Measuring Control Expert Program Section or POU Scan Times in SCADAPack x70 RTUs

Scan time is the amount of time the RTU takes to execute one full cycle of logic.
Understanding scan time helps diagnose performance issues, optimize logic, and verify that time‑critical tasks are running as expected.

To determine how long each section or POU takes to execute in a SCADAPack x70 RTU, use the FREERUN function block. This function block enables a free-running counter, which can be used for run time measurement of sections and application programs.

Measurement using FREERUN function block reflect only the logic execution time of the section being measure. They do not include the RTU system overhead, such as: IO updating  and communication servicing time. These overhead components are included only in the overall task scan time, which can be monitored via system words %SW30.

System Status Words:

FREERUN outputs a counter value that increases every program scan.
By placing a FREERUN block at the start and end of a section, you can calculate the time difference between the two values.

This difference represents the approximate scan time for that section of logic.

Example: Using FREERUN to measure section scan time

Below is a common implementation pattern:

1. Insert a FREERUN block at the beginning of a section.
2. Capture its output into a variable (e.g., StartTime).
3. Insert another FREERUN block at the end of the section.
4. Capture its output into another variable (e.g., EndTime).
5. Subtract StartTime from EndTime to obtain the execution time.

The SUB_DINT instruction is typically used to perform the subtraction.

FREERUN increments once per scan, so the raw difference represents scan cycles, not time in microseconds.

To convert cycles into microseconds:

Time (µs) = CycleCount × ScanPeriod (µs)

Using animation table to monitor the Scan time.

Right click on the OUT of the SUB_DINT and select Initialize Animation Table

Released for: Schneider Electric Netherlands