Industrial control has evolved significantly over the years, transitioning from localized and centralized control systems to modern distributed control systems (DCS). In the past two decades, the process industry has heavily invested in DCS infrastructure and related instrumentation, with the application of DCS proving highly effective. A key feature of DCS is the 4-20 mA signal, which remains a fundamental part of communication between DCS systems and field devices. This analog standard marked a major milestone in control system development.
However, as digitalization and networking have become central to modern control systems, the limitations of traditional analog signals are becoming more apparent. While 4-20 mA provides basic measurement and control data, it lacks the bandwidth to carry additional smart information. Smart transmitters, although capable of embedding some data into the signal, are constrained by the low communication speed of this analog format. As a result, the move toward digitalization and networked control systems is not just a trend—it's an essential evolution.
Fieldbus technology addresses these limitations by offering a fully digital, two-way, multi-node communication link between intelligent field devices and automation systems. It enables greater openness, flexibility, and integration within process control. The introduction of fieldbus has allowed for more powerful field control, improved diagnostics, and a more comprehensive control network that includes measurement, control, execution, and process analysis.
One of the main impacts of fieldbus on traditional DCS lies in its superior technical features. According to IEC and Fieldbus Foundation standards, fieldbus technology offers five key characteristics: it completely replaces the 4-20 mA analog signal with a digital one, allows for fully distributed control functions at the field level, adds non-control information such as diagnostics and configuration data, unifies site management and control, and supports system openness and interoperability.
Beyond being a communication protocol, fieldbus integrates technologies like intelligent instrumentation, computer networks, and open system interconnection (OSI). These features make fieldbus-based control systems (FCS) more efficient, cost-effective, and reliable than traditional DCS systems. FCS simplifies system architecture, reduces costs, enhances on-site autonomy, improves signal accuracy, and ensures full distribution and digitalization. Users also gain greater control over system integration.
The integration of fieldbus into existing DCS systems is now a growing trend. Many users appreciate the benefits of fieldbus but are reluctant to overhaul their current infrastructure. Instead, they prefer gradual integration. One common approach is to connect fieldbus devices to the DCS I/O bus through a fieldbus interface card. This allows the DCS controller to treat fieldbus devices as if they were traditional DCS modules, enabling seamless operation without major changes.
For example, Fisher-Rosemount’s DeltaV system uses this approach. Its I/O cards include a fieldbus H1 communication module (31.25 kbit/s), allowing fieldbus instruments to be connected directly. This integration reduces installation and maintenance costs while maintaining compatibility with both H1 and traditional I/O modules. A specific driver maps fieldbus data to the DCS I/O bus, ensuring smooth data exchange.
Another integration method involves connecting fieldbus to the higher-level DCS network layer. This allows operators to monitor and adjust fieldbus devices from the DCS operator station, decentralizing some control functions and reducing the load on the DCS host. This approach minimizes disruption to the existing system and is used in various applications, including Smar’s 302 series fieldbus products.
In cases where DCS and fieldbus systems coexist in the same facility, a gateway can be used to enable communication between them. This setup allows data from the DCS to be displayed on a new operator interface, and multiple H1 buses can be connected using a bridge. The gateway manages control coordination, alarms, and trend data, making the integration of both systems more efficient.
Overall, fieldbus is set to play a significant role in industrial control. While it may replace some DCS functions, especially in continuous control loops, DCS will still be vital in real-time and complex control scenarios. The coexistence of fieldbus and DCS systems offers users more flexibility and better control solutions, paving the way for a more advanced and integrated industrial future.
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