EtherCAT vs Profinet for Motion Control: Cycle Time and Cost

EtherCAT versus Profinet for Motion Control: Cycle Time and Cost

Key Takeaway

EtherCAT wins on cycle time (tens of microseconds) and fewest switches; PROFINET IRT wins on device availability and engineering ecosystem in India. Choose EtherCAT for sub-millisecond motion, PROFINET for general machine I/O with local support.

EtherCAT versus Profinet for Motion Control ETHERCAT Distributed clocks on every slave On the wire processing Jitter below 1 microsecond Cycle from 250 us up PROFINET IRT with reserved bandwidth Standard GSD engineering Cycle 250 us to 4 ms Widest device availability CHOICE Axis count per segment Redundancy requirement Engineering skill available Motion and plain IO mixed COST Slave hardware price Controller licence model Cable and connector cost Training and spares EtherCAT jitter under 1 microsecond Profinet IRT cycle 250 us to 4 ms Master licence is per channel Same motion works on both, timing differs

Figure 1: EtherCAT versus Profinet – features, choice, cost

1. How the Two Protocols Work

EtherCAT

EtherCAT (Ethernet for Control Automation Technology) is an Ethernet-based fieldbus standardized as IEC 61158 and IEC 61784. It comes from Beckhoff. The master is a single device with two Ethernet ports. A frame leaves port 1, passes through every slave in the line, each slave reads the data addressed to it as the frame passes and writes its own data back into the same frame, and the frame returns through port 2. The entire distributed I/O image is transported in a single frame that physically visits every device.

PROFINET

PROFINET (Process Field Network) is standardized as IEC 61158 and IEC 61784. It comes from Siemens and PNO (PROFIBUS and PROFINET International). It runs on standard Ethernet with three performance classes: RT (Real Time), IRT (Isochronous Real Time) and ERT (Enhanced Real Time). RT uses standard Ethernet switches; IRT requires managed switches with reserved bandwidth; ERT requires PTP-capable switches.

2. Cycle Time and Jitter

EtherCAT cycle time

EtherCAT cycle times are in the tens of microseconds. A 30-axis system with 20 m of cable per segment typically runs at 30 to 100 microseconds. The distributed clock keeps every slave synchronized to within a microsecond.

PROFINET cycle time

PROFINET RT cycle time is typically 1 to 10 ms. PROFINET IRT cycle time is 250 microseconds to 4 ms with reserved bandwidth. PROFINET ERT can reach sub-millisecond with PTP-capable switches throughout the path.

Jitter

EtherCAT distributed clocks give jitter below 1 microsecond. PROFINET IRT jitter depends on switch configuration and is typically higher. For motion control where position loop bandwidth matters, EtherCAT’s lower jitter is an advantage.

3. Topology and Switch Count

EtherCAT topology

EtherCAT is a line topology (daisy chain). One frame visits every slave. Very few switches are needed – often just one or two for a whole machine. This reduces cabinet space, switch cost, and failure points.

PROFINET topology

PROFINET supports line, star and tree topologies with standard Ethernet switches. More switches are needed for star topologies, but the wiring is more flexible and faults are easier to isolate.

4. Device Availability in India

EtherCAT

EtherCAT slaves are available from Beckhoff and many other vendors, but the installed base in India is thinner than PROFINET. Spares and local engineering support may be limited outside major cities.

PROFINET

PROFINET has the widest device availability in India. Siemens has a strong presence, and many Indian machine builders use Siemens PLCs. Training, spares and local engineering are readily available.

5. Choice Criteria

Choose EtherCAT when

  • Cycle time below 1 ms is required
  • High axis count (20+ axes) on a single network
  • Fewest switches and lowest wiring cost is a priority
  • Motion and I/O are on the same network

Choose PROFINET when

  • Cycle time of 1 to 10 ms is sufficient
  • Local engineering support and spares are important
  • Mix of motion and general I/O
  • Star or tree topology is preferred

6. Cost Comparison

Slave hardware

EtherCAT slaves are often slightly cheaper than PROFINET slaves due to simpler silicon. The difference is small per slave but adds up on high-axis-count machines.

Controller licence

PROFINET master licence is typically per channel. EtherCAT master licence varies by vendor; some charge per axis, some per channel.

Switches

EtherCAT needs very few switches. PROFINET IRT needs managed, IRT-aware switches which cost more. Standard PROFINET RT uses standard switches.

Training and spares

PROFINET has more trained engineers and more spare parts available in India, reducing downtime cost.

Sources

  1. EtherCAT Technology Group, EtherCAT Specification
  2. PROFIBUS and PROFINET International, PROFINET Specification
  3. IEC 61158, Industrial communication networks – Fieldbus
  4. IEC 61784, Industrial communication networks – Profile specifications

Key Takeaways

  • EtherCAT cycle time is tens of microseconds with sub-microsecond jitter; PROFINET IRT is 250 microseconds to 4 ms.
  • EtherCAT uses line topology with few switches; PROFINET supports star/tree with standard switches.
  • PROFINET has wider device availability and engineering support in India.
  • Choose EtherCAT for sub-millisecond motion and high axis count; choose PROFINET for general machine I/O with local support.
  • EtherCAT slave hardware is often cheaper; PROFINET master licence is typically per channel.
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