EL6751 communication with a CANopen motor driver depends first on a stable CAN physical layer, matching Node ID and baud rate, and a correct device description; successful discovery alone does not prove that process data is mapped or that the motor accepts commands.
Which path must the request follow before CANopen data can move?
A command originates in the PLC or motion-control logic, passes through the EL6751 CANopen interface, travels on CANH/CANL to the motor driver, and returns as a response or process-data message. Check the path in that order: terminal configuration, bus wiring and termination, node discovery, then data mapping. A break at an earlier hop makes later software checks inconclusive.
At the physical layer, inspect CANH and CANL polarity, connector continuity, common reference and shielding practices specified for the installed equipment, and the bus termination. CAN is a differential bus; a wiring fault or incorrect termination can prevent reliable communication even when the node settings are correct. The available installation notes specifically call for a 120 Ω resistor between CANH and CANL. Confirm the actual network layout and the motor and terminal documentation before changing termination: bus termination belongs at the physical ends, not at every node.
| Check | What to compare | What it tells you |
|---|---|---|
| CAN wiring | CANH to CANH and CANL to CANL; continuity and termination | Whether the physical link can carry frames reliably |
| Node ID | Motor setting versus EL6751 node configuration | Whether the master addresses the intended device |
| Baud rate | Motor setting versus EL6751 bus setting | Whether both devices can decode the same bit timing |
| EDS | Device description versus the actual motor driver | Whether the configured node and its data layout represent the device |
Record the configured Node ID and baud rate at both ends rather than relying on a remembered default. The correct values depend on the motor configuration and bus design; read them from the device settings and project configuration.
How do you prove that the bus and node settings match?
Use the TwinCAT configuration-mode scan as an early commissioning check. It tests whether the EL6751 can discover a node on the bus; it does not by itself validate the application’s command path.
- Put the system in configuration mode.
- In the I/O tree, right-click Device 3 (EL6751) and select Scan.
- Compare the discovered node identity and settings with the motor driver configuration. If the device is absent, recheck CANH/CANL wiring, termination, Node ID, and baud rate before investigating PLC variables.
- If a node appears but its configuration is modified, review the selected EDS against the actual motor model and supported objects and process-data layout.
A successful scan narrows the fault to configuration or data exchange; an absent node sends troubleshooting back to wiring and bus settings. A modified node is a configuration clue, not proof that the EDS is the only fault.
Does the project describe the motor node or only the EL6751?
The EDS for the EL6751 describes the interface terminal, while the motor driver’s EDS describes the CANopen device. Add and configure the motor as a node under Device 3 using the matching device description, then inspect its identity and data configuration. If the project contains only the terminal description, the PLC may lack the correct node objects and process-data definitions.
Compare the configured node’s identity, supported communication objects, and mapped input/output lengths with the motor documentation. When the scan reports a modified node, resolve the mismatch before using the project’s variables: an incorrect EDS can expose the wrong object layout or mapping and make the online view misleading. Do not treat grey or non-updating variables as proof of a wiring failure until the node and its mappings are correct.
Which command path should carry control and feedback?
Separate cyclic process data from acyclic parameter access. CANopen PDOs carry mapped process data; SDO transfers access object-dictionary entries. Decide whether the application controls the drive through mapped process data, an established motion-control integration, or another documented interface. Then verify that the selected mechanism is actually configured and that the PLC logic reads and writes the corresponding variables.
Do not substitute EtherCAT CoE service-data function blocks for CANopen SDO access merely because both use service-data terminology. CoE is an EtherCAT mechanism; the CANopen request must use the supported path for the EL6751 and its configured node. Consult the installed TwinCAT library and terminal documentation for the correct blocks or mapping route rather than guessing function names or memory addresses.
A drive that emits broadcast or process-data messages after configuration may provide feedback without a PLC write to its control word, as reported for one motor setup. Treat that behavior as device-specific: read the motor manual and observe bus traffic to establish what the device sends, what it requires for enable or motion, and which command object is writable.
How do you verify the complete command-and-feedback loop?
After correcting wiring, node settings, and EDS configuration, verify each layer before running a motion command. Confirm discovery first; then confirm that configured process-data inputs update and outputs reach the intended mapped objects. For acyclic access, perform a documented read before attempting a write, and check the returned status or diagnostic information.
- With motion inhibited as required by the machine procedure, confirm the node remains present after activation and that the project reports no node-configuration mismatch.
- Monitor the mapped input data and verify that it changes in response to the motor’s documented status or feedback.
- Write only a documented, safe test value to the intended output or object, then check the motor response and any returned status.
- Enable the normal sequence only after input feedback, output path, and drive state agree with the motor documentation.
If inputs update but commands do not, trace the output mapping and control method. If neither direction updates, return to bus health, node configuration, and the EDS. Finish by observing the intended command reach the motor and the corresponding feedback return through the EL6751 into the PLC.
Frequently asked questions
Can I scan the EL6751 before adding the motor EDS?
Use configuration mode and scan Device 3 (EL6751) to test whether a node is discoverable. Add the correct motor EDS and verify node configuration before relying on application variables.
Does a successful CANopen scan prove the motor is controllable?
No. Discovery confirms communication at the node level, not correct PDO mapping, a valid command path, or motor enable conditions. Verify process-data updates and the documented control sequence separately.
Can I use a 120 Ω resistor between CANH and CANL?
The installation notes identify 120 Ω CANH-to-CANL termination. Confirm the bus layout and equipment documentation, place termination at the physical bus ends, and finish by verifying the command reaches the motor and its feedback returns to the PLC.