Take Advantage of Fieldbus

Digital networks offer both technical and economic benefits.

By Craig McIntyre, Endress+Hauser

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Most engineers undoubtedly have heard the terms "fieldbus" and "digital industrial networks" — but the reasons for using this technology in chemical plants and processes may not be clear. So, here, I'll briefly explain the basics of fieldbus and how it can improve performance, increase uptime and cut costs. I'll also mention potential pitfalls and best practices.

Fieldbus actually is a generic term for a number of digital industrial networks including, but not limited to, Foundation Fieldbus, HART, EtherNet/IP, Modbus TCP and Profibus. In this article, the term fieldbus refers to all these networks.

Fieldbuses are used to link field devices to control, monitoring and enterprise systems (Figure 1), which collectively I'll call the control system.

Field devices most typically are flow (Figure 2), level, temperature and pressure transmitters. Other common field devices are analyzers and valve actuators. Applications range from process control to environmental monitoring to supply chain management.

In the past field devices primarily were linked to control systems via 4–20-ma analog hard wiring. This hard-wired solution did a good job of transmitting process variable information from the field device to the control system. However, capabilities were limited — the only information that could be transmitted from the transmitter to the control system was a one-way 4–20-ma signal proportional to the measured process variable.

Enter HART, perhaps the first digital fieldbus in widespread use at process plants. HART has the advantage of being able to use existing 4–20-ma wiring as the carrier for its digital signal.

This digital signal establishes a two-way link between transmitters and the control system. This two-way link allows data exchange including instrument identification along with monitoring of multiple process variables, calibration parameters and field device health.

Although HART was a big step up from a one-way analog connection, its bandwidth is limited. So, dedicated digital fieldbus networks such as Profibus PA and Foundation Fieldbus H1 emerged, offering much higher speeds and additional capabilities.

Unlike HART, these new fieldbus systems require dedicated networks — necessitating new types of wiring practices, networking components and support tools. Ethernet-based fieldbuses such as Profisafe, HSE and Ethernet/IP then emerged — bringing even higher communication speeds. Wireless versions of these protocols such as WirelessHART were a logical next step to increase capabilities and reduce installation costs and complexity.

Today, different fieldbuses can be integrated together when needed to establish connectivity between field devices and the control system.

The Benefits Of Fieldbus
Chemical plants, first and foremost, employ fieldbus to increase productivity through precise measurements of operating conditions. Two-way high-bandwidth digital communications with each field device provide the type of information needed by the control system to improve productivity and reliability.

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