Error -65536 When Using Scan Engine and FPGA Simultaneously on a CompactRIO

Updated Jul 15, 2018

Reported In


  • LabVIEW FPGA Module

Issue Details

I get the following error from an I/O Variable when I am running my host VI that uses both the Scan Engine and FPGA methods of communication with the C Series Modules in my chassis.

Error -65536 occurred at Shared Variable in


Make sure that you are not accessing an I/O variable before before you have opened a reference to your FPGA VI or after closing a reference to the FPGA VI.

Additional Information

When using Scan Engine and FPGA modes at the same time the compiler has to combine the default RIO Scan Interface bitfile as well as the user FPGA code that was created into one bitfile. Doing this essentially means that the scan engine is running in your bitfile. 

This means that to use an I/O variable, you must ensure the FPGA code is running first. If you try to access an I/O variable before you have opened a reference to the FPGA VI or after you have closed a reference to the FPGA VI, then you will get this error.

To prevent this from happening you must make sure you stop accessing I/O variables before you stop your FPGA VI or close the reference to it. In the image below you can see how the I/O variable loop has to stop before the FPGA reference will be closed.

It is also possible to receive this error in initial calls to the IO variable or UDV after opening the FPGA reference. After ~5 seconds this error should clear.


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