Robot Software
PROFINET, EtherCAT, EtherNet/IP: Robot Fieldbus Compared
Which industrial network belongs in a robot cell, compared on cycle time, jitter, topology and what your plant already speaks. The last criterion usually wins.
Choose what the plant already speaks unless a cycle time below 1 ms forces the issue. EtherCAT reaches cycle times of 50 to 250 microseconds with jitter under a microsecond and dominates inside robot controllers. PROFINET and EtherNet/IP typically run 1 to 8 ms at the cell level, which is ample for handshakes, and they win on plant integration because that is what the PLC and the diagnostics already use.
Almost every cell has two networks
This is the fact that resolves most of the argument. Inside the robot, the controller talks to its own drives over a deterministic bus, very often EtherCAT or a vendor-proprietary equivalent, at high rate. Outside the robot, the controller talks to the plant PLC over whatever the plant uses, at a much lower rate, exchanging start signals, part present flags, program numbers and fault codes.
Those two layers have completely different requirements, and confusing them produces the classic mistake of specifying a microsecond-class bus for an interface that exchanges eight bits every 20 ms.
| Layer | Typical protocol | Cycle | Carries |
|---|---|---|---|
| Drive control, inside the robot | EtherCAT, vendor bus | 50 to 500 us | Position, velocity, torque per axis |
| Cell integration, robot to PLC | PROFINET, EtherNet/IP, EtherCAT | 1 to 8 ms | Handshakes, program select, status |
| Safety, overlaid | PROFIsafe, CIP Safety, FSoE | 4 to 20 ms | Emergency stop, zone status, enable |
| Plant data | OPC UA, MQTT | 100 ms to seconds | Production counts, condition data |
The protocols on their merits
| Protocol | Min cycle | Jitter | Topology | Strength |
|---|---|---|---|---|
| EtherCAT | 50 to 100 us | < 1 us | Line, ring, branch | Fastest, cheap slave hardware, precise sync |
| PROFINET IRT | 250 us to 1 ms | 1 us | Line, star, ring | Determinism plus full plant integration |
| PROFINET RT | 1 to 8 ms | tens of us | Standard ethernet | Ubiquitous in European plants |
| EtherNet/IP | 1 to 10 ms | tens of us | Standard ethernet | Dominant in North America, easy IT integration |
| Powerlink | 100 to 400 us | < 1 us | Line, star | Deterministic, open |
| Sercos III | 31 to 250 us | < 1 us | Line, ring | Motion-focused, strong sync |
| Time-sensitive networking | varies | low | Standard ethernet | Converged traffic on one network |
Safety rides on top
Safety communication is a protocol layer above the fieldbus, not a separate cable. PROFIsafe over PROFINET, CIP Safety over EtherNet/IP and Fail Safe over EtherCAT all add sequence numbers, timeouts, cross-checking and unique addressing so that a corrupted or delayed message is detected rather than acted upon.
Two practical consequences. First, safety cycle times are deliberately slower, typically 4 ms to 20 ms, because the protocol adds redundancy and confirmation. Second, the safety layer and the standard layer usually share the physical network, which means the network design has to account for both, and a safety timeout under load is a machine stop.
Choosing without overthinking it
- What does the plant PLC speak? That answers the cell integration layer in most projects. Fighting it costs gateways, spares and diagnostic blind spots.
- Is there coordinated motion between the robot and external axes? If yes, a deterministic bus with a distributed clock is required, and the robot vendor usually dictates which one.
- How fast is the actual signal exchange? Most cell handshakes need 5 ms to 20 ms. Buying microseconds for that is spending money on nothing.
- Who maintains it at 03:00? Diagnostic tooling and technician familiarity are worth more over a machine's life than a factor of ten in cycle time.
- Is a gateway acceptable? A protocol converter is a legitimate answer, at the cost of one more device, added latency and a second thing to diagnose.
Frequently asked questions
Which fieldbus is best for a robot cell?
For the robot to PLC interface, whichever the plant already uses, since 1 ms to 8 ms is ample for handshakes. Inside the robot controller, a deterministic bus such as EtherCAT is standard, and that choice is usually made by the robot manufacturer.
How fast is EtherCAT compared with PROFINET?
EtherCAT reaches 50 to 250 microsecond cycles with sub-microsecond jitter. PROFINET RT typically runs 1 to 8 ms, and PROFINET IRT reaches 250 microseconds to 1 ms with comparable determinism.
Do I need a separate cable for safety?
No. Safety protocols such as PROFIsafe, CIP Safety and Fail Safe over EtherCAT run on top of the standard fieldbus with added sequence numbers, timeouts and cross-checks. Safety cycles are slower, typically 4 ms to 20 ms.
Why is my cell slow despite a fast network?
Because response time is the sum of the whole chain: PLC scan, robot interface update, network cycle and any gateway. A 1 ms bus inside a 10 ms PLC scan and a 12 ms controller interface produces tens of milliseconds of response.
Is a protocol gateway acceptable?
Yes, and it is common where a robot speaks one protocol and the plant another. The cost is an extra device, additional latency of typically 1 ms to 10 ms, and one more component to diagnose during a fault.
Sources
- IEC 61158, industrial communication networks, fieldbus specificationsInternational Electrotechnical Commission, the standard covering these protocols
- IEC 61784-3, functional safety fieldbusesInternational Electrotechnical Commission, safety communication profiles
- ISO 10218-1:2025, Robotics, Safety requirements, Part 1International Organization for Standardization, requirements for safety-related communication