CLAD Debugging & Troubleshooting Techniques 3 — Questions and Answers
Question 1: In LabVIEW, what is the effect of placing a breakpoint on a wire versus on a node?
- Wire breakpoints pause before the downstream node executes; node breakpoints pause before the node begins (Correct answer)
- There is no difference — both pause at the same point
- Wire breakpoints only work in subVIs
- Node breakpoints display data; wire breakpoints do not
Correct answer: Wire breakpoints pause before the downstream node executes; node breakpoints pause before the node begins
A breakpoint on a wire pauses execution after the upstream node finishes and before the data is consumed by the downstream node, while a node breakpoint pauses just before that node executes.
Question 2: Which built-in LabVIEW VI is commonly used to display the call chain when an error occurs, helping identify which subVI generated the error?
- Simple Error Handler
- General Error Handler (Correct answer)
- Error Cluster From Error Code
- Clear Errors
Correct answer: General Error Handler
The General Error Handler provides more detail than the Simple Error Handler, including the full call chain source string that shows which VI in the hierarchy generated the error.
Question 3: You suspect a race condition in a parallel loop structure. Which LabVIEW mechanism is the correct fix to ensure safe shared variable access?
- Add a sequence structure around both loops
- Use a functional global variable or shift register pattern to serialize access (Correct answer)
- Increase the loop timing with a Wait function
- Use the Error Wire to serialize data flow between loops
Correct answer: Use a functional global variable or shift register pattern to serialize access
Functional global variables (action engine pattern) serialize access to shared data by using an uninitialized shift register inside a subVI that can only be called by one caller at a time.
Question 4: When LabVIEW reports 'Warning: VI is reentrant; data may not be preserved between calls,' what does this mean for debugging?
- Each reentrant clone has its own uninitialized memory, so probes may show different values per clone (Correct answer)
- The VI cannot be debugged with probes
- Reentrant VIs never retain wire values
- The VI's front panel controls are read-only during debugging
Correct answer: Each reentrant clone has its own uninitialized memory, so probes may show different values per clone
Each reentrant clone maintains its own data space, so probes on different simultaneous calls may display different values, making debugging more complex.
Question 5: What is the primary purpose of the 'Find All Instances' feature when debugging a global variable usage issue?
- It renames all instances of the variable across all VIs
- It locates every node in the project that reads or writes the global variable (Correct answer)
- It converts the global variable to a local variable
- It traces the data type of the global variable
Correct answer: It locates every node in the project that reads or writes the global variable
Find All Instances shows every location in the project where a specific global variable is read or written, helping you track down unexpected writers causing data corruption.
Question 6: A LabVIEW VI runs without a broken arrow but produces incorrect numeric results. Which technique should you use FIRST?
- Rebuild the entire VI from scratch
- Place probes on key wires and use Execution Highlighting to trace intermediate values (Correct answer)
- Check the connector pane for missing terminals
- Enable Error Handling on every node
Correct answer: Place probes on key wires and use Execution Highlighting to trace intermediate values
For logic errors (no broken arrow, wrong results), tracing intermediate values with probes and Execution Highlighting is the most efficient first step to isolate where the incorrect value appears.
Question 7: Which LabVIEW data type, when used on a wire, can mask type coercion dots that indicate potential precision loss during debugging?
- Variant data type
- Cluster data type
- Dynamic data type (DDT) (Correct answer)
- Enum data type
Correct answer: Dynamic data type (DDT)
The Dynamic Data Type (DDT) used by Express VIs can hide coercion dots because it auto-converts internally, making precision loss less visible on the block diagram.
CLAD Debugging & Troubleshooting Techniques 3
In LabVIEW, what is the effect of placing a breakpoint on a wire versus on a node?