Configuring Different Units of Measure in Specific PCS 7 SFC Instances

To assign a different engineering unit of measure to a specific Sequential Function Chart (SFC) instance without modifying the master SFC Type or creating duplicate types, configure the instance-specific characteristics directly within the SFC Instance Editor. In the CFC chart containing the SFC instance, open the instance properties and select the 'Characteristics' (Merkmale) editor tab. Locate the target setpoint, process value, or parameter, and change the assigned unit in the 'Unit' column to the required engineering unit. The SFC instance retains this unique unit in its runtime faceplate and batch recipe interface while sharing the identical state logic of the master SFC Type.

SFC Types vs SFC Instances: Architectural Flexibility

In SIMATIC PCS 7, Sequential Function Charts (SFC) can be implemented either as standalone SFC charts or as standardized SFC Types. Standardizing on SFC Types is an ISA-88 best practice: an SFC Type defines the state machine, sequencers, phases, and technological control strategies for an entire class of equipment (such as Dosing Units, Agitators, or CIP Skids).

A common engineering requirement occurs when multiple physical units share identical sequence logic but operate under different physical dimensions. For example, a master Dosing SFC Type might define mass setpoints in kilograms (kg), but a specific small-scale additive dosing unit requires grams (g), or a bulk tank measures volume in liters (L). Creating separate SFC Types for each measurement unit creates code duplication and engineering maintenance overhead.

The Role of the Characteristics Editor in SFC Instances

PCS 7 SFC provides the Characteristics Editor (Merkmals-Editor) specifically to allow instance-level customization without breaking type inheritance:

Type Definition: The SFC Type defines the existence of characteristics—such as Setpoints (Sollwerte), Process Values (Istwerte), Control Values, and Timers—along with default limits and default engineering units.
Instance Customization: When an SFC Type is instantiated into a CFC chart, the instance inherits all sequencers and logic. However, the engineer can open the instance's Characteristics view and override operational properties: specific engineering units, operating limits, and initial values can be tailored exclusively for that instance.
Type-Instance Synchronization: When the master SFC Type is subsequently updated with new logic steps or transitions, updating block types in the project synchronizes the logic across all instances while preserving the customized instance units.

Customizable Instance Attributes in PCS 7 SFC

The table below illustrates which SFC attributes are locked to the master type versus those that can be customized per instance.

SFC Technological ElementMaster SFC Type PropertyInstance Customizable?Where OverriddenEngineering Unit of MeasureMaster default (e.g., kg)YESSFC Instance Characteristics Editor -> Unit columnHigh / Low Operating LimitsMaster default limitsYESSFC Instance Characteristics Editor -> Low Limit / High LimitDefault Setpoint ValueMaster default setpointYESSFC Instance Characteristics Editor -> Initial ValueSequencer Step Logic & TransitionsMaster sequential codeNO (Locked)Defined exclusively in Master SFC TypeOperating Modes (Auto/Manual)Master state machineNO (Locked)Governed by standard SFC runtime state machineSIMATIC BATCH Interface CategoryEPH / EOP classificationNO (Locked)Configured in Master SFC Type header

Comparison of SFC Type Level vs SFC Instance Level Properties

The table below contrasts the scope and impact of configuring properties at the Type level versus the Instance level.

Comparison DimensionSFC Type Definition (Master Data Library)SFC Instance (Plant Hierarchy / CFC)Scope of ImpactImpacts every instance throughout the multiprojectRestricted exclusively to the selected equipment instanceLogic ModificationAllows adding/removing steps, transitions, and actionsRead-only; cannot alter internal sequencer logicUnit CustomizationEstablishes the global default unit for new instancesAdapts the unit for specific physical instrumentationCompilation RequirementRequires type compilation and block type updateRequires CFC compilation (Changes Only)WinCC Faceplate DisplaySets default unit placeholder in typical templatesWinCC faceplate dynamically displays instance unit

Step-by-Step Procedure to Change Units of Measure in an SFC Instance

Follow this procedure to assign a unique unit of measure to a specific SFC instance:

Locate the SFC Instance in CFC:
- In SIMATIC Manager, navigate to the CFC chart containing the instantiated SFC Type.
- Double-click the SFC instance block header (or right-click and select Open).
Open the Characteristics Editor:
- In the SFC Instance window, click the Characteristics icon on the toolbar (or select Edit -> Characteristics).
- The editor displays a table listing all configured setpoints, parameters, and process values.
Modify the Engineering Unit:
- Locate the target parameter line (e.g., Setpoint DoseAmount).
- Click in the Unit column: a dropdown list appears displaying standard engineering units from the STEP 7 unit table.
- Select the desired unit (e.g., change from kg to g or L).
- Adjust Low Limit and High Limit if the scale factor changed (e.g., change 0..100 kg to 0..100000 g).
- Click Apply and close the editor.
Compile and Download:
- Perform a delta compilation of the CFC charts and download to the AS controller.
- Compile the OS (Changes Only) to propagate the updated unit to WinCC tag management and SFC faceplates.

Verification

Verify instance unit customization through the following checks:
1. In the SFC Instance Characteristics Editor, confirm that the Unit column retains the customized value.
2. In the CFC chart, inspect the instance block pins: confirm that the unit string attribute on the setpoint pin reflects the new unit.
3. In WinCC Runtime, open the SFC instance faceplate: verify that the setpoint and actual value fields display the customized unit label.
4. In SIMATIC BATCH (if used): verify that the recipe phase allocated to this unit displays the updated unit of measure in the recipe editor.

Limitations and Operational Precautions

Changing the engineering unit in the Characteristics Editor alters the display string and engineering unit code; it does NOT automatically scale underlying mathematical calculations! If downstream valve pacing or pump timing inside the SFC type was coded assuming mass in kilograms, converting the unit to grams without scaling the raw setpoint in logic will result in a 1,000-fold dosing error. Always ensure that the physical instrumentation, scaling blocks, and calculation formulas match the selected unit.

Engineer FAQ

Will updating block types from the Master Data Library overwrite my custom instance unit?

No. When you update SFC Types from the Master Data Library, SIMATIC Manager preserves instance-specific characteristic values, units, and limits while updating sequencer logic and block interfaces.