C135 PCS 7 (PCS7)
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Activating External Setpoint Mode on PIDConR Controllers via Feature.Bit21 in SIMATIC PCS 7 APL

Activating External Setpoint Mode on PIDConR Controllers via Feature.Bit21 in SIMATIC PCS 7 APL

On the SIMATIC PCS 7 Advanced Process Library (APL) continuous controller PIDConR, the Setpoint mode selection dropdown and buttons (Internal / External setpoint toggle) are completely hidden on the WinCC faceplate when Feature.Bit21 is set to 0. Although operator authorizations OS_Perm.Bit4 and Bit5 allow switching between internal and external setpoints, Feature.Bit21 ('Setpoint mode specification') fundamentally governs the algorithmic availability and graphical visibility of the external setpoint feature. Setting Feature.Bit21 = 1 activates the SP_Ext input pin and immediately exposes the Setpoint Internal/External selection on the faceplate.

Table of Contents

Executive Summary

In continuous process control applications utilizing the SIMATIC PCS 7 Advanced Process Library (APL), engineers frequently deploy the PIDConR function block (continuous controller with external reset feedback) for critical cascade control, ratio control, and anti-reset windup loops. During commissioning, an engineer may find that while standard controllers such as PIDConL readily display the Internal / External setpoint selection dropdown on their WinCC faceplates, PIDConR faceplates omit the setpoint mode selection entirely. Even when operator permissions (OS_Perm.Bit4 and OS_Perm.Bit5) are explicitly set to True, the mode toggle remains absent. This technical article explains the interaction between APL feature bits and faceplate dynamic scripts, focusing on Feature.Bit21.

APL Controller Architecture: PIDConR vs PIDConL

The SIMATIC PCS 7 Advanced Process Library provides two primary continuous PID controllers tailored for distinct technological tasks:

  • PIDConL (Standard Large Controller): The universal continuous controller featuring integrated smith predictors, disturbance variable feedforward, and standard internal/external setpoint switching.
  • PIDConR (Controller with External Reset): Specifically designed for master-slave cascade loops and processes with external actuator feedback. It uses an explicit tracking/reset input (PosT) to prevent integral windup when slave actuators saturate or when operating in split-range systems.
  • Because PIDConR is frequently deployed as a dedicated secondary controller in cascade loops, its default feature bit configuration differs from PIDConL.

Root Cause Analysis: Why the Setpoint Selection Option Disappears

When viewing an APL faceplate in WinCC runtime, the visibility of operational controls is determined by script evaluation:

  1. Faceplate Script Evaluation: Upon opening @pg_pidconr.pdl, the faceplate script queries the block's Feature word (specifically Feature.Bit21).
  2. Feature.Bit21 Defined: In the APL engineering specification, Feature.Bit21 is defined as 'Setpoint mode specification' (Enable bumpless switchover / external setpoint capability).
  3. Hiding Inactive Controls: When Feature.Bit21 = 0, the block algorithm treats the controller as an internal-only setpoint device. To prevent operator confusion, the faceplate script sets the visibility property of the Setpoint mode dropdown to FALSE. The button does not appear grayed out—it is completely removed from the graphical layout.
  4. The OS_Perm Disconnect: OS_Perm.Bit4 ('1 = Operator can switch to Internal setpoint') and OS_Perm.Bit5 ('1 = Operator can switch to External setpoint') only dictate user click permissions. If the feature itself is disabled via Feature.Bit21, the permission bits have no effect.

The Role of Feature Bits vs Operator Permissions (OS_Perm)

Engineers must distinguish the structural hierarchy of APL configuration words:

  • Feature Word: Defines the fundamental algorithmic capabilities, system behavior, and interface appearance of the block. Feature bits are typically established during engineering design and remain static during runtime.
  • OS_Perm Word: Dictates the operational rights of the logged-in operator on the WinCC station. It controls whether visible buttons are enabled or grayed out based on operator privileges.
  • Rule of Operation: A function must be enabled in Feature before OS_Perm can grant access to operate it.

Step-by-Step Procedure to Enable External Setpoints

To enable external setpoint selection on PIDConR instances across the plant:

  1. Open the CFC Chart: In SIMATIC Manager, locate the target CFC chart and double-click to open it.
  2. Locate the PIDConR Block: Select the controller block instance.
  3. Access Object Properties: Double-click the block or right-click and select Object Properties.
  4. Modify Feature Parameter:
    * Navigate to the Inputs tab and locate the parameter Feature.
    * Expand the structure to view individual bits, or calculate the hexadecimal value.
    * Set Feature.Bit21 = 1 (True).
  5. Verify Interconnection: Ensure the external setpoint signal (e.g., from an upstream master controller or recipe system) is interconnected to input pin SP_Ext.
  6. Verify OS_Perm Settings: Confirm that OS_Perm.Bit4 = 1 and OS_Perm.Bit5 = 1.
  7. Delta Download: Compile and download changes to the AS station (Changes only).

APL PIDConR Setpoint Control and Mode Configuration Matrix

The following table details the interaction between feature bits, permission bits, and faceplate presentation:

Feature.Bit21OS_Perm.Bit4 / Bit5SP_Ext Connected?Faceplate Setpoint Dropdown StateController Operating Behavior0 (Disabled)Any stateIrrelevantHidden / InvisibleInternal setpoint only (SP_Int); external setpoint ignored1 (Enabled)0 / 0YesVisible but Grayed Out (Disabled)External setpoint active algorithmically; operator cannot switch modes1 (Enabled)1 / 1YesVisible and Fully FunctionalOperator can freely toggle between Internal and External setpoints1 (Enabled)1 / 1No (Open pin)Visible; shows error if switched to ExtOperator can select External, but block uses default initial value

Verification

To verify that external setpoint switching operates correctly:

  1. Open WinCC Faceplate: In WinCC Runtime, open the faceplate for the modified PIDConR block.
  2. Verify Dropdown Display: Confirm that the Setpoint mode selection dropdown is now visible in the standard view header.
  3. Switch to External: Click the dropdown and select External. Confirm that the setpoint value field displays the value provided by the SP_Ext input pin and indicates external control mode.
  4. Switch to Internal: Select Internal. Confirm that the operator can enter an internal setpoint set value manually.
  5. Bumpless Transfer Check: Observe the process value during switchover to confirm that the setpoint transitions smoothly.

Bumpless Transfer and Tracking Characteristics on PIDConR

When configuring external setpoints on PIDConR, consider the related feature bits that govern mode switching dynamics:

  • Feature.Bit23 (Bumpless Switchover to Automatic Mode): When switching from Manual to Automatic mode with an external setpoint, Feature.Bit23 = 1 ensures the internal setpoint tracks the process variable (PV) to prevent sudden control step changes.
  • SP_TrkExt Parameter: If SP_TrkExt = 1, the internal setpoint SP_Int continuously tracks SP_Ext while in external mode, ensuring bumpless reversion if the loop returns to internal setpoint mode.

Limitations and Operational Precautions

Do not activate Feature.Bit21 = 1 unless an active, validated signal is connected to SP_Ext. If an operator switches a controller to external setpoint mode with an unlinked SP_Ext pin, the controller will immediately drive output toward zero or default limits, potentially disrupting the process.

Modifications to Feature bits take effect immediately upon delta download to the AS controller; ensure that external signal values match current plant operating ranges prior to downloading.

Engineer FAQ

Does PIDConL require Feature.Bit21 to use external setpoints?

On PIDConL, Feature.Bit21 also governs external setpoint behavior, but standard library templates often have it enabled by default, unlike customized PIDConR instances.

Can an external setpoint be forced via CFC without operator permission?

Yes. Connecting a digital signal to the input parameter SP_ExtOp = 1 forces the controller into external setpoint mode, overriding operator faceplate selection.

Why did setting OS_Perm bits 4 and 5 not make the button visible?

OS_Perm bits only grant click authorization to an existing UI element. If Feature.Bit21 is 0, the element is not rendered in the faceplate layout.

Does changing Feature.Bit21 require a CPU STOP?

No. Modifying feature parameter bits is a standard online parameter change that can be downloaded via online delta download.

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