Foundation Fieldbus

What is Foundation Fieldbus?

Foundation Fieldbus is a fully digital, bidirectional fieldbus for process automation in which sensors, transmitters and actuators share a single two-wire cable for both power and data with a DCS, and in which control functions can even run inside the field devices themselves. It was developed as the digital successor to the analogue 4-20 mA signal and is managed by the FieldComm Group. Foundation Fieldbus has two levels: H1 at the field level and HSE (High Speed Ethernet) as the backbone. You will mainly find it in refineries, oil and gas facilities, chemical plants and power stations, often in hazardous areas, and also in water treatment and the food and beverage industry.

🧠 How did Foundation Fieldbus come about?

The technology grew out of the work of the ISA SP50 committee, which worked on an international fieldbus standard from the 1980s onwards. The Fieldbus Foundation was founded in 1994 to bring the competing initiatives together and released the first H1 specifications in 1996, followed by the HSE specifications in 1999. The protocol is part of the international standard IEC 61158 (H1 as type 1, HSE as type 5) and of IEC 61784-1 as Communication Profile Family CPF 1.

In August 2014 the members of the Fieldbus Foundation and the HART Communication Foundation approved a merger. Since 1 January 2015 the FieldComm Group in Austin, Texas, has managed Foundation Fieldbus as well as HART and WirelessHART. In 2024 the FDT/DTM activities of the FDT Group were added.

🔧 How do H1 and HSE work?

H1 runs at 31.25 kbit/s over a shielded twisted pair according to IEC 61158-2, the same physical layer used by Profibus PA. The signal is Manchester-encoded and travels over the same two wires that power the instruments (9–32 V DC). This multi-drop architecture, with one trunk and several spurs, typically consists of:

• H1 interface card in the control system, often redundant
• Fieldbus power supply (power conditioner) that feeds the bus and decouples the signal from the supply
• Two terminators, each a 100 Ω resistor in series with 1 µF, at both ends of the trunk
• Device couplers or segment protectors that connect the spurs and limit short circuits per spur
• Field devices such as transmitters and valve positioners, each a field device with its own address

HSE runs on standard Ethernet (100 Mbit/s or faster) using TCP/UDP/IP. A linking device connects several H1 segments to the HSE network so the host system can communicate with all of them at once. HSE does not supply power over the cable.

⏱️ How does communication on an H1 segment work?

Every H1 segment has exactly one active Link Active Scheduler (LAS), usually the H1 card in the host. The LAS maintains a fixed schedule, the macrocycle, in which each function block publishes its value at a fixed moment. Other devices with link master capability can take over as backup LAS if the primary one fails.

• Scheduled traffic (publisher/subscriber) - deterministic transfer of process variables, one publisher and several subscribers
• Unscheduled traffic (client/server) - configuration, parameter reads and diagnostics in the free time of the macrocycle
• Report distribution - time-stamped alarms and events

The length of the macrocycle depends on the number of function blocks and links; in practice it is often between a few hundred milliseconds and one second. If you put many fast control loops on one segment, the cycle gets longer.

🎯 What is control in the field?

Foundation Fieldbus is more than a transport protocol: it also defines standardised function blocks. Each device has a resource block (device data), one or more transducer blocks (the link to the physical measurement or output) and function blocks such as AI (analogue input), AO (analogue output), DI, DO and PID.

Because these blocks behave the same in every device, you can run a complete control loop in the field: the AI block in the transmitter publishes the measured value, the PID block in the valve positioner calculates the new position and the AO block drives the valve. This is called control in the field. If the host fails, the loop keeps running, provided a field device with link master capability takes over the LAS role. Many owners still prefer to control in the DCS controller, because that is easier to manage and fits change procedures better.

Each device is described by a Device Description (DD) written in EDDL (IEC 61804). Vendors register devices after testing them with the Interoperability Test Kit (ITK), so that a transmitter from brand A works with a host from brand B. Newer hosts also support FDI packages (IEC 62769), which combine the DD with a graphical user interface.

🔄 How does Foundation Fieldbus compare with other process protocols?

Compared with classic 4-20 mA wiring, with one wire pair per signal, Foundation Fieldbus needs far fewer cables and much less marshalling, and it delivers several process variables plus NE 107-style diagnostics per asset. On the other hand, a segment fault affects several instruments at once, and design and troubleshooting require more specialist knowledge.

🏭 How do you design an H1 segment in practice?

Good segment design determines the reliability of the entire plant. A proven approach:

1. Group by process area and criticality - put control loops that form one unit on the same segment and spread critical loops across several segments.
2. Calculate power and voltage drop - add up the current draw (typically 15–26 mA per device) and check that the furthest device still gets at least 9 V over type A cable (about 0.8 mm²).
3. Respect the lengths - a maximum of 1,900 m for the trunk and all spurs combined; spurs up to 120 m, shorter as more devices are added to the segment. With up to four repeaters you can reach about 9,500 m.
4. Choose the Ex concept - in hazardous areas you use intrinsic safety according to FISCO or the Entity concept (IEC 60079-11 and -25), or a high-power trunk with Ex barriers per spur. FISCO limits spurs to 60 m and a segment to a handful of devices.
5. Leave spare capacity - design with 12 to 16 devices in practice rather than the theoretical maximum of 32, so you can expand later.
6. Plan for redundancy - redundant H1 cards and power conditioners prevent a single defect from taking down a whole segment.

In the Netherlands and the wider EU, such installations fall under the ATEX requirements and the IEC 60079 series. Chemical and petrochemical companies covered by the Seveso rules must also include the fieldbus in their cybersecurity; see Seveso and Cybersecurity.

🔐 How do you secure Foundation Fieldbus?

Foundation Fieldbus was designed for availability and determinism, not for security. H1 and HSE have no authentication and no encryption: anyone with access to the segment, the HSE network or an engineering tool can read and change parameters. HSE uses ports registered with IANA (1089 annunciation, 1090 Fieldbus Message Specification, 1091 system management and 3622 LAN redundancy), which you can explicitly allow or block in firewall rules.

Practical measures:

• Segmentation - place HSE and H1 hosts in their own zone according to the zones and conduits model of IEC 62443, with no direct route to office networks
• Restrict engineering access - only authorised engineering stations may configure devices; disable write access on devices where possible
• Monitor the network - use OT monitoring at the HSE level and compare configurations against a baseline periodically
• Protect the data path - deliver data to SCADA, the Historian and asset management or condition monitoring through a controlled interface such as OPC UA, or via a second channel according to NAMUR Open Architecture

For Dutch organisations covered by the Cybersecurity Act, which entered into force on 15 August 2026, these measures are part of the duty of care for network and information systems, including the OT layer and its connections to IT. Overall cybersecurity for the plant therefore has to reach down to the fieldbus.

🧱 What is the future of Foundation Fieldbus?

New projects increasingly choose Ethernet-APL, which uses the same type A cable and trunk-and-spur topology but brings 10 Mbit/s and IP all the way to the field. The FieldComm Group is therefore working on a migration path: report TR10365 (2024) describes an APL field switch with dual-mode ports, on which existing H1 devices and new APL devices work side by side.

There is a caveat: APL devices draw more power, so in a worked example the maximum trunk length drops from about 1,500 m to about 450 m. Existing H1 installations will therefore remain in service for years. Migration fits best within lifecycle management and a brownfield strategy.

❓ Frequently asked questions

What is the difference between Foundation Fieldbus H1 and HSE?

Foundation Fieldbus H1 is the instrument-level fieldbus at 31.25 kbit/s that combines power and data on two wires. HSE is the Ethernet backbone (100 Mbit/s or faster) that connects several H1 segments to the host system through linking devices. HSE does not power field devices.

How many devices can you connect to one Foundation Fieldbus segment?

A Foundation Fieldbus H1 segment supports 32 devices in theory, but power and voltage drop limit that in practice. Many designs assume 12 to 16 devices per segment. With intrinsic safety according to FISCO, it is often only four to five.

Is Foundation Fieldbus the same as Profibus PA?

No, but Foundation Fieldbus H1 and Profibus PA share the same IEC 61158-2 physical layer at 31.25 kbit/s. The protocol layers above it differ, and only Foundation Fieldbus supports control in the field with standardised function blocks. The two protocols therefore cannot be mixed on one segment.

What does the Link Active Scheduler do?

The Link Active Scheduler (LAS) is the device that controls traffic on a Foundation Fieldbus H1 segment according to a fixed macrocycle. Each segment has exactly one active LAS, optionally with a backup link master. Without a LAS there is no scheduled communication.

Is Foundation Fieldbus secure against cyberattacks?

Foundation Fieldbus has no built-in authentication or encryption, so its security has to come from the environment around it. Zoning according to IEC 62443, restricted engineering access and monitoring of the HSE network are the most important measures. Physical access control for cabinets and junction boxes is part of that too.

Is Foundation Fieldbus being replaced by Ethernet-APL?

Ethernet-APL is gaining ground in new builds, but Foundation Fieldbus has a large installed base that will remain in service for years. The FieldComm Group is developing a migration concept in which APL field switches also serve H1 devices. This lets you move step by step on your existing cabling.

📌 In summary

Foundation Fieldbus is a fully digital fieldbus that carries power, process data, diagnostics and even control functions over a single two-wire cable between field devices and the DCS.

H1 connects instruments at 31.25 kbit/s over up to 1,900 m per segment, and HSE links the segments to the host via Ethernet. Function blocks, the Link Active Scheduler and standardised device descriptions deliver deterministic, vendor-independent communication. Because the protocol has no security of its own, you protect it with zones, restricted engineering access and monitoring. Looking ahead, there is a migration path to Ethernet-APL on the same cable.