Take advantage of communication opportunities provided by HART signals: consistent information flow increases availability

Every manufacturer and user active in the area of process engineering is familiar with the HART protocol but experience has shown that the procedure is commonly known but rarely used.

Every manufacturer and user active in the area of process engineering is familiar with the HART protocol. This technology modulates an analog 4-20 mA signal to a digital signal using FSK (frequency shift keying) for the purpose of performing configuration work or collecting diagnostics data. This is possible not only during commissioning but also within the scope of maintenance and operation. A precondition is that all devices involved are HART-capable. Experience has shown that the procedure is commonly known but rarely used.

Statistics from the HART Communication Foundation (HCF) show that around 64.9 million devices were installed throughout the world in 2008. Some 58% of these devices are equipped with communication capabilities such as HART or a fieldbus interface. The great majority (some 30 million devices) are HART-capable. Although users want to know more about devices in terms of a secure, efficient and sustainable process, HART capabilities are used for only about 10% (approximately 3 million) of the devices. The majority of users thus not only forgo the option of simplifying commissioning and maintenance but also miss out on the ability to gather important diagnostic data. One reason for this is that many planners were initially only interested in the device’s 4-20 mA signal. Its HART capabilities, while not being considered a bother, were not included in any list of requirements. HART-capable I/O cards were rarely installed.

However, the communication capabilities of devices are playing an ever greater role. While increasingly networked production environments provide many advantages, they also require a consistent information flow. In other words, what is the use of a smart device if the relevant data is not available? In this context, HART communication represents an interesting option.

Bidirectional communication between a PC and up to 32 field devices

When it comes to configuring and maintaining individual devices, users can perform the necessary tasks with a HART modem or a HART handheld. The disadvantage of this approach is that users are only able to access HART-capable devices and, when using a handheld, only temporarily. While these restrictions are tolerable for configuration, the procedure is not viable for the ongoing maintenance and monitoring of multiple important field devices. If the HART signal is to be put to more comprehensive use at a later time, normal I/O cards need to be replaced with HART-capable cards to enable communication with the higher-level asset management system. Replacing a number of cards is, however, highly expensive and has repercussions for the existing equipment and its function.

An alternative solution is HART multiplexing. A HART multiplexer organises bidirectional data exchange between a maximum of 32 field devices and a separate PC or controller. Users need to wire the 4-20 mA signal to the controller and also implement a link to a transfer board (Figure 1). This does not impact the 4-20 mA signal in any way due to capacitive decoupling of the information. The HART multiplexer takes up the decoupled HART signal and relays it via an RS485 interface. This enables consecutive communication with the devices via the HART multiplexer. If the connection volume is not sufficient, the transfer board can be extended by a second board.

Figure 1: The HART multiplexer receives capacitively decoupled digital information from the HART transfer board.

Simple integration into the ethernet infrastructure

The available communication configurations between the multiplexer and PC depend on the application scenario.

• Implementation using an RS485-to-RS232 interface converter
• Data exchange with a device server via ethernet and TCP/IP

The ethernet-based option is best explained using an example. The temperature is measured using a temperature head transducer. Communication was previously restricted to 4-20 mA signals; HART signals were not used. To improve plant monitoring and simplify the configuration of temperature ranges, the operator now wants to be able to communicate with the field devices. A number of the head transducers installed are HART-capable. The sensors are located in an explosion hazard zone. For this reason, the operator installed an intrinsically safe head transducer and an additional Ex-i repeater power supply to separate the intrinsically safe and non-intrinsically safe circuit. This is not a safety function in the sense of functional safety in accordance with IEC 61508. Nevertheless, the operator wishes to improve the availability of the unit and its monitoring by using the HART information. Moreover, the data is to be transferred via TCP/IP. The effort involved is to be kept to a minimum.

Figure 2: The individual components of a solution based on transmitting HART information via TCP/IP.

To this end, the operator requires HART-capable devices and HART-transparent Ex-i isolation amplifiers that do not degrade the signal through filtering. Clips for transmitting the 4-20 mA signal to the control and the transfer board may be required as well, in addition to the transfer board for uncoupling the signals, the HART multiplexer as well as a device server for communication via ethernet. A PC with a suitable program is also required (Figure 2). The control cabinet can accommodate a HART multiplexer with two transfer boards and a device server such as a Phoenix Contact COM Server. The additional devices, with a total width of 22 centimetres, enable the connection of 32 signals. The HART-transparent isolation amplifiers do not need to be installed next to each other. The signals can instead be collected on the HART transfer board in the control cabinet and transferred to the multiplexer via ribbon cables. If the information is then to be communicated via ethernet, the device server converts the signal from RS485 to ethernet and relays the data to the PC via the existing network structure using the TCP/IP protocol suite (Figure 3).

Figure 3: Integration of the HART multiplexer in the existing installation.

Comprehensive monitoring and flexible parameterising of the field devices

After the HART multiplexer has been connected to the PC via ethernet by means of the device server, the operator can call up the device server web interface for system configuration. To do so, the operator enters the IP address printed on the side of the device into the browser’s address list. In the case of the Phoenix Contact COM server, this enables communication with multiple multiplexers. The operator will also require a HART OPC server. Once all connected multiplexers and the field devices attached to them have been read-in, they are displayed in the software. Operators can now select devices and choose the ‘Properties’ menu option to view all constituent data, such as the present process value, the date as well as the upper and lower measuring range limits or to enter a description. Further information on the status of the field device can be displayed if required. The HART OPC server supports all basic and general HART commands. In addition, the manual entry of device-specific HART instructions for parameterising is also possible if the manufacturer supports this function. The operator can finally save the HART information using suitable commercial software.

The HART multiplexer enables improved use of the HART signals for optimising both new and existing systems. By exchanging data with the HART devices in the field based on a multiplexer, operators gain more complete information on the process and are able to visualise the data, for instance via TCP/IP, at any desired location.