Why PCS 7 CFC Test Values Can Lag Behind S7-PLCSIM

A CFC test display is a periodically refreshed view of CPU values, not a live oscilloscope. Siemens' CFC V9.0 SP5 manual gives a default 2 s watch cycle for monitored I/Os and says an I/O tooltip refreshes once per second, independently of that setting. A signal that changes on a similar or faster timescale can therefore look stale or appear to hold one state. The CFC watch interval, CPU task execution and PLCSIM scan-cycle watchdog are separate settings.

Three Different Cycles

Keep these timings separate when diagnosing a mismatch:

TimingWhat it controlsWhat it does not meanCPU task cycleWhen OB1 or a configured cyclic interrupt OB executes its assigned codeThe refresh speed of the CFC online displayCFC watch cycleHow often registered I/Os are refreshed in CFC test mode; Siemens documents a 2 s default in CFC V9.0 SP5The time between PLC scansPLCSIM scan-cycle monitoring timeThe maximum time allowed for a simulated OB1 scan and relevant I/O update before the simulated CPU enters STOP; the cited V5.4 manual gives 6000 ms as the defaultA requested 6 s scan interval or a clock-memory period

Why a One-Second Signal May Look Mismatched

A polled display can miss transitions that occur between refreshes. When the watch interval is close to the signal's on/off duration, consecutive reads can repeatedly land in the same state, or the displayed state can lag the simulator's more frequently refreshed view. Siemens documents an independent CFC tooltip read at a one-second cycle in CFC V9.0 SP5. Therefore hovering an I/O can provide a newer value than the registered watch display, but it still does not provide continuous capture or guarantee that every transition will be seen.

Check the CFC Test Display

Confirm that the program was compiled without errors and downloaded to the simulated CPU before entering CFC Test mode; this is a stated prerequisite in the Siemens CFC function manual.
In CFC, review Test > Settings and note the watch cycle for the CPU. A shorter watch interval refreshes registered I/Os more often if the installed CFC version permits it; it does not change OB execution.
Move the cursor over the relevant block I/O or connection line. In CFC V9.0 SP5, Siemens documents that the tooltip actively retrieves an online value on a one-second cycle, independent of the watch cycle, and changes its background when an online value is available.
Compare the same I/O, data type and polarity in CFC and PLCSIM. Check whether CFC is showing an input, an output, or an interconnected value; these can represent different points in the signal path.
If the visible state duration is too short for the available display refresh, temporarily use a longer test pulse or a controlled latched test indication in an approved simulation copy. Do not treat the CFC watch window as a high-speed capture instrument.

OB1, OB35 and Process-Image Updates

OB1 is the cyclic program organization block, while OB35 is an example of a cyclic interrupt OB configured to run at an interval. Moving a CFC chart between them changes when its logic executes; it does not force the CFC test display to refresh at that same interval. For hardware signals in PCS 7, process-image partition assignment also matters: the configured I/O addresses and signal-processing blocks must be assigned and executed consistently with the intended OB. This is an engineering configuration question separate from online display polling. The source does not identify the CPU or show its process-image partition configuration, so an exact OB/PIP prescription cannot be derived for this project.

What the PLCSIM Cycle Setting Means

The reported 1000 ms minimum and 6000 ms default correspond to a scan-cycle monitoring limit in the S7-PLCSIM V5.4 manual: it limits how long one simulated OB1 scan and relevant I/O update may take before PLCSIM enters STOP. It is not the period at which OB1 is scheduled. Because the user did not provide a PLCSIM version or identify the exact dialog, confirm the label and meaning in the installed simulator documentation before changing it. Siemens documentation for PCS 7 also cautions against changing the configured scan-cycle monitoring value without version- and CPU-applicable guidance.

Verification

Observe the output for several full signal cycles in both views, using the same point and data interpretation. Confirm that the CFC watch or tooltip changes when polled and that the PLCSIM value continues to change. If increasing the CFC refresh rate is unavailable or still too slow, lengthen the simulated state duration for inspection rather than interpreting a sampled display as proof that the logic is stopped. Verify task assignment and process-image configuration separately if the logic's actual execution or input acquisition is in question.

Limitations and Operational Precautions

The ticket reports PCS 7 V9.1 on Windows 10 in VMware, but omits the AS CPU, exact CFC revision, PLCSIM version, OB settings, process-image assignments and screenshots. The opened CFC timing reference is CFC V9.0 SP5; check the exact CFC component documentation installed with PCS 7 V9.1 before relying on its intervals. The PLCSIM watchdog details cited are specifically from S7-PLCSIM V5.4 and are not a guarantee for later simulator versions. Avoid changing watchdog, OB or process-image settings on a production system to improve a display; those settings affect execution and I/O behavior.

Engineer FAQ

Why does PLCSIM change faster than CFC appears to? CFC test values are polled. Siemens documents a 2 s default watch cycle for CFC V9.0 SP5, while a tooltip refreshes at one-second intervals.

Does changing the CFC watch cycle change OB1 or OB35? No. It controls the online display refresh, not CPU task scheduling.

Does the 6000 ms PLCSIM setting run OB1 every six seconds? No. In the cited PLCSIM V5.4 manual it is a maximum scan-cycle monitoring limit; exceeding it causes simulated STOP.

Will hovering over an I/O catch every edge? No. The documented tooltip refresh is once per second in CFC V9.0 SP5, so brief transitions may occur between reads.

Should a one-second display cycle be used to verify a one-second toggle? It is not sufficient to guarantee visibility of every state. Use a faster applicable observation method or lengthen the test state duration.

Technical References

Siemens, CFC for SIMATIC S7 (V9.0 SP5), Function Manual, 11/2020, A5E41356073-AF, sections 9.11 “Tooltips” and 14.1 “Working in Test Mode” (watch-cycle default and tooltip refresh behavior): https://cache.industry.siemens.com/dl/files/631/109782631/att_1035867/v1/s7cfcs7b_en-US.pdf
Siemens, S7-PLCSIM V5.4, Operating Manual, 07/2011, A5E00992424-03, section 4.18 “Monitor cycle time” (simulation scan-cycle watchdog semantics): https://cache.industry.siemens.com/dl/files/828/54667828/att_110511/v1/s7wsvhdb_en-US.pdf

Why PCS 7 CFC Test Values Can Lag Behind S7-PLCSIM