How the Intlock Input Governs PIDConL Manipulated Variables and Feature Bit Behaviors in PCS 7 APL

In the PCS 7 Advanced Process Library (APL), the Intlock (Interlock without reset) input on PIDConL serves as an operational trip condition that immediately suspends closed-loop control when unfulfilled (Intlock = 0). By default, when Intlock drops to 0, the controller drives its manipulated variable (MV) to 0.0% (or safe position) and locks the faceplate against manual operator setpoints. This default behavior can be customized using APL Feature bits—specifically Feature Bit 1 (reaction to interlock), which determines whether the controller transfers to safe position, freezes at the last valid value, or moves to a neutral value—while manual forcing (MV_ForOn / MV_Force) can override the locked state under supervisory conditions.

Functional Significance of the Intlock Input on PIDConL

In the SIMATIC PCS 7 Advanced Process Library, the continuous controller block PIDConL (FB1874) incorporates built-in equipment protection logic via standardized interlock inputs:
* Intlock (Interlock Without Reset): Represents an operational interlock condition.
* Unlike basic start permits (Permit), which only act before equipment starts, Intlock monitors operating boundaries continuously during active closed-loop runtime.
* If upstream process conditions become hazardous (e.g., burner flame failure on a temperature loop, or extreme low suction pressure on a pump speed loop), Intlock goes to 0 (unfulfilled).
* The controller algorithm immediately aborts normal closed-loop PID error calculation to protect the mechanical actuator and downstream process.

Manipulated Variable (MV) State When Intlock = 0

When Intlock.Value = 0 (with valid signal status ST = 16#80):
1. Manipulated Variable Output:
* By default, the manipulated variable output MV is driven immediately to 0.0% (or the configured safe resting value SafePos).
* The internal integral action is halted and frozen to prevent mathematical windup.
2. Faceplate State in WinCC Runtime:
* The controller faceplate displays a prominent yellow Interlock active icon.
* Operator manipulation buttons (Manual setpoint entry, Auto/Manual toggles) are visually greyed out and locked. Operators cannot override the interlock from the HMI.
3. Automatic Restoration:
* Because Intlock is an interlock without reset, as soon as process conditions clear and Intlock returns to 1, the block automatically exits the interlocked state and resumes active control.

Configuring Interlock Reactions via APL Feature Bits

Engineers can customize how PIDConL responds to an active interlock by configuring the 32-bit double word Feature:

Feature Bit 1 (Reaction to Interlock State):
Feature.Bit1 = 0 (Default): The controller drives MV to safe position (0.0%) and locks out operator commands.
Feature.Bit1 = 1: The controller freezes MV at its last valid operating value (Hold Last Value). This is vital in thermal processes where cutting valve setpoint to zero would cause catastrophic thermal shock or flame blowout.
Feature Bit 0 (Semantics of Interlock Blocks):
Governs whether the input signal is evaluated with standard boolean logic or inverted safety logic.
Feature Bit 24 (Bumpless Transfer to Auto After Interlock Clearance):
Ensures that when Intlock returns to 1, the controller resumes automatic control smoothly without output step jumps.

Overriding Interlocks with Forced Manipulated Variables (MV_Force)

Under specific plant startup or emergency scenarios, supervisory logic must override an active interlock:

PIDConL includes dedicated forcing pins:
MV_ForOn: Forces the controller output when set to TRUE.
MV_Force: Real value applied to MV during forcing.
Execution Priority: In standard APL priority hierarchy:
Safety Interlock (Protect) > Manual Forcing (MV_ForOn) > Operational Interlock (Intlock) > Operator Manual Mode > Automatic Control
If MV_ForOn = 1, the controller applies MV_Force directly to MV, overriding an unfulfilled Intlock = 0 condition, provided higher-level safety interlocks (Protect) are clear.

Comparison of PIDConL Interlock and Tracking Inputs

The table below compares the behavioral characteristics of interlock, tracking, and forcing inputs on PIDConL.

Block ParameterPurpose / TriggerResulting MV BehaviorOperator Overwritable?Requires Reset?IntlockOperational trip conditionDriven to 0.0% or held at last valueNo (Locked out)No (Auto-resumes)ProtectSafety interlockDriven to safe positionNo (Locked out)Yes (Operator reset)MV_TrkOnActuator trackingClamped to MV_Trk valueNo (Tracking mode)NoMV_ForOnSupervisory forcingClamped to MV_Force valueOverrides IntlockNoManModLiSwitch to ManualHanded over to manual setpointYes (Manual control)No

Verification

To confirm that Intlock governs PIDConL properly:
1. In CFC test mode, verify that PIDConL is controlling in Auto mode with MV = 55.0%.
2. Force Intlock.Value = 0 (ensure ST = 16#80): observe that MV drops immediately to 0.0% (or holds 55% if Feature Bit 1 is enabled).
3. In WinCC Runtime, open the faceplate: verify that the yellow interlock icon displays and setpoint input fields are disabled.
4. Attempt to write a manual setpoint from the faceplate: confirm the command is rejected.
5. Return Intlock.Value = 1: confirm that the interlock icon clears and the controller resumes automatic closed-loop control.

Limitations and Operational Precautions

Never connect raw boolean inputs without valid signal status to Intlock; entering 0 without status 16#80 causes the block to report an invalid status fault rather than an interlock trip.
If configuring Feature Bit 1 to 'Hold Last Value', ensure thermal runaway protection is guaranteed by an independent hardwired safety system.
Do not use Intlock for personnel or environmental safety trips; safety-critical functions must be implemented using Protect in safety F-CFC charts.
Modifying Feature bits online requires caution; changing bit values can immediately alter active control responses.

Engineer FAQ

Why did setting Intlock = 0 not move the valve to zero?

Check the setting of Feature.Bit1. If Feature Bit 1 is set to TRUE, PIDConL is configured to 'Hold Last Value' during an interlock, keeping the output at its current operating position rather than driving to zero.

Can an operator in WinCC override an active Intlock?

No. When Intlock = 0, the block algorithm enforces an absolute lockout of operator manual setpoints on the faceplate to protect process equipment.

What is the difference between Intlock and Protect on PIDConL?

Intlock is self-resetting: when the interlock condition clears, the controller automatically resumes operation. Protect is latching: even after conditions clear, the controller remains locked out until an operator manually presses the 'Reset' button on the faceplate.