Executing CFC Readback After Engineering Changes - Preserving Field-Modified OCM Parameters in SIMATIC PCS 7

To transfer modified CFC charts from a Factory Acceptance Test (FAT) backup into a live running plant project without overwriting online OCM parameters (such as alarm limits, PID parameters, and setpoints tuned in the field), follow a structured four-stage readback workflow. First, connect to the online AS and execute 'Chart > Read Back... > OCM-capable parameters only' on the live plant project to synchronize offline charts with field tuning; archive this baseline. Second, copy or cut the modified charts from the FAT project into the plant project. Third, perform a second Read Back on the updated plant project; the engineering system will read the live controller memory and overwrite default values in the newly pasted charts with the exact online values. Fourth, execute 'Target System > Download > Changes only' to deploy logic changes bumplessly.

The Engineering Concurrency Dilemma: FAT Modifications vs Live Plant Tuning

In operational industrial plants, automation engineering faces a continuous synchronization dilemma. An engineer takes an archive backup of the multiproject to develop new functionality or test modifications during a Factory Acceptance Test (FAT) on an offline simulator. Meanwhile, the real plant continues running 24/7. During this interval, control room operators and process engineers continuously tune parameters via WinCC faceplates—adjusting PID gains (Gain, TI), alarm thresholds (SP_HiLim, PV_AH_Lim), filter time constants, and valve ramp rates.

If the engineer simply copies the modified charts from the FAT project back into the plant project and executes a download, the offline default values from the FAT environment will overwrite months of field tuning, potentially causing catastrophic process upsets or plant trips. SIMATIC PCS 7 resolves this through CFC Parameter Readback ('Rücklesen').

Mechanics of CFC Readback: 'All Parameters' vs 'OCM-Capable Parameters Only'

The CFC Editor provides a dedicated online upload function (Chart > Read Back...) that queries the CPU's active load/work memory and writes current runtime values into the offline CFC chart database. The dialog presents two distinct scope options:

All Parameters: Uploads the current value of every single block parameter in the AS program. Caution: This includes internal state variables, calculation flags, and intermediate counters, which can cause erratic behavior if downloaded back into different execution cycles.
OCM-Capable Parameters Only (Recommended): Restricts readback strictly to parameters tagged for Operator Control and Monitoring. These are the exact operational values that plant operators and engineers tune from WinCC faceplates.

System Attributes Governing Readback (S7_m_c and S7_read_back)

The readback engine evaluates system attributes defined in block declarations:

S7_m_c := 'true': Identifies parameters exposed to WinCC OS for operator control. By default, all OCM parameters are included in OCM readback.
S7_read_back := 'true': An explicit compiler pragma assigned to inputs/outputs indicating that the parameter must be included in readback operations, even if S7_m_c is false.
Parameters that are interconnected in CFC (e.g., fed by an upstream block output) are excluded from readback overwrites because their values are cyclically calculated by the upstream block rather than held as static parameters.

The Four-Stage Safe Readback and Chart Integration Workflow

To integrate modified charts from an offline FAT project into a running plant project without data loss, follow this standardized four-stage protocol:

Stage 1 (Baseline Readback & Archive): On the live plant project, execute Read Back for OCM parameters. Archive the project (Plant_BeforeMerge.zip). This guarantees that if anything goes wrong, a 100% accurate snapshot of live plant tuning exists.
Stage 2 (Chart Transfer): Copy or cut the modified charts from the FAT project into the plant project, replacing old charts.
Stage 3 (Secondary Readback): Execute Read Back again on the plant project. Because the block instance names match the running CPU, the readback engine immediately queries the online CPU and populates the newly pasted charts with the live field values!
Stage 4 (Changes Download): Perform a CFC Changes Download to the running CPU. The new algorithmic logic is downloaded while all field-tuned parameters remain completely undisturbed.

Comparative Workflow Matrix: Parameter Preservation Methods

The following table evaluates methods for managing parameter updates during offline chart integration in PCS 7:

MethodRisk to Field TuningEngineering EffortCPU STOP Required?SuitabilityDirect Cut/Paste & DownloadExtreme (All tuning wiped)MinimalNoUnacceptable in live plantsManual Parameter Copy from FaceplatesHigh (Human transcription error)Extremely HighNoUnreliable for >5 blocksStandard 4-Stage CFC ReadbackZero (Automated reconciliation)ModerateNo (Changes Download in RUN)Recommended Industry StandardFull Upload to New StationModerate (Loses chart formatting)HighNoDisaster recovery only

Step-by-Step Procedure to Read Back and Merge Modified Charts

Follow this exact sequence in SIMATIC Manager and CFC Editor:

Read Back the Live Plant Project:
- Open the live plant project connected to the operational AS.
- Open any CFC chart and select Chart > Read Back...
- Select scope: 'OCM-capable parameters'.
- Click OK. The progress bar reads back all live values from CPU memory.
- Archive the project (File > Archive).
Paste Modified FAT Charts:
- Open the FAT project in a secondary SIMATIC Manager window.
- Copy the modified charts (e.g., 12 modified charts).
- Paste them into the live plant project's chart folder, confirming overwrite of existing charts.
Execute Secondary Readback on Plant Project:
- In the live plant project, select Chart > Read Back...
- Select 'OCM-capable parameters'.
- The compiler reconciles the pasted charts against the live AS memory, overwriting FAT test defaults with active plant values.
Compile and Download Changes:
- Select CFC > Compile > Changes only.
- Select Target System > Download > Changes only.
- Confirm that the download finishes in RUN mode without process disturbance.

Verification

Verify parameter integrity through these checks:

Open one of the modified charts in CFC Test Mode (Ctrl+T).
Inspect critical tuned parameters (e.g., PIDConL.Gain, TI, SP_LiOp). Confirm that values match the control room operator faceplates.
Verify that the new logic functions introduced during FAT operate as designed.
Confirm that the CPU remains in RUN and no alarm spikes occur in WinCC AlarmControl following download.

Limitations and Operational Precautions

Renamed Blocks: Readback matches parameters based on exact block instance names. If you renamed block instances in the FAT version, readback cannot match them to the old online DBs, and parameters will revert to defaults.
Structural Interface Changes: If you modified the interface of an FB (added/removed pins), readback must be performed before compiling new block types, otherwise interface mismatches will prevent online memory mapping.
SFC State Data: Readback applies to continuous CFC parameters. Active SFC sequence step pointers cannot be read back; never download SFC charts while critical batch sequences are running.

Engineer FAQ

What happens if I select 'All parameters' during Read Back?

Selecting 'All parameters' reads back every internal state variable and diagnostic flag. While technically comprehensive, it can inadvertently lock dynamic algorithm states that should be allowed to initialize fresh upon chart execution.

Does Read Back require stopping the CPU?

No. Read Back is a non-intrusive online upload operation executed while the S7-400 CPU is running at full production.

Can I read back parameters for a single chart instead of the whole AS?

No. In standard PCS 7 CFC, Read Back operates at the S7 Program / Chart Folder level, reading back all charts assigned to that CPU.

Where is the official Siemens guidance for parameter readback?

Detailed instructions are documented in the PCS 7 Compendium Part A - Configuration Guideline (Section 'Downloading the user program') and Engineering System manual Section 10.

Why are interconnected block inputs not updated during Read Back?

Interconnected inputs receive their values dynamically from upstream block outputs during cyclic CPU execution. Writing an offline value to an interconnected pin is meaningless because the connection immediately overwrites it on the next scan.