HART-IP simplifies Ethernet-APL transition

Ethernet Insights

• HART-IP works to preserve end user investments in technology and training related to HART.
• Ethernet-APL is based on two-wire Ethernet ( 10BASE-T1L) and transmits data and power to process network transmitters and actuators.
• Ethernet-APL combines existing digital communication technologies in the process manufacturing industry.

The HART communication protocol is a digital information exchange between a smart process control field device and a control or monitoring host system and works on traditional 4-20mA analog current loops.

However, as newer digital media options are becoming available for field instrumentation service—most notably Ethernet advanced physical layer (Ethernet-APL)—many designers and engineers are keen to chart the best path forward. Ethernet-APL is based on Ethernet standard 10BASE-T1L two-wire Ethernet, plus extensions for deployment in hazardous areas. Adopters want to ensure Ethernet-APL media provides equal or better functionality than 4-20mA, especially considering the challenging and sometimes hazardous locations target field devices must work in.

Developers also must determine how data handling will preserve compatibility with extensive existing installations and what capabilities will soon be available. HART-IP was introduced in 2007 to support industrial Ethernet communications among input/output (I/O) systems, muxes, WirelessHART gateways and other instruments and devices (Figure 1).

Figure 1: HART, WirelessHART and now HART-IP are evolutions of the leading industrial wireless protocol for process automation communications and control. Courtesy: FieldComm Group

Ethernet-APL can deliver process automation signaling options and HART-IP works to preserve end user investments in technology and training related to HART.

Migrating from 4-20mA to Ethernet APL

HART superimposes digital communications on top of legacy point-to-point 4-20mA signals. The 4-20mA signal is the transport media while HART is the “communications protocol.” With this configuration, users keep the known functions and preserve existing investments, while adding new means of communication. HART is the most widely-used digital communication technology deployed in the process industries, provides data access including multiple process variables, instrument status, calibration and diagnostic information.

Wired Ethernet in general was commercially introduced in the 1980s and soon became dominant for consumer and commercial networking uses. However, even as wired Ethernet evolved and exponentially gained capabilities, it retained many limitations with respect to distance, power, and other factors, making it less suitable for industrial field device connectivity.

Ethernet continues growing closer to the plant floor from enterprise networks to plant networks to control/field networks. In fact, it is now used as a media for industrial automation applications, especially between PCs, controllers, and remote I/O. There are many Ethernet-based industrial communication protocols.

The quest to adapt modern digital computing and networking technologies  for industrial use led to the development of Ethernet-APL media, and ideally positioned to bring HART-IP to the field instrument layer.

Ethernet-APL benefits

Ethernet-APL combines the best of existing digital communication technologies, with consideration for specific industrial needs such as extended cable lengths, suitability for hazardous areas and robust noise immunity.  The Ethernet-APL familiar two-wire installation approach for process automation and instrumentation has a reach of up to 1,000 m between network switches and an additional 200 m to each field device.

Ethernet-APL is based on two-wire Ethernet ( 10BASE-T1L), and transmits data and power to the process network transmitters and actuators. Ethernet-APL builds on the Ethernet standard IEEE 802.3cg with a data rate of 10 Mbit/s. The Ethernet-APL Port Profile Specification (FCG_TS10186 pending IEC TS 63444) further defines power and other parameters for use in the process industry including hazardous areas (Class 1 Div 1, Zone 0 and 1; as well as less hazardous areas). To support installation of products, the Ethernet-APL Engineering Guide (FCG_AG10337) provides engineers with critical information for planning, installation, and acceptance testing of Ethernet-APL networks. Successful deployment of Ethernet-APL requires a well-engineered solution following the relevant planning steps.

Ethernet-APL resides on layer 1 of the protocol stack, the physical layer as shown in the ISO/OSI protocol stack of Ethernet according to ISO/IEC 7498-1. Ethernet-APL is one of many physical layers that support Ethernet. Unlike prior automation technologies, the Ethernet-APL physical layer is independent of the other layers of the OSI model.

Ethernet-APL also can be used in conjunction with any Ethernet based protocol such as HART-IP. Ethernet-APL follows the requirements of the NAMUR recommendations NE 74 and NE 168 to define a network architecture for use across the process automation industry.

As with 4-20mA signaling, device power is provided over the media. This is a significant benefit for industrial instrumentation, which generally has low power requirements, because removing the need to install separate power circuits means large capital and installation cost savings. Also, similar to 4-20mA, it is possible to deploy Ethernet-APL in hazardous areas.

Communications over Ethernet-APL operate at 10Mbit/s full-duplex with cycle times on the order of 10 to 2,000ms. This is slower than commercial Ethernet but is by far fast enough for industrial communications, and the performance level easily supports real-time control. Ethernet-APL uses standard industrial Ethernet switches for upper layer communications, and specific Ethernet-APL switches for the control/field network. The technology uses one physical layer supporting the simultaneous interoperation of many protocols (Figure 2).

Figure 2: Ethernet-APL is an industrial-grade evolution of standard Ethernet media types. It is ideally suited for connecting process instruments with multiple host systems, and supports simultaneous operation of many protocols. Courtesy: FieldComm Group

Ethernet-APL and HART-IP

To address the emergence of Ethernet-APL for industrial process automation usage, FieldComm Group has extended HART-IP with security and other features that enable deployment of HART-IP in field instrument layers of the plant network for applications including control. HART-IP is simple to use, is independent of underlying media, and works with redundant media, including in mesh and ring topologies, at any network speed.

HART-IP allows any properly credentialed host system—whether it is a controller, asset management application, or other—to access and integrate with measurement information, device configuration, and device diagnostics. Any properly credentialed host system connected on the IP network with an instrument can securely access the instrument independently, and multiple hosts can interact with an instrument at the same time.

Field devices supporting HART-IP are becoming available, and the next generation of instruments will be used for monitoring, asset management, and direct control applications. FieldComm Group is ready with a test system and associated tools to support and enable registration of HART-IP products on Ethernet-APL. HART-IP testing builds upon FieldComm Group’s experience with HART and Foundation Fieldbus testing. Registration helps suppliers ensure compatibility of devices they bring to market.

HART-IP supports the commonly understood HART data model, making it backwards compatible, but it also extends the architecture further for Ethernet support. Existing HART tools can use HART-IP products, and benefit from HART data at Ethernet speeds. HART has always been capable of two-way communication, and HART-IP is designed to support direct control of field devices.

A particular benefit is how HART-IP simplifies and flattens the control network, so higher-level controllers, applications and resources can directly access field device information. Previous generations of industrial automation hardware, software, and networking required complex and problematic data-passthrough to get field device information through intermediate controllers and up to the enterprise level. HART-IP effectively and cleanly addresses this common need (Figure 3).

Figure 3: The HART-IP protocol operating over Ethernet-APL flattens the industrial control network, and enables many higher-level systems to access field device information for monitoring and direct control without any complex data-passthrough schemes. Courtesy: FieldComm Group

There are other industrial Ethernet protocols used at the control network level for field instrument service. However, large groups of process users are already familiar with the data model and work practices associated with HART. When moving to new technologies, it is less risky to leverage and expand upon tools already at hand and work practices already in place, instead of ripping-and-replacing systems, which requires users to learn new concepts and protocols.

HART has a long heritage within the process industry and  holds much untapped potential. HART-IP enables an even greater expansion of data availability and access. While companies can develop many new tools to take advantage of the higher bandwidth, these users do not need to wait because the existing HART tools can be used with HART-IP products.

Assuring conformance to Ethernet-APL

The previously mentioned Ethernet-APL Port Profile Specification defines power and other parameters to extend Ethernet-APL use into the process industry, including hazardous areas. Ethernet-APL networks include a variety of infrastructure devices (power switches, field switches, media converters) and field devices (including instruments, actuators, analyzers, motor controls, gateways, and more).

Conformance ensures the variety of products consistently adhere to specific port profile requirements with regards to power, protection, and other characteristics. The co-owners of Ethernet-APL technology—which include ODVA, OPC Foundation, PI Organization, and FieldComm Group—agreed to offer one form of physical layer testing to assure conformance to physical layer specifications and requirements.

Each organization also offers testing of their supported automation protocols. FieldComm Group now offers registration services for network infrastructure and HART-IP field devices (instruments, actuators, and more) so vendors and end users can be assured of compatibility and performance.

Why HART-IP for Ethernet-APL?

Industrial process automation end users already have significant investments in existing HART and WirelessHART tools, technology and training. HART and WirelessHART lead among wired and wireless installations.End users, and the original equipment manufacturers (OEMs) and systems integrators serving them, wrestle with the best ways to adopt newer and higher-performance technologies in practical and economical ways, while preserving existing investments. Ethernet-APL and HART-IP enable this, as modernizations of existing commercial and industrial technologies to meet the needs of industrial automation.

Ethernet-APL adapts commercial technology to make it ideal for industrial use. For users familiar with HART or WirelessHART, transitioning to HART-IP is straightforward because it uses many of the same data structures, and its software tools are similar. End users can leverage their understandings of HART and WirelessHART networking and communication technologies, applying them to even more capable wired HART-IP over Ethernet-APL installations.

Sean Vincent is director of technology programs of the FieldComm Group. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, cvavra@cfemedia.com.

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