Magnetostrictive Displacement Sensor Fieldbuses: CANopen vs Profibus for Position Feedback

CANopen and Profibus both look like serial buses, but CANopen runs on a CAN differential pair, not the Profibus RS-485 layer. Serial fieldbuses already include two types: 194 CANopen and 195 Profibus DP. Roles: CANopen is controller and node; Profibus is master and slave. This page compares those two only.

Separate the physical layers first

Terminate CAN with 120 ohm at both ends; on Profibus the end plug must sit on a powered device, not a parked one. Treating CAN as RS-485 when you buy plugs, terminators and repeaters shows up as error frames and intermittent faults. Termination and length: why it has to be 120 Ω.

CANopen (194) — advantages

• Mobile controllers on excavators, implements and access platforms almost always have CAN, so joining the machine network is cheap.
• Several rods share one shielded pair, unlike one analogue pair per axis.
• Position on PDO, parameters on SDO; faults can be raised with EMCY without polling.
• DSP406 already places the position objects, so a type swap need not rewrite the mapping.
• Several magnets on one waveguide can map to several PDOs; analogue cannot.

CANopen — disadvantages

• Bandwidth and cycle determinism are below EtherCAT / PROFINET; it is not a drop-in for industrial Ethernet.
• SYNC is a medium synchronisation tool; strict simultaneous sampling belongs on the Ethernet pair.
• Baud must be identical on the whole network; one wrong node can take the bus down.
• Topology is a line with short drops, not a star. Missing end termination or extra resistors in the middle both reflect.

Object dictionary and baud: 194 explained, PDO/SDO.

Profibus DP (195) — advantages

• Many S7-300 / S7-400 lines still run DP; replacing the rod need not replace the PLC or the trench.
• GSD import, station numbers and diagnostic telegrams are familiar to electricians.
• Polled diagnostics can tell “no response” from a parameter or configuration mismatch — better than a single analogue current.
• The dedicated plug is a feed-through; unplugging a middle station usually leaves the trunk intact (end plugs excepted).

Profibus — disadvantages

• New projects rarely choose it on purpose; a new Siemens CPU speaks PROFINET.
• Master–slave polling has no DC / IRT sample-time alignment.
• The terminating plug must sit on a powered end device; parking it on a dead station is the same as no match.
• At high baud, spurs must stay short — same as CAN, you cannot star-wire by habit.

GSD and diagnostics: 195 explained, GSD configuration.

One-line choice

• Mobile machine, CAN already on the controller: 194.
• Installed Siemens DP network: 195.
• New S7-1200/1500: do not keep Profibus just because it is still Siemens — see 199 PROFINET.
• Fast multi-axis sync: neither serial bus is first choice; look at EtherCAT.

Four-bus table: comparing the whole fieldbus range. Output types: output signals compared.