Industrial use of Ethernet and TCP/IP is growing rapidly, outpacing conventional industrial communications protocols, according to a 2013 IMS Research study, because it runs on widely available and standard physical and wireless media, and is often easier to integrate with office and information systems. So it's no surprise that many fieldbus specifications have adopted corresponding industrial Ethernet protocols.
In 2012, the HART Communication Foundation officially added a HART over Internet protocol (HART-IP) capability to the HART Protocol, a specification for transmitting the HART Protocol over the increasingly common IP using either Transmission Communication Protocol (TCP) or User Datagram Protocol (UDP).
In practical terms, the existing HART protocol can now run over an IP-based connection, so the standard Ethernet infrastructure that is used today in most control systems is now able to run the HART application layer and protocol over the same Ethernet and TCP/IP layer. Through HART-IP, third-party packages such as asset management, control and historian systems can access information from, for example, multiplexers and WirelessHART gateways, without data mapping.
"Before HART-IP, you had three options for integrating HART data. You could use HART I/O and controllers, HART multiplexers or serial interfaces," says Vibhor Tandon, product marketing manager, OneWireless, Honeywell. Tandon's experience includes 10 years working with HART technology and the HART Communication Foundation. "You have issues with capacity, scalability and complexity. Larger systems take a lot of data mapping.
"With HART-IP, you don't need hardware converters or interface boxes with their limited number of serial ports. Using Ethernet, you have all the HART data (for example all process variables, device diagnostics, device status information, control valve signatures, level echo curves, etc.) mapping right onto IPv4 and IPv6 — the most standard interface in the industry. It comes down to three things: simplicity, scalability and ease of use."
Today, vendors and users are beginning to implement and appreciate the power of HART-IP. Much of that power derives from the fact that HART-enabled devices are already dominant in most process plants. HART began as, and remains, a straightforward and relatively simple technology that provides low-cost, highly capable digital communications. Ease of use and compatibility with existing 4-20 mA systems, combined with the benefits of remote device configuration and diagnostics, contribute to the technology's status as the standard for smart instrument communication (see sidebar, HART Basics).
"From its first version, the HART standard has been easy. HART took that first version and maintained, upgraded, improved and added to it," says Bob Karschnia, vice president, Emerson Smart Wireless. "HART begat HART 5, HART 7 and WirelessHART. Now HART-IP is the next evolution. Everyone's excited."
The Impact of IP
HART-IP primarily serves as a high-bandwidth connection between host applications (e.g., process automation systems, plant asset management systems, data historians, CMMS, etc.), HART-enabled I/O (e.g., multiplexers, remote I/O, gateways) and HART-IP-enabled instrumentation. It combines the ubiquitous IP network infrastructure and the HART network and application layers. As a result, plant personnel can use infrastructure they have already deployed and understand, such as Ethernet and Wi-Fi, to provide HART-compatible system connectivity and integration of all available HART information — not just the primary variable.
Figure 1: In a typical HART architecture, a HART device (server) may be a stand-alone field device, a gateway connected to WirelessHART field devices, or an I/O system connected to traditional 4-20mA HART-enabled field devices. Devices such as a serial-to-HART-IP converter are also possible.People often refer to "Ethernet" when they are really using shorthand for not only the physical layer of a network known as Ethernet, but also for the suite of protocols and network structure defined by the seven-layer Open Systems Interconnect (OSI) Reference Model (see sidebar, The OSI Model and TCP/IP). This model was developed by the International Standards Organization and adopted in 1983 as a common reference for the development of data communication standards. It does not attempt to define an implementation, but rather it serves as a structural aide to understanding "what must be done" and "what goes where." What we commonly call "Ethernet" is really just layers 1 and 2 of the seven-layer stack.
HART-IP is an application protocol. It defines rules for organizing and interpreting data and is essentially a messaging structure that is independent of the underlying physical layer. HART enables communication with smart process instrumentation and controls, and supports both wired and wireless network topologies. The TCP/IP communication transport extends the applicable physical layers to those that support TCP/IP communication. Possible physical layers include Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11b/g), RS232 using PPP-, 4G- and 4G-LTE-enabled smart phones and even SCADA OPS over VSAT.
General Architecture for HART-IP
The general architecture may include different device types (Figure 1). A HART device (server) may be a standalone field device, a WirelessHART gateway connected to WirelessHART field devices, or an I/O system connected to traditional 4-20 mA HART-enabled field devices. Devices such as a serial-to-HART-IP converter are also possible.
The HART client implements information exchange with a remote device. The client builds a HART request with information sent by the user application to the client interface. The request is then sent to UDP or TCP port of the server. The server response will be returned via the same transport (UDP or TCP) as the request. The basic message format includes a header to describe the content of the HART-IP payload and the Token-Passing Data-Link Layer PDU format message (Figure 2).
A HART field device implementing HART-IP, for example, might be a process analyzer, flow computer, mass flowmeter or sophisticated level device connected using standard Cat-5 cable. With power-over-Ethernet (POE), no separate power wiring is required. High-performance types of these devices that include a lot of process and device information could even have a web browser interface for quick and easy setup.
Likewise, a HART-enabled system can use standard HART commands to access HART information from any and all devices connected to it using a HART-IP-enabled multiplexer, I/O or WirelessHART gateway. This is done by simply connecting the system and the HART-IP-enabled device using the installed plant IP infrastructure. Mapping of Modbus registers and other tasks are eliminated.
Figure 2: The basic message format includes a header to describe the content of the HART-IP payload and the HART pass-through Protocol Data Unit (PDU) using the Token-Passing Data-Link Layer frame format.HART-IP standardizes the entire path from field device to asset management. "We have the HART, the tools, the knowledge. We can add Profibus, Profinet, DCS integration, and the 4-20 mA stays the same. No protocol switching, no tunneling. Seamless," says Gerrit Lohmann, manager, product group Remote Systems, Pepperl+Fuchs. "HART-IP keeps the seamless capability and extends it to the Internet. You have all the same mechanisms, now with fast communication. You don't have to use it for field devices, but it's needed for backbone communications and multiplexers."
Third-Party Systems: The Killer App
Today, HART-IP's killer application is streamlining access to HART data for asset management. Historian and control systems are in the pipeline, and it's expected that ready access to HART data on IP, through WirelessHART and without hardware limitations, will lead to as-yet-unheard-of applications in widely distributed sensor networks (pervasive sensing) and the industrial Internet of Things (IoT).
Without HART-IP to get data from an instrument or multiplexer into an asset management system, you need a communication link, often with proprietary standards, one for each kind of instrument or multiplexer. "With HART-IP, there's one common protocol. You don't have to worry about different drivers for different multiplexers. That makes it easier to implement the asset management package," says Emerson's Karschnia.
That complexity has kept many users from integrating HART, and just using it for configuration in the field. "What makes HART-IP exciting is, they don't have to go out to the device," says Jeff Dobos, president, ProComSol. "If they have access to the gateway, they have access to the device, without interfacing with control or through the control system. This is great for maintenance, data logging or alarms."
"Now you have a clear infrastructure to connect your tools by Ethernet or wireless LAN for device asset management," says Frank Fengler, head of device integration management, ABB. For example, "If you have a HART topology using a wireless gateway-to-backbone Ethernet and then connect to tools for device management, now you can use a single tool to access all devices with one network."
The HART Protocol is used in various modes to communicate information to/from smart field instruments and central control or monitoring systems. In most applications, HART digital information is communicated simultaneously with the traditional 4-20 mA analog signal. The process variable is transmitted continuously on the 4-20 mA, while additional measurements, calculated values, device configuration, diagnostics and device status information are sent by HART digital communication on the same wires. This field proven technology uses the same physical signaling as caller ID does on a telephone line.
Unlike other digital communication technologies, the HART protocol maintains compatibility with existing 4-20 mA systems, and in doing so, provides users with a uniquely backward-compatible solution. The process variable continues to be transmitted by the 4-20 mA analog signal, and additional information pertaining to other variables, parameters, device configuration, calibration and device diagnostics can be transmitted digitally at the same time. Thus, a wealth of additional information related to plant operation is available to central control or monitoring systems through HART communication.
The HART Communication Protocol is the industry standard (IEC; China GB/T) for digitally enhanced 4-20 mA field communication. The enhanced communications capability of intelligent field instruments employing the HART protocol offers significantly greater functionality and improved performance over traditional 4-20 mA analog devices.
"The case for using HART-IP for asset management is clear," says Amit Ajmeri, consultant, asset management solutions, Yokogawa. "We support HART-IP for asset management. You can bring data over Modbus-TCP for determinism and use HART-IP to bring the data from different vendor transmitters and multiplexers for asset management."
With HART-IP users can expect easier access to all the HART data in field devices to allow asset management systems to work smarter. "Advanced diagnostic platforms have to be able to deal with so many alarms," says Jim Shields, marketing manager, calibration, Fluke. "Software can use this data to analyze the alarms so only the needed information is presented. Today, maintenance people have to look at them, and there are so many, they often just turn them off."
HART-IP connectivity is coming to historian and process control systems, in part to simplify connectivity, but also to access more information. "Smart devices offer secondary and tertiary and quaternary information, and HART-IP access allows you to collect this data," says Glenn Gardener, business unit manager, process tools, Fluke. "Plant historian companies will probably be getting requests."
You'll probably find HART-IP in your next DCS, especially if you demand it. "Honeywell is building it into their systems now," says Emerson's Karschnia. "It's the simplest interface to work with in DCS and AMS. We're testing it in AMS, and it's logical to put it in the DCS."
Field Connectivity: The Sleeper App
In the traditional OSI model, the functions of communication are divided into layers, with each layer handling precisely defined tasks. For example, Layer 1 of this model is the physical layer and defines the physical transmission characteristics. Layer 2 is the data link layer and defines the bus access protocol. Layer 7 is the application layer and defines the application functions (this is the layer that defines how device data is to be interpreted).Most experts who are enthusiastic about HART-IP at the system level don't see a lot of need for it at the device level, where there's little need for its high speed or the relative cost and complexity of TCP/IP.
But others see opportunity for HART-IP in the field. "HART-IP won't change the workflow in the medium term. People will continue measuring directly, with HART-IP on the back end of a gateway or multiplexer," says Fluke's Gardener. "But if OEMs start issuing HART-IP-compatible transmitters with an Ethernet port, instead of direct communication at the screw terminals or with a loop sensor, we'll have another option for how to connect."
Above and Beyond: The Future Apps
When information becomes available on IP, we can start imagining it being accessed by business applications and even via the Internet from around the world. So do the vendors. "I sit on the Internet of Things board with Cisco," says Emerson's Karschnia. "In our industry's situation, the ability to translate from the existing installed base to Internet protocol, which HART-IP does, is critical. It allows it to go across the globe."
Systems to do this are already available. "Our product allows configuration," says ProComSol's Dobos. "I assign an IP address to a WirelessHART gateway, and anyone in the world can access device information on that gateway's wireless network by downloading it. Just having that connection — that ability to access information — is very exciting."
Surveys of Control readers in June 2013 and July 2014 show that HART remains by far the leading digital protocol in percentages of facilities and of devices (Figure 1). Almost two-thirds of respondents have integrated HART for more than isolated configuration and calibration (Figure 2), and the percentage of devices that are integrated has risen to 55% in 2014 from 43% in 2013 (Figure 3). The degree of integration has also risen in the past year, with 41% now using real-time connectivity for alerts and other automated actions (Figure 4), and 12% extending integration to business and enterprise systems. Click here to see the figures.
Regarding security, Karschnia says we can rely on industry best practices to be secure. "On wireless, data is encrypted from the devices to the gateways then re-encrypted via SSH for transmission. SSH is an Internet banking system. We just use what's available today," he says. "That's the way of the future, leveraging Internet best practices."
This was the intent with HART-IP from the beginning. By design, HART-IP does not specify what security to use but rather requires use of industry best practices.
There's work to do, but the potential for HART to be the default protocol for bringing field device information all the way to business systems is solid. "Man, does it simplify how to get information into a system. EAM, ERP — IBM is trying to make a protocol to go from devices to business systems. HART-IP looks ideal for doing that," says Karschnia. "Three elements make this approach much more stable over time: The Ethernet physical medium is very stable; TCP/IP messaging is very stable; and the HART protocol is also a stable piece. Together they will meet the needs of anything we can think of for the foreseeable future."