Ethernet-based protocol is thus useful in overcoming fieldbus difficulties and objections.
Configuration data: Configuring and documenting a process device in an automation system can be a very time intensive task. EtherNet/IP gives users of these devices several options for configuration and documentation by giving them different access points and letting them use different tools to configure and maintain device configurations.
Ethernet 802.3 provides a large data packet—up to 1,500 bytes—that opens up a large chunk of data in a frame, enabling device vendors to serve up more device attributes than can be communicated over typical fieldbus protocols. This configuration data for a process device is communicated at the I/O data level to the automation system.
This gives the automation system access to the configuration parameters of a process device, allowing the user to determine which, if any, configuration parameters can be accessible to system programmers or operators at the operator workstations. This provides flexibility during start-up and commissioning for personnel to monitor or change parameters while working from within their system configuration programs.
Using EtherNet/IP does not require all users to use the same set of tools. Most devices on Ethernet have a built-in Web server that gives users access to device parameters. This is useful for the IT technician who may not have access to, or training for, process control software or device configuration software tools.
Because the Ethernet/IP protocol uses the standard OSI model, other toolsets become available, and can coexist and function synchronously throughout the architecture.
Maintenance personnel also have at their disposal their own tools, such as asset configuration software and asset management software, for documentation and change management requirements. All this software can reach devices throughout the EtherNet/IP network.
EtherNet/IP provides network access beyond the local area network (LAN) to a wide area network. I/O data can now traverse from one network to the other through standard IT hardware. This gives support personnel access from virtually anywhere in the world, allowing manufacturers and vendors to support their customers remotely.
It also provides segmentation and optimization of networks using tools that IT companies commonly provide to the marketplace. Traditional fieldbus implementations constrain data to their physical network; that data must be accessed through the host or a third-party communication interface.
The volume of data on the network is increasing as users begin to merge their business/financial networks with the plant automation system network. This creates an ever increasing need to segregate, constrain, and secure the traffic so that it does not impact the data throughput of the automation networks. IT suppliers have been providing the hardware and tools to support these needs, and that technology is now employed on industrially hardened Ethernet-based devices.
Some IT vendors are also providing switch diagnostic data as I/O data in the CIP library. This commercially available technology allows the engineer to segregate network traffic inside the common hardware appliances, allowing for even faster propagation of critical data inside the network topology.
There will be some applications where a user may not be able to completely segregate or constrain the data to a virtual LAN or local subnet. The issue now becomes being able to compete for the data packets to be processed in the switches throughout the network. EtherNet/IP has identifiers in the CIP library to allow a switch, configured for QoS, to prioritize these packets over the voice, data, and media packets on the network. Being able to perform these QoS tasks within the network provides the best optimization of the network for the automation network data.
Security is a wide and deep topic and is not addressed in this article, other than to note that EtherNet/IP is able to leverage all of the commercially available security features that are delivered in the IT market today for Ethernet-based networks. There are several publicly available documents for securing converged networks, and the ODVA website has a publication that discusses securing Ethernet networks.
Ethernet has been the dominant commercial network for the past 40 years, and will continue to be in the future. As the convergence of the plant floor to the front office continues its progress, leveraging this future in automation devices will be essential. Process devices will get more intelligent—the past and present demonstrate this. A process device will have a lot of information to share, and will need ever more network capacity and capabilities.
EtherNet/IP will meet these needs by leveraging Ethernet advances, taking advantage of Ethernet’s huge economies of scale. More Ethernet nodes will be connected this—or any other—month than have been connected in the entire history of fieldbus. This economy of scale and the tremendous technological advancements that go along with it is what makes EtherNet/IP more capable than a fieldbus network, now and especially in the future.
Michael Robinson is director of solutions for the Endress+Hauser Sales Center, US. He has 18 years of experience in factory and process automation as a project engineer, product manager, and business development manager. Robinson has a BS in agricultural engineering technology from California Polytechnic State University, San Luis Obispo, Calif.