140-0033

Access PLC Error Log

Press MOD function key, select 'System Information' > 'PLC' > 'Error Log' to view detailed job execution times and which specific job is failing.

Look for the job name and timestamp pattern - if timestamps show regular intervals matching your cycle time, this confirms timing overrun.

Check Job Execution Statistics

In PLC programming environment (PlcProgramDesigner or TNCopt), open Job Monitor to view actual vs. configured execution times for all cyclic jobs.

Maximum execution time should not exceed 70-80% of cycle interval to allow for system overhead and variations.

Optimize PLC Code

Review the failing job's program code for inefficient loops, excessive calculations, or unnecessary wait states. Break complex jobs into smaller sub-jobs if needed.

Always backup PLC program before making modifications using MOD > 'Data Management' > 'Backup PLC'.

Adjust Cycle Time

If code cannot be optimized further, increase the job's cycle time parameter in the PLC job configuration table to allow adequate execution time.

For non-critical background tasks, cycle times of 100-500ms are typical; critical servo-related jobs may need 5-20ms cycles.

Review PLC Compiler Messages

In PLC programming environment, check the compiler output window for errors or warnings related to the failing job. Note specific line numbers and error codes.

Even warnings can sometimes prevent job execution - aim for zero warnings in production code.

Verify Variable Declarations

Check that all PLC variables used in the job are properly declared in the variable table with correct data types (BOOL, INT, REAL, etc.) and memory allocation.

Undeclared or incorrectly typed variables may compile but fail at runtime when job attempts to execute.

Test Job in Isolation

Temporarily disable other PLC jobs and test the failing job alone to determine if the issue is inherent to the job code or caused by interaction with other tasks.

Use the PLC job enable/disable flags in the job configuration table rather than deleting code.

Check Function Block Availability

Verify that all function blocks (FBs) called within the job are available in the control's PLC library and compatible with current TNC software version.

After software updates, some deprecated function blocks may need replacement with newer equivalents.

Check System Resource Utilization

Press MOD > 'System Information' > 'System Load' to view current CPU load, memory usage, and PLC cycle time. Look for values consistently above 80%.

System CPU load above 90% can cause unpredictable behavior and additional alarms.

Review Active PLC Jobs

Access PLC job configuration table to count total active jobs and their cycle frequencies. Calculate total theoretical system load from all jobs.

TNC 640 can typically handle 20-30 PLC jobs depending on complexity; older TNC 320/530 may be limited to 10-15 jobs.

Check for Memory Leaks

Monitor free PLC memory over time using System Information. If free memory decreases continuously, a job may have memory leak from improper pointer handling or dynamic allocation.

Power cycling the control temporarily resolves memory leaks but doesn't fix the underlying code issue.

Disable Non-Essential Jobs

Temporarily disable lower-priority background jobs to free resources, then re-enable one at a time to identify which combination causes the resource conflict.

Jobs related to data logging, display updates, or communication can often run at longer intervals without affecting machine function.

Identify Resource Dependencies

Review the failing job's code to list all hardware resources accessed (specific I/O addresses, M-functions, axis parameters, system variables).

Use PLC code comments to document resource usage for each job - helps identify conflicts during troubleshooting.