Resolving WinCC Flexible Trend View 1-Second Update Limit

1. Problem Summary

On a Siemens Multi Panel 377 (MP377) running WinCC Flexible 2008 / 2008 SP2 / SP3 / SP4 / SP5, a configured trend view bound to a real-time PLC tag refreshes the visible curve only every 1 second, even when the underlying tag is updated on the PLC at 100 ms. The PLC archive data may be present and dense, but the HMI screen repaints the trace slowly, giving a stepped, non-fluid curve when compared with competing platforms such as Allen-Bradley PanelView Plus.

Typical observed behavior in runtime:

• PLC tag update rate: 100 ms (or 10 ms with WinAC RTX, S7-300/400, or ET 200S).
• Archive configured with Acquisition cycle 100 ms or 500 ms.
• Display update of the f(t) trend view: fixed at ~1 s regardless of project setting.
• Visible curve looks like discrete dot-clusters every second rather than a continuous line.

This article explains the architectural reason for the cap, documents the supported workarounds inside WinCC Flexible, and shows the migration path to TIA Portal / WinCC Comfort, WinCC Advanced, and WinCC Professional where the update cycle constraint no longer applies in the same way.

2. Affected Products and Software Versions

3. Root Cause Analysis

Siemens documents the limit directly in their support database. Per Siemens Support entry 34230359:

Basically, it is not possible to have a trend display updated continuously, because upon calling it, only the available archive data can be read out.

Internally, the f(t) trend view on a WinCC Flexible runtime reads from a tag buffer that is updated on a screen-cycle basis, not on the PLC's acquisition cycle. The screen cycle is determined by the schedule of the picture that hosts the trend. On MP377 the default scheduling sets the Update parameter of the trend view to 1 s (the minimum value exposed in the project dialog is 1 s). The PLC can absolutely deliver 100 ms values to the tag; the values are stored, but the HMI widget redraws them in 1 s batches.

Three structural reasons converge to produce the cap:

1. Picture update cycle - the trend is a child object of a picture; the picture scheduler drives the trend's redraw.
2. Read cycle vs. acquisition cycle - the acquisition rate of the archive tag may be 100 ms, but the read cycle of the trend view is governed by the Update property on the view, not by the tag.
3. CE kernel / COM container - the WinCE 5.0 runtime on the MP377 cannot schedule a UI thread faster than ~1 Hz without starving the tag polling, alarm logging, and recipe services. WinCC Flexible protects the system by enforcing a 1 s floor.

4. WinCC Flexible Trend Architecture

A WinCC Flexible trend view on MP377 has four timing-related properties exposed in the configuration dialog:

The trap: setting Acquisition cycle to 100 ms does not change the Update field. The data is captured at 100 ms, but the trend widget cannot render it faster than the picture cycle, which in turn cannot fall below 1 s on WinCC Flexible runtime for MP377.

5. Solution 1 - Archive-Based Trend with Tight Read Cycle

Switch the trend view from a real-time tag source to a tag archive source. The tag is then logged by the HMI archive system, and the view reads from the archive buffer.

1. Project tree → Tags → select the tag → Properties → Archive tab → enable Tag archive.
2. Set Acquisition type = Cyclic, Acquisition cycle = 500 ms (or 1 s for safety).
3. Select Archiving → Cyclic continuous with the desired Archive size (e.g. 600 entries ≈ 5 min).
4. Open the trend view → Source dropdown → select the archive, not the raw tag.
5. Set Update on the trend view to 1 s (still the floor), but the Display cycle to 500 ms so the X-axis labels resolve correctly.

This will not lift the 1 s screen redraw, but it ensures that the data feeding the curve is dense and continuous so the rendered line is geometrically accurate even if it repaints slowly.

6. Solution 2 - Buffer-Bit Trend (PLC-Side Acquisition)

This is the pattern Siemens forum veterans and Siemens support both point to for "smoother-looking" trends at sub-second update rates. The PLC collects samples at full speed into a DB array; the HMI simply displays the buffer. The HMI never tries to do high-rate acquisition itself.

PLC code (S7-300/400 STL, OB35 cyclic interrupt 100 ms):

// DB100 - Trend buffer, 200 REAL samples
// DB100.DBD0   = nIndex
// DB100.DBD4   = current sample (REAL)
// DB100.DBD8.. = ring buffer of 200 REALs

// OB35 - 100 ms interrupt
      L     "Trend".nIndex
      +     1
      T     "Trend".nIndex            // wrap at 200
      L     200
      MOD
      T     "Trend".nIndex

      L     "Trend".nIndex            // pointer into ring buffer
      SLD   3                          // *8 bytes per REAL
      LAR1
      L     "ai_Pressure".Actual       // sample the process value
      T     DBD [AR1,P#8.0]            // store in DB100 at offset 8 + idx*4

HMI configuration in WinCC Flexible:

1. Insert a f(t) trend view.
2. Source = array tag "Trend".Buffer[0..199] (REAL array).
3. Index tag = "Trend".nIndex (or use the array-mode index property of the trend view).
4. Set Update = 1 s; set Display to a 10 s window so all 200 points span the visible width.

Result: the trend looks like a true continuous line because the HMI is reading 200 already-collected points per redraw. The redraw is still 1 s, but each redraw carries 200 fresh points spaced 100 ms apart, so the visible curve is smooth.

7. Solution 3 - Tighten the Picture Update Cycle Project-Wide

For users who need the trend to react immediately to setpoint changes (e.g. the curve re-originates on a button press):

1. Project tree → Screens → select the screen that hosts the trend → Properties → Update → 1 s (lowest value WinCC Flexible exposes).
2. Ensure no other animation, IO field, or alarm control on the same screen has a longer cycle; the slowest child dictates the picture's redraw.
3. Under Device → Properties → Cyclic services, reduce the Minimum cycle time if exposed in your WinCC Flexible build (SP3 and later).

There is no supported method to drop below 1 s on MP377. Attempting to script an Update cycle of 200 ms via VBScript in the project produces a runtime warning and a throttle back to 1 s on the next picture change.

8. Solution 4 - Bit-Triggered Update (Event-Driven Repaint)

Instead of relying on the 1 s cyclic redraw, trigger the redraw on a PLC bit. WinCC Flexible supports the UpdateTag property on a trend view. Toggle a bit in the PLC whenever a meaningful event occurs (e.g. batch end, alarm). The HMI picture scheduler will redraw the trend as soon as the bit transitions.

// PLC: in OB1
      SET
      S     "HMI".Bit_RefreshTrend   // 1 = force HMI repaint
      L     S5T#200ms                // pulse 200 ms
      SD    "HMI".BitTimer
      U     "HMI".BitTimer
      R     "HMI".Bit_RefreshTrend

WinCC Flexible configuration:

1. Trend view → Properties → Update = By tag.
2. Tag = "HMI".Bit_RefreshTrend.
3. On the rising edge the curve re-reads the archive and re-paints.

This does not increase the line density, but it does eliminate the perceived "stale" look of a 1 s cyclic trend because the curve always shows the freshest data on demand.

9. Migration Path - TIA Portal and WinCC Comfort/Advanced/Professional

The 1 s floor is specific to WinCC Flexible on Windows CE panels. On the TIA Portal side, the f(t) trend view object documented in WinCC Professional / RT Professional online help for the f(t) trend view exposes:

• Start/End update runtime buttons (operators can freeze the curve and resume).
• Trend buffer configurable from 100 ms to 1 h without a 1 s floor.
• Ruler for time-aligned read-out of any sample on the curve.

Migration procedure:

1. In TIA Portal → Project → Migrate WinCC Flexible project.
2. Select the source .hmi file. The wizard imports tags, alarms, screens, archives, and recipes.
3. Open each trend view; the Update property has a 100 ms minimum on the new target panel (TP700 Comfort, TP1500 Comfort, MTP700, MTP1200, MTP1900).
4. Re-compile and transfer. Typical migration time: 1-2 hours for a 30-screen machine project.

10. Comparison - WinCC Flexible Trend vs Allen-Bradley PanelView Plus

Customers who migrate from AB often cite the smoother AB trend as a reason to standardize on Siemens. The visual difference is real and the technical reason is platform-specific.

After migrating to TIA Portal with a Comfort Panel, Siemens HMIs match or exceed the AB visual fluidity, and the operator gets the runtime Start/End update and ruler features the MP377 cannot expose.

11. Verification and Benchmarks

Procedure to verify the fix on the bench with a S7-315-2 PN/DP and a MP377 12" Touch running WinCC Flexible 2008 SP5:

1. Generate a ramp on the PLC: L 0.0; T "test".ramp; LOOP: L "test".ramp; + 0.5; T "test".ramp; BEA; LOOP in OB35 at 100 ms.
2. Configure an archive tag with Acquisition = 100 ms, Archive size = 600, Logging = continuous.
3. Drop a f(t) trend view bound to the archive; Update = 1 s; Display cycle = 1 s; X-axis = 30 s window.
4. Trigger a screen change to force a redraw, then capture a screenshot at 5 s intervals.
5. Inspect the archive in Start → SIMATIC → WinCC Flexible → Archive Viewer. The ramp samples should be exactly 100 ms apart.
6. If the HMI curve shows dot-clusters every 1 s, the trend widget is operating to spec - apply the buffer-bit pattern from §6 to smooth it.

12. Troubleshooting Matrix