Series 194 CANopen Explained: Putting the DSP406 Profile into Practice

Series 194 is Germanjet’s CANopen fieldbus magnetostrictive displacement sensor, following the CiA DSP406 encoder profile. Against a point-to-point interface, its core value is several axes on one cable: one twisted-pair shielded cable strings several sensors and greatly reduces the harness; it also supports online parameter read/write and fault diagnostics. CANopen is most widely used on mobile machinery, hydraulic equipment and small-to-medium multi-axis systems, because mobile controllers almost all natively support CAN and the bus itself is tolerant of the electrical environment.

What the DSP406 profile solves

The CANopen base specification (DS301) defines only the communication mechanism, not “which object the position value sits in”. If each manufacturer defines its own, the master must write a parser for every brand. DSP406 is the device profile for encoders / position sensors; it unifies the object-dictionary location and data type of position value, resolution, preset, counting direction and other parameters.

The practical meaning is direct: the master is configured to DSP406, so when the sensor brand or model is changed the mapping need not be rewritten. Engineering should confirm three things — the sensor supports DSP406, the master / higher-level software also parses DSP406, and the EDS file version matches the firmware on the device. The CANopen object dictionary and the division of PDO/SDO are in CANopen introduction: object dictionary and the division of PDO/SDO.

Division of PDO and SDO

Item	PDO (process data object)	SDO (service data object)
Use	Cyclic / event transfer of real-time position	Read/write parameters, configure the object dictionary
Message overhead	Small; data sit directly in the message	Large; request–response handshake required
Real-time performance	High	Low
Typical timing	Used continuously in operation	Used in configuration and tuning
Common misuse	—	Polling position with SDO, filling bus bandwidth

The most common engineering error is to read position cyclically with SDO. Each SDO read is a question and an answer, so message count multiplies; once the axis count rises, bus load saturates quickly, appearing as slower refresh and occasional timeouts. The correct approach is position on PDO (cyclic transfer or event-triggered); SDO is used only in configuration and maintenance.

Node address, baud rate and bus length

Three basic settings decide whether the bus will come up:

Node-ID must be unique on the same bus. The typical symptom of an address clash is a large number of error frames and intermittent drop-out of some nodes. Before several sensors go live in a batch, set each one and record it on a label.

Baud rate must be the same across the whole network. Any node at a different baud rate floods the bus with error frames and brings the whole network down — the most common cause of “the whole line went dead after we added a new device”.

Baud rate is inversely proportional to bus length: the higher the baud rate, the shorter the allowed total bus length. This is a physical constraint of CAN bit arbitration (the signal must traverse the whole line within one bit time) and cannot be freely broken by adding a repeater. In design, fix length from the site distance first, then work back to the usable baud rate.

Practical configuration is in CANopen node address and baud-rate configuration in practice.

Terminating resistors: 120 Ω is not optional

A CAN bus needs a 120 Ω terminating resistor at each end, to match the characteristic impedance of the twisted pair and absorb reflections. Three typical errors:

• None fitted: reflections are severe; at short distance and low baud rate it may “look as if it works”, but at long distance or high baud rate a large number of error frames appear at once;
• Three or more fitted: total load impedance is too low, the transceiver cannot drive, and the dominant level cannot be pulled down;
• Fitted on a middle node: they must be at the two physical ends of the line; in the middle they do not match.

In addition, CANopen requires a linear (trunk-and-short-drop) topology; it must not be made star or ring. Drop-line length must be kept as short as possible; long drops likewise introduce reflections. Terminating-resistor principle and measurement are in Terminating resistors and bus length: why 120 Ω is required.

Where Series 194 is suitable

Mobile hydraulic equipment. Mobile controllers on excavators, loaders, agricultural machines and aerial work platforms almost all have a CAN port. Series 194 can join the machine CAN network directly, saving extra analog input modules and harness. Product reinforcement for mobile duty is in Series 13 mobile hydraulics explained.

Multi-axis hydraulic equipment. Multi-cylinder synchronised hydraulic presses and lift platforms read every axis position on one bus, far tidier than a pair of analog wires per axis.

Need to read several magnet rings. Positions of several magnet rings on one waveguide wire can be reported together through several PDO mappings — analog cannot do this. The principle is in Multi-magnet / multi-position measuring principle; configuration is in CANopen multi-magnet read configuration.

Need for diagnostic information. Sensor status and error codes can be reported actively by emergency messages (EMCY), which helps predictive maintenance.

Where it sits against other fieldbuses

Dimension	CANopen (194)	Profibus DP (195)	EtherCAT (197/198)	PROFINET (199PROFINET)
Physical layer	CAN differential twisted pair	RS-485 twisted pair	Ethernet	Ethernet
Configuration file	EDS	GSD	ESI (XML)	GSDML
Synchronisation	SYNC message	Master–slave polling	DC distributed clocks	IRT isochronous synchronisation
Typical strength	Mature mobile-machinery ecosystem, low cost	Good compatibility with existing lines	Strongest real-time performance, fast scan	Smooth integration with Siemens systems
Terminating resistor	120 Ω at each end	Plugs with termination at each end	Not required	Not required

The full trade-off among the four fieldbuses is in Comparing the whole fieldbus range. Product pages: Series 194 CANopen, Series 195 Profibus, Series 197 EtherCAT. General fieldbus wiring practice is in Fieldbus practical guide.