What Is Fieldbus?

Fieldbus is the name for a family of digital networks that link industrial controllers to the devices “in the field”: sensors, transmitters, actuators, motor drives, valve terminals and remote I/O. Instead of running a separate cable pair from every device back to the controller, many devices share one bus cable and exchange data as messages, each tagged with the device’s address.

The term covers many competing protocols rather than one technology. The international standards IEC 61158 and IEC 61784 collect them as communication profile families, so PROFIBUS, Foundation Fieldbus, CANopen and others are all fieldbuses, each with its own wiring, speed and user organization.

Before fieldbus, every measurement travelled on its own pair of wires, usually as a 4–20 mA current loop, and every on/off signal had its own wire to a PLC input card. A large plant needed thousands of cores, terminal blocks and marshalling cabinets, and each wire carried just one value.

A fieldbus carries many values on one cable, plus information that analog wiring cannot: device status, diagnostics, engineering scaling, alarms and configuration. Most fieldbuses use a controller–device arrangement, in which a controller (historically called the master) polls each device in turn, and run on twisted-pair cable in a line or tree, with terminators at the ends of each segment.

The table lists the fieldbuses and industrial Ethernet protocols most often met on USA, European and US sites. Speeds are typical ranges set by each protocol’s specification and cable length.

Serial Modbus is usually counted as a fieldbus because it does the same job on the same RS-485 wiring, and our explainer on how Modbus works covers it in detail. CANopen and DeviceNet both run on CAN bus, which suits machines and vehicles. HART is often mentioned alongside them, but it is a digital signal superimposed on a 4–20 mA loop rather than a full fieldbus.

Since the 2000s, most new factory automation has moved from serial fieldbus to industrial Ethernet protocols. PROFINET, EtherNet/IP and EtherCAT keep the deterministic, cyclic data exchange of a fieldbus but run on standard Ethernet cabling and switches at 100 Mb/s or more, so the same network can carry I/O data, diagnostics and IT traffic. Time-Sensitive Networking extends this further, as our guide to Time-Sensitive Networking explains.

Classic fieldbus is far from gone. Process plants still install PROFIBUS PA and Foundation Fieldbus H1 because they carry power and data on two wires and suit intrinsically safe zones, and huge installed bases of PROFIBUS DP and DeviceNet run for decades. Most sites end up with both, linked by protocol gateways or proxies.

The two names cause the most confusion. PROFIBUS is a serial fieldbus on RS-485 or two-wire MBP cabling. PROFINET is an industrial Ethernet protocol from the same organization, PROFIBUS & PROFINET International. PROFINET is not PROFIBUS sent over Ethernet: it has its own device model, real-time classes and conformance classes, and runs on standard Ethernet hardware. The two connect through a proxy that maps PROFIBUS devices into a PROFINET network, which is how many plants migrate one line at a time.

Industrial PCs connect to fieldbus networks in three common ways: a fieldbus interface card in a PCIe or Mini PCIe slot, an external protocol gateway, or a software controller that uses the PC’s own Ethernet ports for EtherCAT or PROFINET. Fanless PCs with expansion slots, such as the Advantech MIC-770 V3 series, leave room to add a fieldbus card later in a project.

Our CAN bus I/O modules include isolated interfaces with CANopen firmware, and some of our managed industrial switches support PROFINET and EtherNet/IP for industrial Ethernet networks.

Where a computer has to talk to a fieldbus, we fit the matching interface card or gateway and install its drivers as standard, so the system arrives ready to join the plant network.