Fieldbus replaced analog wiring
One serial bus serves many devices. PROFIBUS DP and Modbus RTU remain in widespread use.
9.2 Fieldbus Systems
The previous chapter introduced OT networks and the protocols that run on them today. Those protocols did not appear from nothing. They evolved from serial fieldbus systems that replaced analog wiring in the 1980s. Understanding fieldbus explains why Industrial Ethernet protocols work the way they do and why billions of dollars of fieldbus equipment remain in production.
Why Fieldbus Exists
Section titled “Why Fieldbus Exists”Before fieldbus, every sensor and actuator required its own dedicated wiring back to the controller. A machine with 500 I/O points needed 500 pairs of wires. Fieldbus replaced this with a single serial bus carrying data for many devices, reducing cabling cost and installation time dramatically.
PROFIBUS
Section titled “PROFIBUS”PROFIBUS (Process Field Bus) is the most widely deployed fieldbus globally. Siemens developed it. The IEC standardizes it as IEC 61158 Type 3. PROFIBUS DP (Decentralized Periphery) is the most common variant, providing fast cyclic I/O exchange between a master (PLC) and slaves (I/O modules, drives, sensors).
| Parameter | Value |
|---|---|
| Physical layer | RS-485 (2-wire, differential) |
| Max speed | 12 Mbps |
| Max distance at 12 Mbps | 100 m |
| Max distance at 187.5 kbps | 1000 m |
| Max devices per segment | 32 (126 with repeaters) |
| Topology | Bus (linear) |
| Cycle time | 1 to 10 ms |
The master polls each slave in sequence. Slaves cannot initiate communication. This master-slave architecture guarantees deterministic timing but limits flexibility.
PROFIBUS dominated European factory automation for two decades. In North America, a different protocol filled the same role.
Modbus RTU
Section titled “Modbus RTU”Modbus RTU was developed by Modicon (now Schneider Electric) in 1979. It is the oldest and simplest industrial protocol still in widespread use.
| Parameter | Value |
|---|---|
| Physical layer | RS-232 or RS-485 |
| Speed | Up to 115.2 kbps |
| Max devices | 247 (RS-485) |
| Architecture | Master-slave |
Modbus defines four register types:
| Register Type | Address Range | Access |
|---|---|---|
| Coils (discrete outputs) | 00001 to 09999 | Read/Write |
| Discrete Inputs | 10001 to 19999 | Read only |
| Input Registers (16-bit) | 30001 to 39999 | Read only |
| Holding Registers (16-bit) | 40001 to 49999 | Read/Write |
DeviceNet
Section titled “DeviceNet”DeviceNet was developed by Allen-Bradley (Rockwell Automation) in 1994. It is based on CAN (Controller Area Network) and standardized as IEC 62026-3.
| Parameter | Value |
|---|---|
| Physical layer | CAN (ISO 11898) |
| Speed | 125, 250, or 500 kbps |
| Max devices | 64 |
| Power | Bus-powered (24 VDC) |
DeviceNet is common in North American manufacturing with Rockwell/Allen-Bradley PLCs.
All three fieldbus systems share the same fundamental limitations, which drove the transition to Industrial Ethernet.
The Transition to Industrial Ethernet
Section titled “The Transition to Industrial Ethernet”Fieldbus systems have fundamental limitations: low bandwidth (kbps versus Gbps for Ethernet), limited distance without repeaters, proprietary and incompatible standards, and no standard diagnostic tools.
Industrial Ethernet (PROFINET, EtherNet/IP) addresses these limitations. Fieldbus remains in use because of billions of dollars of installed equipment, proven reliability, and intrinsic safety certifications that are difficult to achieve with Ethernet.
Gateways (PROFIBUS-to-PROFINET, Modbus-to-EtherNet/IP) integrate fieldbus devices into Industrial Ethernet networks without replacing the field devices.
Key Takeaways
Section titled “Key Takeaways”Use gateways for integration
Gateways bridge fieldbus and Industrial Ethernet. Replace fieldbus only when there is a clear business case.
What Comes Next
Section titled “What Comes Next”Fieldbus systems operate at the field level. The standards that govern how industrial networks are structured, how substations communicate, and how time is synchronized span multiple levels. The next chapter covers IEC 61850, IEC 62443, and IEEE 1588 PTP, the standards that define industrial network architecture.
References
Section titled “References”- IEC 61158 — Industrial communication networks: Fieldbus specifications
- IEC 62026-3 — Low-voltage switchgear and controlgear: DeviceNet
- Modbus Organization. (2012). Modbus Application Protocol Specification V1.1b3.
- PROFIBUS & PROFINET International. (2022). PROFIBUS Technology and Application.