Fieldbuses and Device Networks
PROFIBUS, CAN, CANopen, DeviceNet, AS-Interface, IO-Link, HART and CC-Link: how each works and where it fits.
What a fieldbus is
A fieldbus replaces a bundle of individual wires with one cable that serves many field devices. The controller (the master) exchanges small, frequent messages of process data with each device (the slaves): outputs go out, inputs come back, many times a second. Fieldbuses were designed in the 1980s and 1990s for exactly that job, before Ethernet was rugged and fast enough, and they remain widespread in existing plants. New installations tend to choose Ethernet-based networks (see Industrial Ethernet: EtherNet/IP, PROFINET and EtherCAT), but you will meet fieldbuses for decades.
| Network | Physical layer | Typical speed | Sweet spot | Strong in |
|---|---|---|---|---|
| PROFIBUS DP | RS-485, 150 Ω shielded pair | 9.6 kbit/s to 12 Mbit/s | Remote I/O, drives, process instruments (PA) | Siemens and process plants worldwide |
| DeviceNet | CAN, trunk and drop cable with 24 V power | 125, 250, 500 kbit/s | Sensors, valves, drives on a machine | North America, Rockwell |
| CANopen | CAN | up to 1 Mbit/s | Machine control, mobile equipment, drives | Europe, motion, medical, vehicles |
| AS-Interface | Two-wire flat cable with data and power | cycle 5–10 ms | Binary sensors and actuators | Material handling, packaging |
| IO-Link | Point-to-point 3-wire sensor cable | 4.8–230.4 kbaud | Smart sensors and valves | Any machine, any brand of PLC |
| HART | Digital signal on a 4–20 mA loop | 1 200 bit/s | Setup and diagnostics of instruments | Process industries |
| CC-Link | RS-485 (10 Mbit/s), CC-Link IE on Ethernet | 156 kbit/s to 10 Mbit/s | Mitsubishi systems | Asia |
PROFIBUS DP
PROFIBUS DP (decentralized periphery) is a master/slave network on RS-485. A class 1 master (the PLC) polls each slave in a fixed cycle, sending outputs and receiving inputs in one exchange; a class 2 master (an engineering tool) may read and set parameters. Each slave has an address from 0 to 125, and the process data of each slave is described in a GSD file that the engineering software reads.
| Item | Value |
|---|---|
| Cable | Violet, shielded twisted pair, 150 Ω (type A); connectors with a termination switch and an optional programming port |
| Speeds | 9.6, 19.2, 45.45, 93.75, 187.5, 500 kbit/s; 1.5, 3, 6, 12 Mbit/s |
| Stations | 32 per segment (including repeaters), up to 126 addresses with repeaters |
| Termination | 390 Ω to +5 V, 220 Ω across A and B, 390 Ω to ground: provided in the bus connector, powered from the device, so the terminated device must stay powered |
| Process data | Up to 244 bytes of input and 244 bytes of output per slave |
| PROFIBUS PA | The same protocol on a two-wire intrinsically safe medium (31.25 kbit/s) for process instruments in hazardous areas, joined to DP through a segment coupler |
Worked example: roughly how long is a PROFIBUS cycle?
- Ten slaves, each exchanging 8 bytes of output and 8 bytes of input. A telegram is 9 bytes of overhead plus the data, each byte is 11 bits (start, 8 data, parity, stop), and the master waits about 100 bit times of idle time before and after each exchange.
- Per slave: ((9 + 16 + 9) × 11 + 200) bit times = 574 bit times. At 1.5 Mbit/s that is 0.38 ms, so the ten slaves take about 3.8 ms.
- At 12 Mbit/s the same cycle is about 0.48 ms; at 93.75 kbit/s it is about 61 ms. The estimate ignores acyclic traffic and token passing, so a real cycle is longer; the configuration software shows the exact bus cycle.
- The lesson: baud rate buys cycle time, but cable length falls as the baud rate rises (1 200 m at 93.75 kbit/s and below, 100 m at 12 Mbit/s).
CAN, CANopen and DeviceNet
CAN (controller area network) was developed for cars and is now used in machines, mobile equipment, medical devices and drives. It is a multi-master bus: any node may start sending when the bus is idle, and when two start together the one with the lower identifier wins without losing a bit (arbitration). The identifier says what the message is, not who it is for; any node may listen.
| Layer | Defines | Examples |
|---|---|---|
| CAN (ISO 11898) | The frame, arbitration, error handling and the electrical layer | The same silicon in every device |
| CANopen (CiA 301) | An object dictionary, process data objects (PDO), service data objects (SDO), network management, heartbeat | Drives (CiA 402), I/O modules (CiA 401), encoders |
| DeviceNet (ODVA) | CIP over CAN with a trunk and drop cable that includes 24 V power | Sensors, valve manifolds, starters; Rockwell and others |
| J1939, NMEA 2000 | Vehicles, engines, marine | Mobile machines |
Worked example: how loaded is a CAN bus?
- A standard CAN frame with 8 data bytes is about 111 bits (44 bits of overhead, 64 bits of data, 3 bits of spacing); bit stuffing can add up to about 24 more.
- At 500 kbit/s a bit lasts 2 µs, so one frame takes about 222 µs; at 125 kbit/s it takes 888 µs.
- Twenty nodes each sending one full frame every 10 ms: 20 × 111 bits ÷ 0.01 s = 222 000 bit/s, a load of 44 percent at 500 kbit/s and 89 percent at 250 kbit/s.
- Keep CAN loads under about 30 to 50 percent so that a high-priority message does not wait behind a long queue, and so that retransmission after errors has room.
- Wiring. Two twisted pairs for DeviceNet (signal and power), one for CANopen; 120 Ω terminators at both ends of the trunk; drops short (6 m at most for DeviceNet).
- Node numbers and speed are set by switches, parameters or both; every node on the bus must use the same speed.
- Bus-off. A node that sees too many errors disconnects itself; if one device keeps dropping off, suspect its wiring or its termination first.
- DeviceNet power is carried on the cable: voltage drop on a long trunk with many devices is a common fault. Measure at the far end under full load.
AS-Interface
AS-Interface is a simple network for binary sensors and actuators. A flat two-wire cable carries both data and 30 V power; slaves attach with piercing connectors that bite through the insulation, so a sensor can be added anywhere along the cable without cutting it. The master polls all slaves in a fixed cycle (5 ms for 31 slaves, 10 ms for 62 with A/B addressing). Each slave exchanges four bits of input and four of output; analog and more complex devices use special profiles.
| Item | Value |
|---|---|
| Cable | Yellow flat cable: data and 30 V. A black flat cable carries 24 V auxiliary power for actuators. |
| Length | 100 m, extendable to 300 m with repeaters |
| Slaves | 31 (standard), 62 (extended addressing) |
| Addressing | Addresses are written to the slave with a handheld addressing unit or by the master; a new slave of an existing address can be replaced and set up automatically |
| Safe version | ASi Safe carries safety signals (such as e-stops) on the same cable, to the safety category of the devices |
IO-Link
IO-Link (IEC 61131-9) is a point-to-point link between an IO-Link master port and one smart sensor or actuator over a standard unshielded three-wire cable of up to 20 m. It is not a bus: the master has several ports, and the master connects to the PLC over a fieldbus or Ethernet network. What it adds is data about the device: parameters can be read and set from the PLC, diagnostics are reported, and when a device is replaced, the master downloads the stored parameters to the new one automatically.
| Mode | Speed | Typical minimum cycle |
|---|---|---|
| COM1 | 4.8 kbaud | 18 ms |
| COM2 | 38.4 kbaud | 2.3 ms |
| COM3 | 230.4 kbaud | 0.4 ms |
- Backward compatible. The C/Q wire can work as a plain switching input or output (SIO mode), so an IO-Link port can run an ordinary sensor.
- Process data (up to 32 bytes per device) is sent cyclically; events and on-request data carry diagnostics and parameters.
- Class A and B ports. Class B ports provide an additional power supply for actuators.
- IODD file. Each device ships with a device description file that the master or engineering tool reads.
- Use where it pays: adjustable sensors (set the switching point from the HMI), valves and RFID heads, and anywhere a device is hard to reach.
HART
HART (Highway Addressable Remote Transducer) is a digital signal laid on top of the ordinary 4–20 mA analog loop. The loop current still carries the measured value; the digital signal, a small frequency-shift-keyed tone of ±0.5 mA at 1 200 or 2 200 Hz, carries configuration, diagnostics and extra variables at 1 200 bit/s. Because its average is zero, it does not disturb the analog reading. A handheld communicator or a HART modem connected across the loop (the loop needs at least about 230 Ω of resistance) can read and set the device.
- A HART message takes time. A 25-byte exchange at 1 200 bit/s (11 bits per byte) takes about 229 ms, so HART suits set-up and slow data (about two exchanges per second), not control.
- Point to point or multidrop. In multidrop mode up to 15 devices share one loop with the current fixed at 4 mA; the analog value is then not used.
- I/O support. HART-capable analog input cards pass the digital data to the control system; ordinary cards ignore it or have a filter that blocks it. Check before you rely on it.
- Common uses. Re-ranging a transmitter without a trip to the field, reading device diagnostics, and keeping a record of calibrations.
Other device networks and serial protocols
| Name | What it is | Where you meet it |
|---|---|---|
| CC-Link / CC-Link IE | Mitsubishi’s open network: CC-Link on RS-485, CC-Link IE on gigabit Ethernet | Mitsubishi PLCs and many Asian drives and sensors |
| DF1 | Allen-Bradley serial protocol, full or half duplex | SLC, MicroLogix and older ControlLogix serial ports; legacy HMIs |
| DH-485, DH+, ControlNet | Older Rockwell networks | Legacy plants |
| Host Link, FINS (serial) | Omron command protocols | Omron CP1, CJ series; HMIs |
| Computer link, MC protocol | Mitsubishi serial and Ethernet commands | Mitsubishi PLCs with HMIs and PCs |
| BACnet MS/TP, LonWorks | Building automation on RS-485 or twisted pair | HVAC controls; gateways to PLCs |
| M-Bus, DALI, KNX | Metering, lighting and building control | Energy and building projects |
| POWERLINK, Sercos, MECHATROLINK | Real-time Ethernet motion networks | Motion control; see Industrial Ethernet: EtherNet/IP, PROFINET and EtherCAT |
| Wireless: WirelessHART, ISA100, Zigbee, Wi-Fi, LoRaWAN, 5G | Cable-free links | Instrumentation retrofits, mobile equipment, remote sites |
Distance, speed and node limits
All of these networks trade distance against speed, for the same reason as RS-485: the cable’s capacitance rounds the edges, so faster bits need shorter cable. Longer distances need repeaters (which add delay and count as stations) or slower speeds. The figures are typical; the cable manufacturer’s data and the network specification are the final authority.
Choosing and migrating
| Situation | Suggestion | Why |
|---|---|---|
| New machine, Siemens controller | PROFINET | Native; supports the whole device range; same cable as Ethernet |
| New machine, Rockwell controller | EtherNet/IP | Native |
| Extending an existing PROFIBUS line | Keep PROFIBUS; add slaves | A working bus rarely pays to replace |
| Old PROFIBUS or DeviceNet system reaching the end of life | Gateway first (PROFIBUS to PROFINET, DeviceNet to EtherNet/IP), then replace section by section | Spreads cost and risk |
| Many binary sensors in one conveyor | AS-Interface or remote I/O blocks | Cheapest per point |
| Many adjustable or smart sensors | IO-Link | Remote set-up and automatic replacement |
| Mobile machines and vehicles | CANopen or J1939 | Rugged, low cost, event-driven |
| Process instruments, hazardous areas | HART, PROFIBUS PA, Foundation Fieldbus, or Ethernet-APL | Intrinsic safety, existing instrument base |
Addresses, files and configuration
| Network | Address set by | Configuration file | Notes |
|---|---|---|---|
| PROFIBUS DP | Switches, a software tool or a bus connector | GSD (device data file) | Duplicate addresses stop the bus; the master must have the correct GSD and module list |
| CANopen | Switches or parameter (node ID 1 to 127) | EDS (electronic data sheet) | Node ID and bit rate often need a power cycle or a “save” command to take effect |
| DeviceNet | Switches (MAC ID 0 to 63) and a rate setting | EDS | Duplicate MAC IDs show as a “duplicate node” fault |
| AS-Interface | Addressing unit or the master | Profile of the slave | A new slave takes the address of the replaced one automatically if the master supports it |
| IO-Link | Port number on the master | IODD | Parameters are stored in the master for automatic device replacement |
Typical faults
| Symptom | Likely cause | Check |
|---|---|---|
| One PROFIBUS slave shows a diagnostic or drops out | Wrong address or GSD, loose connector, no power at a terminated station | Station list in the master, the connector, the segment termination |
| All slaves fail beyond a point | Cable break or short, a missing terminator, a segment over its length limit | Measure A to B and shield with the segment disconnected; use a bus tester |
| PROFIBUS works for a while then faults | Poor shielding or grounding, noise from a drive, loose connector | Check the shield bonding and the cable routing |
| CAN bus-off or high error counters | Wrong bit rate on one node, no termination, two terminators too many, long drop | Bit rates, 60 Ω across CAN-H and CAN-L (power off), drop lengths |
| DeviceNet nodes drop when many outputs switch | Voltage drop on the trunk | Measure 24 V at the far end under load |
| AS-Interface slave not found | Address 0 (new, unaddressed) or a duplicate, damaged cable, overloaded power supply | The master’s slave list and the AS-i power supply |
| IO-Link device not recognized | Wrong cable or port class, a device with a different IODD or revision | Port status on the master and the identification data |
| HART does not appear | Loop resistance below 230 Ω, an analog card that filters HART, a loop with no power | Add a 250 Ω resistor and a handheld across the loop |
Related
- Industrial Communication: the overview of this guide.
- Serial Communication: RS-232, RS-422 and RS-485: RS-485, the electrical layer of PROFIBUS and many serial networks.
- Industrial Ethernet: EtherNet/IP, PROFINET and EtherCAT: where new machines are going.
- Troubleshooting Industrial Networks: a layered method for network faults.
- Sensors and Motion Control.
- Analog Scaling Tool for the 4–20 mA signal that HART rides on.
FAQ
Are fieldbuses obsolete?
Not in the field: millions of PROFIBUS, CANopen, DeviceNet and AS-Interface nodes are in service and still sold. New designs lean toward Ethernet, but a working fieldbus is usually left alone and extended, with gateways when the controller changes.
What is the difference between a fieldbus and Industrial Ethernet?
Fieldbuses use their own cables, connectors and speeds. Industrial Ethernet uses standard Ethernet hardware with real-time protocol extensions, so it is faster, uses cheap cabling and tools, and connects directly to IT systems.
Can I mix CANopen and DeviceNet on one bus?
No. They use the same physical layer but different protocols and bit-rate settings. Use separate buses or a gateway.
Does IO-Link replace a fieldbus?
No. IO-Link connects one device to an IO-Link master. The master then connects to the PLC through a fieldbus or Ethernet, as a remote I/O block would.
Why does my PROFIBUS network need a powered terminator?
The termination resistors in the bus connector are powered from the device. If the end device is switched off, the termination is lost and the bus can become unreliable. Some active terminators have their own supply.
References
- IEC 61158 and IEC 61784, Industrial communication networks: Fieldbus specifications; Profile sets.
- PROFIBUS & PROFINET International, PROFIBUS Installation Guideline for Cabling and Assembly, and PROFIBUS DP Specification (profibus.com).
- ISO 11898-1 and -2, Road vehicles: Controller area network (CAN); CAN in Automation, CiA 301 CANopen application layer.
- ODVA, DeviceNet Specification and CIP Networks Library (odva.org).
- IEC 61131-9, Single-drop digital communication interface for small sensors and actuators (SDCI); IO-Link Community specification.
- AS-International Association, AS-Interface specification; FieldComm Group, HART Communication Protocol.
Last reviewed: October 2026. Speeds, lengths, node counts and menu names are typical: confirm against the specification and the manufacturer’s documentation for the devices you use.