Comparing the Whole Fieldbus Range: How to Pick CANopen / Profibus / EtherCAT / PROFINET

The only first criterion for choosing the protocol of a fieldbus magnetostrictive displacement sensor is the master system: whatever fieldbus master the controller is, that is the protocol the sensor should use. There is no relationship of “more advanced therefore more suitable” among protocols — adding a gateway across systems only adds a fault point, delay and maintenance cost. Once the master is fixed, the remaining judgement dimensions in order are: synchronisation requirement, segment length and topology, diagnostic need, cost. This article compares CANopen, Profibus DP, EtherCAT and PROFINET item by item. The corresponding products are 194, 195, 197 / 198, 199PROFINET.

Four fieldbuses compared on every dimension

Selection advice by application

Mobile hydraulic equipment (construction machinery, agricultural machines, marine, aerial work platforms) → CANopen. Mobile controllers almost all natively have a CAN port; the machine already has a CAN network, so joining cost is lowest; CAN’s electrical-environment tolerance also suits mobile duty. Product and configuration are in Series 194 CANopen explained; profile basics are in CANopen introduction: object dictionary and the division of PDO/SDO; mechanical reinforcement for mobile duty is in Series 13 mobile hydraulics explained.

Existing Siemens S7-300/400 lines → Profibus DP. The master is not changed and the network backbone is not changed, so conversion cost is lowest. New projects generally will not choose it of their own accord, but on existing-plant conversion it is often the most economical scheme; see Series 195 Profibus explained.

High-speed multi-axis synchronisation (servo-hydraulic pressing, multi-cylinder synchronisation) → EtherCAT. Shortest cycle, highest DC synchronisation accuracy. When space is tight or nodes are dense, choose the single-M12 198; see Series 197 EtherCAT explained and Series 198 single M12 explained.

New Siemens-system lines that need to connect to MES → PROFINET. Based on standard Ethernet, convenient to fuse with upper systems; diagnostics can locate to a port; see 199PROFINET explained.

Synchronisation capability: an easily overestimated item

Both DC and IRT can align the sample instants of the slaves, but neither can remove the sensor’s own measuring delay. Magnetostriction must wait for the torsion wave to travel from the magnet ring back to the electronics head; the longer the measuring range, the longer it takes. Typical update rate is about 0.5 ms (shorter ranges) to 5 ms (very long ranges), depending on measuring range.

Two engineering conclusions follow:

• The bus cycle should not be shorter than the sensor update rate, or repeated values are read in succession and the controller will misjudge the position as stalled;
• A multi-axis synchronised system should as far as possible choose sensors of the same measuring range; a range difference brings measuring-delay differences on each axis that DC / IRT cannot recover.

How to match update rate to the control cycle is in How to match response time / refresh rate to the control cycle; overall configuration of a multi-axis system is in Selecting for multi-axis synchronisation: how many axes one bus can carry stably.

Three frequent physical-layer pitfalls

Terminating resistors. CANopen and Profibus must impedance-match at both ends of the segment; middle nodes are not fitted. On the CAN side, with power off, resistance between CAN_H and CAN_L of about 60 Ω means both 120 Ω resistors are in place; on the Profibus side note that the device carrying the terminated plug must be powered, or the match fails. The principle is in Terminating resistors and bus length: why 120 Ω is required.

Baud rate and length. Segment length of CANopen and Profibus is inversely proportional to baud rate — a physical constraint. If site distance is long, lower the rate deliberately; forcing a high rate causes intermittent “sometimes fine, sometimes not” faults that are harder to find than a complete failure.

Topology form. CANopen and Profibus run linear with short drops; EtherCAT is strung in IN/OUT order — reversing it causes identification order to be wrong; under IRT, PROFINET topology is locked, and a wiring change must be accompanied by a configuration change. Systematic investigation of interference and earthing is in Fieldbus interference: earthing, shielding, topology; investigation of lost data is in Lost fieldbus data: terminating resistor / address clash / water ingress.

Which other interfaces are available

Besides the four fieldbuses, the interfaces also include analogue, SSI (see Series 192 SSI explained) and Start-Stop. How Modbus and IO-Link read a value is in Modbus and IO-Link. Pros and cons of each output type: output signals compared. If the controller is not a fieldbus master, or the system has only one or two axes, fieldbus is not mandatory — interface classes: analogue vs digital; wiring: fieldbus practical guide.