Industrial Ethernet protocols are the top way to gather smart-instrument data, and advancements will allow better implementation with two-wire instruments.
By Keith Riley, national product manager – Pressure Products, Endress+Hauser
The Industrial Internet of Things (IIoT) is the use of smart instruments to enhance industrial processes through real-time analytics. It requires the generation of massive amounts of big data, which is collected and stored in host systems for evaluation. Main sources of this data are the smart instruments installed throughout plants and facilities.
These instruments transmit basic process variables — along with extended data such as calibration parameters, diagnostics and other information — over digital networks. This data becomes valuable when analyzed by end users in the context of a specific process, permitting them to perform predictive maintenance, reduce downtime and make other operational improvements.
This description assumes every smart instrument is connected via a digital network, but in many instances, the information present in the instruments is unavailable because of communication protocol limitations. Bridging the gap between smart instruments and data repositories is where industrial Ethernet protocols such as EtherNet/IP™ and PROFINET® enter the picture.
This data-gathering aspect of analytics is the focus of this article, showing why industrial Ethernet protocols are so popular. It explains why two-wire instruments with power and data available via the same 2-wire Ethernet cable (commonly referred to as Power over Ethernet [PoE]) are not yet available, and then discuss developments with two-wire instruments to enable connectivity.
Industrial Ethernet Leads the Way
Most end users prefer industrial Ethernet protocols over other traditional industrial fieldbus protocols for three main reasons:
- Facilitates unification of an Ethernet infrastructure throughout a plant or facility — single network from field to enterprise level.
- Provides extensive bandwidth, enabling access to more information at a faster speed.
- Supports ease and economy of setup and use due to widespread familiarity with and availability of Ethernet.
Will a plant be able to unify its operational technology (OT) and IT networks using industrial Ethernet? Yes, but only with the proper network architecture. Smart switches or routers expressly designed to manage the information traffic from both networks are essential.
One example is the Stratix® family of industrial managed Ethernet switches from Rockwell Automation. These switches separate office Ethernet TCP/IP traffic from industrial EtherNet/IP traffic by using freely assignable ports (see Figure 1).
Ethernet TCP/IP is a common IT network protocol, and several industrial Ethernet OT network protocols exist. All these protocols follow the same OSI Layer Architecture model and comply with the IEEE 802.3 communications specifications.
However, industrial Ethernet protocols modify the application layer. For example, with the EtherNet/IP protocol, the application layer in the OSI model is slightly different from IT protocols because it includes the Common Industrial Protocol (CIP™). CIP improves access to data used for control of network devices by separating it into implicit and explicit data packets, or messages (see Figure 2).
Explicit messages are for node-to-node communication and use TCP delivery. They move large amounts of data, with variable payload sizes, only as needed. Implicit messages are for compact high-speed communication and use UDP delivery. They are most commonly used for preconfigured high-speed input/output (I/O) messaging. EtherNet/IP adds this explicit or implicit distinction to the information packets to optimize performance for these types of transmitted data.
A standard IT Ethernet TCP/IP network communicates information upon request. Managing this traffic without negatively affecting network speed or resulting in significant delays is relatively easy through the strategic use of the switches and routers commonly applied in IT networks.
However, OT industrial Ethernet networks are used for both I/O and control, with information broadcast continually. If a system is trying to transmit both Ethernet TCP/IP and industrial Ethernet messages on the same network, it would quickly become overwhelmed and the speed would degrade to an unacceptable level.
Therefore, proper management of the available bandwidth by using special managed switches or routers is crucial. Routers can be used to segment networks, and managed switches can be used to manage traffic on each segment.
Plug and Play: Not Yet
Industrial Ethernet protocols such as EtherNet/IP are useful for transmitting large amounts of data quickly, making them a good fit for smart instruments. While an EtherNet/IP network is user friendly, it hasn’t yet reached the same level of functionality most people have come to expect from a standard home or office Ethernet TCP/IP network.
Compared to the maturity of Ethernet TCP/IP, industrial Ethernet networks are still relatively young, meaning the critical mass necessary for the market to provide expected true “plug-and-play” functionality has not yet arrived.
If a plant has an EtherNet/IP network installed, the physical connection of four-wire smart instruments into the network is much like any other Ethernet device. The host system also is connected to this network and can process data from the instruments. Hosts can include control systems, asset management systems, process historians and others.
Integration of smart instruments directly into a host system requires coordination among the smart instrument, the host system and the smart switches. For example, three main tools are needed to easily integrate smart instruments into a Rockwell Automation PlantPAx® control system based on a ControlLogix® hardware platform:
1. An Add On Profile (AOP) Level 3 is required data integration. AOPs are software packages loaded into the control system and are:
- specific to individual instruments.
- developed in coordination with the control system manufacturer.
- supplied to the end user or system integrator by the smart instrument manufacturer.
2. Add On Instructions (AOIs) are supplied as a pre-engineered function blocks, and each must be configured by the end user. AOIs are used to define data, arguments, parameters, algorithms and other functions required for processing data from the instruments.
3. Instrument faceplates and Global Objects are supplied to provide human-machine interface (HMI) visualization of instrument data, transparency of extended field instrument information and assistance with diagnostics. These typically are supplied by the control system manufacturer.
If multiple instruments from more than one instrument manufacturer are connected, then AOPs, AOIs and faceplate software will need to be loaded and configured for each type of smart instrument. In addition to the software requirements listed above, hardware connectivity considerations arise with two-wire instruments.
Connecting Two-Wire Instruments with Industrial Ethernet
Most users have heard of PoE, and many question why manufacturers have not implemented this functionality into their smart two-wire instruments. A PoE standard exists in the form of IEEE 802.3 AF/AT, and there are also commercially available switches on the market to manage and distribute PoE.
The main issue is that IEEE 802.3 AF/AT-compliant switches are rated for up to 52VDC at 2.5A. This relatively high level of power makes it difficult to use these switches in the hazardous areas commonly found throughout process plants and facilities. This is true regardless of whether the installation is designed using an intrinsically safe or explosion-proof concept.
Therefore, only areas classified as nonhazardous are viable for PoE installations, but even here, challenges exist. Per NEC® 500 (National Electrical Code®), working on live energized parts operating with a voltage greater than or equal to 50V is a safety issue and requires the use of personal protective equipment (PPE). In many instances, the benefits of PoE implemented in compliance with IEEE 802.3 AF/AT simply don’t provide sufficient value to justify the effort or expense required for design, installation and maintenance.
Addressing the Two-Wire Instrument Issue
IEEE P802.3cg 10 mb/s Single Pair Ethernet is a physical layer specification and management parameter for 10 Mb/s operation and associated power delivery over a single balanced pair of conductors (see Figure 3). Implementation of this standard will make Industrial Ethernet protocols suitable for use in hazardous area locations.
Ethernet-APL is exactly what the name implies: a new physical architecture designed to work in conjunction with industrial Ethernet technology (see Figure 4). Ethernet-APL won’t affect the protocol stack, so it will be suitable for industrial Ethernet technologies such as PROFINET, HART-IPor EtherNet/IP Industrial Ethernet protocols, and opens their use to two-wire topologies.
Together, Ethernet-APL and IEEE P802.3cg 10 Mb/s Single Pair Ethernet will support industrial Ethernet-based communication protocols for two-wire field instrumentation by:
- Allowing power and data to be carried on the same two-wire shielded cable.
- Permitting installation in plant areas classified as hazardous.
- Providing high bandwidth of 10 Mb/s.
The work being conducted by the Ethernet-APL Project is ongoing and expected to be finalized soon. Once this work is completed, instrument manufacturers and component suppliers can respond with the equipment necessary for a complete network.
Until this solution is available, the alternate method for integrating legacy two-wire instruments to an industrial Ethernet control system is through a gateway. While highly functional, this method does limit the data available from each instrument to typical primary or HART values. One example of a device with Industrial Ethernet gateway capability is the RSG45 from Endress+Hauser.
Smart instruments are a vital component for process owners to implement IIoT strategies. However, users can only realize the full value of these devices and the subsequent potential of IIoT analytics when their systems can quickly and easily access the growing amount of available data.
Industrial Ethernet enables this digital transformation by providing simple and fast access to the required real-time instrument and process data. Today, four-wire instruments can easily connect to industrial Ethernet networks, and traditional two-wire instruments can be integrated with gateways.
In the future, two-wire instruments will be able to connect more directly as a result of Ethernet-APL and IEEE P802.3cg 10 Mb/s Single Pair Ethernet. This will enable improved operations through the use of analytics applied to data gathered using industrial Ethernet networks.
Endress+Hauser , a global provider of process measurement instrumentation, services and solutions, is a Strategic Alliance Partner in the Rockwell Automation PartnerNetworkTM program. Together, Rockwell Automation and Endress+Hauser deliver integrated pre-engineered, pre-tested, supported, and maintained instrumentation and control and information solutions that provide plant-wide advanced diagnostics and process system life-cycle management.
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