APL Parameter Arbitration: Faceplate Operator Control vs Interconnection in PIDConL Ramping

In SIMATIC PCS 7 Advanced Process Library (APL), parameters on blocks like PIDConL (such as the setpoint ramping inputs RmpOn and RmpTarget) do not possess native Linking vs Operator (LiOp) arbitration switches. If an input pin is hard-wired in CFC, the cyclic chart scan overwrites operator faceplate modifications on every execution cycle. To allow operators to control ramping from the WinCC faceplate during normal operation while enabling CFC logic to override ramping during emergency events, insert APL operator interface blocks (OpAnL and OpDi01) which feature native LiOp parameters in front of the ramping inputs, or route the emergency ramp through the external setpoint interface (SP_Ext with SP_LiOp) using an external ramp generator (RampP).

APL Parameter Architecture: Cyclic Overwrite Rules in CFC

A core operational principle of SIMATIC PCS 7 Continuous Function Charts (CFC) is the cyclic data flow model. When an algorithm executes within an Organization Block (e.g. OB35 - 100 ms), the controller evaluates block interconnections from left to right. If an input parameter on a block is interconnected to another block's output, the CFC execution engine unconditionally writes the source value into the target instance DB on every scan cycle.

If an operator attempts to change that same parameter from a WinCC faceplate, WinCC writes the new value into controller RAM asynchronously. However, on the very next 100 ms OB35 execution cycle, the CFC chart interconnection overwrites the operator's value back to the wired source. The faceplate appears interactive, but operator setpoints are obliterated within milliseconds.

The Missing LiOp Switch on PIDConL Ramping Parameters

To accommodate both operator control and programmatic interlocking, standard APL blocks provide Linking vs Operator (LiOp) switches for primary operational modes:

ModLiOp: Arbitrates Manual vs Automatic mode selection (Operator faceplate vs CFC logic).
SP_LiOp: Arbitrates Internal vs External setpoint selection.

However, secondary technological functions—specifically the Setpoint Ramping function on PIDConL (RmpOn to activate ramping and RmpTarget to set the ramp destination)—were designed primarily as operator convenience tools. Siemens did not provide a dedicated RmpLiOp switch on PIDConL. Consequently, hardwiring emergency ramping logic in CFC permanently locks out operator faceplate manipulation.

Architecture 1: Operator Interface Blocks (OpAnL / OpDi01) with Native LiOp

The most elegant method to restore operator faceplate access without modifying the standard APL library employs Siemens APL Operator Interface blocks:

OpAnL (Checkback and Operator Analog): Inserted in CFC ahead of PIDConL.RmpTarget.
OpDi01 (Digital Operator Block): Inserted in CFC ahead of PIDConL.RmpOn.

Both OpAnL and OpDi01 incorporate native LiOp parameters:

Normal Operation (LiOp = 0): The operator controls the ramp target setpoint and start/stop buttons directly from the OpAnL/OpDi01 faceplate on WinCC. The blocks pass these operator values through to PIDConL.
Emergency Override (LiOp = 1): When an emergency event or supervisory sequence trips, CFC logic asserts LiOp = 1, disabling operator buttons and forcing the pre-programmed emergency ramp setpoint into PIDConL.

Architecture 2: External Setpoint Override via SP_Ext and RampP

An alternative proven architecture segregates normal operator ramping from emergency ramping by utilizing the native external setpoint channel:

Normal Plant Operation: PIDConL operates with internal setpoint (SP_ExtAct = 0). The operator uses the standard native PIDConL faceplate ramping function (RmpOn, RmpTarget, RmpModTime) without any CFC interconnections blocking the pins.
Emergency Ramping via RampP: In CFC, an APL ramp profile generator block RampP is configured with emergency ramp rates (e.g., controlled reactor cooling slope). The output of RampP.Out is connected to PIDConL.SP_Ext.
Automatic Transfer: When the emergency condition occurs, safety logic asserts PIDConL.SP_ExtLi = 1 (with SP_LiOp = 1). PIDConL switches bumplessly to SP_Ext, following the emergency ramp profile flawlessly. Once the emergency clears, logic returns the controller to operator control.

Comparative Architecture Matrix: APL Input Arbitration Methods

The following table compares architectural methods for arbitrating operator vs logic control:

Architectural StrategyImplementation ComplexityOperator Faceplate ExperienceCFC Logic Override CapabilityBest Applied InDirect CFC InterconnectionMinimal (1 wire)Broken; faceplate overwritten in 100ms100% logic control onlyAutomated sequences without operator roleArchitecture 1: OpAnL / OpDi01Low (Insert 2 helper blocks)Full faceplate control via Op-blocksComplete override via native LiOpMulti-mode setpoint and ramp adjustmentsArchitecture 2: SP_Ext + RampPMedium (Configure RampP block)Standard native PID faceplate rampClean bumpless emergency switchoverCritical thermal/pressure emergency cooldownCustom Logic MultiplexingHigh (Custom logic gates)Confusing; faceplate buttons remain openOverride possible; visual lagNon-standard legacy workarounds

Step-by-Step CFC Implementation Workflow

To implement Architecture 2 (External Setpoint Ramp Override), follow this procedure:

Open the CFC chart containing the PIDConL controller.
Insert block RampP (FB1886) from the APL library.
Configure RampP with the emergency ramp rate (RampRate) and emergency target setpoint.
Interconnect RampP.Out to PIDConL.SP_Ext.
Interconnect your emergency trip signal to RampP.Start and to PIDConL.SP_ExtLi.
Set PIDConL.SP_LiOp = 1 to authorize logic-based setpoint switching.
Leave parameters PIDConL.RmpOn and PIDConL.RmpTarget completely disconnected in CFC, allowing operators full interactive control in normal operation.

Verification

Verify parameter arbitration in CFC test mode and WinCC Runtime:

Open the PIDConL faceplate in WinCC Runtime during normal operation. Verify that the controller runs on SP_Int.
Open the Ramp view in the faceplate; enter a target setpoint and click Ramp On. Verify that the setpoint ramps smoothly without being overwritten.
In CFC test mode, simulate the emergency condition by asserting the trip signal.
Confirm that PIDConL immediately switches to SP_Ext, begins following the emergency RampP profile, and locks out operator setpoint edits.
Clear the emergency signal; verify bumpless return to operator-selected setpoint mode.

Limitations and Operational Precautions

Adhere to the following operational precautions:

Bumpless Setpoint Transitions: Configure SP_TrkExt = 1 or proper tracking options on PIDConL to prevent process setpoint steps when transitioning between emergency ramps and manual operation.
Never Modify Standard APL Blocks: Do not attempt to open and edit standard APL blocks (FB1874) in SCL to add custom LiOp pins; modifying library blocks breaks manufacturer version support and invalidates future library migrations.
Operator Notification: Ensure that when logic overrides operator ramping, a clear status message or alarm appears on WinCC to alert operators that automated safety profiles are active.

Engineer FAQ

Can I use OpAnL to arbitrate operator setpoints vs logic?

Yes. OpAnL provides native LiOp switching; when LiOp = 0, operators adjust the value from the faceplate; when LiOp = 1, CFC logic forces the setpoint.

What happens if both RmpOn and RmpTarget are interconnected in CFC?

The operator can still open the ramping dialog, but any modification made by the operator is overwritten within 100 ms by the cyclic CFC chart link.

Does switching from an emergency external ramp back to internal setpoint cause a process bump?

Not if bumpless tracking is configured. By enabling setpoint tracking (SP_TrkExt), the internal setpoint tracks the external setpoint, ensuring smooth transitions.