Error 56 Occurs When Using The NI Modbus API

Issue Details

Solution

This error indicates that the Modbus slave device did not respond to your read request within the timeout specified for the Modbus session. 

If using Modbus TCP:

• Make sure that the device is powered on and ready to receive communication.
• Make sure that you have specified the correct IP address and port of the slave device.
• If you have multiple devices on the same network, remove all the other devices but one, and try them one after the other. This issue can be caused by one of the devices and can impact all the networks.
• It could be the case that the slave response is not entirely formatted according to the Modbus TCP standard, to confirm this use network packet sniffers like WireShark to check if the slave is responding at all to the master queries.

If using Modbus RTU:

• Reinstall NI-VISA driver, sometimes it happens that installing 3rd party software instruments drivers interferes with NI-VISA driver
• Make sure that you have specified the correct serial port, Unit ID, parity, and flow control settings. Refer to the manual for your device for information on how these should be set.
• Make sure that the connection between the serial pins on each end is correct. 
• Make sure you are properly closing the VISA sessions, avoiding doing this could end in a multiple reference issue.
  • You can enable the automatic closure of VISA sessions in your Real-Time target properties from the LabVIEW project in the Miscellaneous category.

Additional Information

In case of using Modbus API with Arduino as slave device, older version of libraries deployed on Arduino can cause error 56. Using the latest library version on Arduino usually resolves the timeout error.