Synchronizing Manual and Automatic Manipulated Variable Limits on PIDConL in PCS 7 V9.1

In SIMATIC PCS 7 APL PIDConL blocks, manipulated variable high limits in automatic mode (MV_HiLim) and manual mode (ManHiLim) are architecturally decoupled. Interconnecting ManHiLim directly in CFC locks the parameter, preventing operator adjustments from the WinCC faceplate. To keep both limits synchronized bidirectionally while preserving operator write access, engineers implement external CFC tracking logic with edge-detection triggers or configure Feature.Bit23 to disable manual limits.

Functional Separation of MV_HiLim and ManHiLim in PIDConL

In the SIMATIC PCS 7 Advanced Process Library (APL), the PIDConL block (FB1874) provides continuous PID control with independent operational boundaries:
* MV_HiLim / MV_LoLim: Enforces manipulated variable (MV) output limits when the controller is in Automatic mode (AutAct = 1). These limits prevent the closed-loop algorithm from over-saturating actuators.
* ManHiLim / ManLoLim: Enforces manipulated variable limits when the controller is in Manual mode (ManAct = 1). These boundaries prevent operators from driving valves beyond physical or process safety thresholds during manual plant steering.
* By design, the APL intentionally decouples these parameters: a process may permit an automated loop to modulate between 10% and 90%, while operators in manual mode are restricted to 0% and 50% (or vice versa).

The CFC Interconnection Lockout: Why Faceplate Edits Are Overwritten

When an engineering specification requires that changing the high limit in automatic mode automatically updates the manual limit (and vice versa):
* If an engineer directly connects MV_HiLim to ManHiLim in the CFC chart:
* The CFC interconnection asserts absolute priority over the input parameter. At every execution cycle of OB35, the block overwrites the internal instance data with the interconnected value.
* If an operator enters a new value into ManHiLim via the WinCC OS faceplate, the value is immediately overwritten on the very next CPU cycle ($100 ext{ ms}$ later), rendering faceplate input impossible.

Solution 1: Disabling Manual Limits via Feature.Bit23

When separate manual limits are unnecessary and automatic limits should govern all operating modes:
1. In the CFC chart, inspect the PIDConL block input parameter Feature.
2. Locate Feature.Bit23:
* Limits disabled in manual mode.
3. Set Feature.Bit23 = 1 (TRUE).
4. When Feature.Bit23 = 1:
* The manual limits (ManHiLim, ManLoLim) are deactivated entirely.
* The automatic limits (MV_HiLim, MV_LoLim) govern the controller output in both automatic and manual modes.
* Operators adjusting limits on the faceplate modify MV_HiLim, achieving 100% unified limit enforcement without custom chart logic.

Solution 2: Bidirectional Tracking Logic in CFC / SCL

If distinct manual limits must be retained but dynamic synchronization is required:
1. Create a lightweight tracking function block in SCL or assemble standard APL comparator blocks.
2. Monitor value changes:
* Maintain internal historical copies of both variables: MV_HiLim_Old and ManHiLim_Old.
* If an operator modifies MV_HiLim via faceplate ($\Delta ext{MV}
e 0$), pulse a transfer to ManHiLim.
* If an operator modifies ManHiLim via faceplate ($\Delta ext{Man}
e 0$), pulse a transfer to MV_HiLim.
3. Feed the synchronized outputs to the unlinked block pins using memory transfer pointers, ensuring neither pin has a permanent static CFC connection.

Verification

In WinCC OS runtime, open the PIDConL faceplate and navigate to the Parameter View.
If using Feature.Bit23 = 1: verify that manual limit input fields are hidden or grayed out, and modifying MV_HiLim from 80% to 75% constrains actuator travel in both Auto and Manual modes.
In Manual mode, attempt to drive the manipulated variable to 90%: confirm that the output clamps cleanly at 75%.
In CFC dynamic test mode, confirm that no parameter overwrites or oscillation occur.

Limitations and Operational Precautions

Applying Feature.Bit23 = 1 removes operator freedom to open a valve beyond automatic tuning limits during plant startup; ensure process operations accept unified limits.
Never create circular feedback interconnections in CFC (MV_HiLim connected to ManHiLim and ManHiLim connected back to MV_HiLim), as this creates undefined compiler loops and numerical freezing.
Verify actuator travel feedback (MV_Trk) is properly aligned to avoid reset windup during mode transitions.

Engineer FAQ

What is the difference between MV_HiLim and ManHiLim in PIDConL?
MV_HiLim limits controller output in Auto mode; ManHiLim limits controller output in Manual mode.
Why does interconnecting ManHiLim in CFC prevent operator faceplate entries?
Static CFC interconnections overwrite block instance parameters every execution cycle, immediately erasing faceplate writes.
What does Feature.Bit23 do on PIDConL?
It disables manual limits entirely, enforcing automatic limits (MV_HiLim / MV_LoLim) across both Auto and Manual modes.
How do I enable Feature.Bit23 in CFC?
Open block properties for PIDConL, locate the Feature structure, and set Bit 23 to 1.
Can Feature bits be modified online in RUN mode?
Yes, Feature parameter words can be adjusted online via delta download or dynamic testing without a CPU STOP.
Where are manipulated variable limits displayed in WinCC faceplates?
In the Parameter View and as limit markers on the manipulated variable bar graph in the Standard View.
What parameter defines the low manipulated variable limit in Auto mode?
The parameter MV_LoLim.
What parameter defines the low manipulated variable limit in Manual mode?
The parameter ManLoLim.
Does PIDConL support tracking external manipulated variable limits?
Yes, using inputs MV_HiLim_Li and MV_LoLim_Li when controlled by higher-level supervisory logic.
Can an SCL wrapper block update ManHiLim without a static CFC connection?
Yes, by accessing the instance DB offset directly or calling the parameter conditionally during change events.

Technical References

SIMATIC PCS 7 Advanced Process Library (APL) Function Manual (PIDConL / FB1874)
Siemens Industry Online Support Entry: PIDConL Manipulated Variable Limits (Entry ID 299492)
SIMATIC PCS 7 Standard Routing and Closed-Loop Control Guidelines
ISA-5.1 Instrumentation Symbols and Identification