Understanding Parameter Read Back in SIMATIC PCS 7 CFC and Preventing Data Loss During Entire Downloads

In SIMATIC PCS 7, the Continuous Function Chart (CFC) 'Read Back' (Rücklesen) function transfers operational parameter values—specifically setpoints, alarm limits, and controller tuning parameters modified online by operators or commissioning engineers via WinCC faceplates—from the Automation Station's (AS) live Data Blocks (DB) back into the offline CFC charts. If an engineer performs a full compilation and 'Entire program' download without first executing a Read Back, the offline CFC chart defaults permanently overwrite the controller's runtime memory, erasing all field-tuned values. To preserve plant tuning, engineers must select 'Chart > Read Back...', choose 'OCM-capable parameters' or 'Parameters designated for read back', recompile the charts, and then download.

Online vs Offline Parameter Divergence in SIMATIC PCS 7

In an operational process plant, automation controllers execute continuously while operators and process engineers adjust control parameters in real time. Through WinCC faceplates, users modify:
- PID Controller tuning constants: Proportional Gain (Gain), Integral Time (TI), Derivative Time (TD).
- Alarm and warning thresholds: High-High (PV_AH_Lim), High (PV_WH_Lim), Low (PV_WL_Lim), Low-Low (PV_AL_Lim).
- Operating setpoints and operational timers.

These live modifications are written directly into the S7-400 CPU's active Data Blocks (instance DBs) in controller RAM. Crucially, these changes do NOT automatically write back to the offline CFC charts on the Engineering Station.

Over months of operation, the offline engineering project and the live running controller diverge substantially. If an engineer opens the project on the ES and initiates a full program download (Download > Entire program), the engineering system generates fresh instance DBs initialized with the obsolete offline CFC values, overwriting plant tuning and potentially destabilizing active chemical or thermal processes.

Mechanics of the CFC Read Back Function

To eliminate parameter divergence and protect plant tuning, SIMATIC PCS 7 provides the Read Back utility in the CFC editor (Chart > Read Back...).

The Read Back mechanism connects to the target online AS controller over Industrial Ethernet, reads the current values of block input parameters from the CPU's instance DBs, and updates the corresponding block inputs within the offline CFC charts (.s7p database).

Once the Read Back completes:
1. The offline CFC charts reflect the exact operational values currently running in the plant.
2. The engineer can safely archive the project, knowing that the backup contains live field tuning.
3. Subsequent full program compiles and entire downloads will maintain existing process setpoints.

Read Back Scope Selection: OCM vs Designated vs All Parameters

When initiating a Read Back in the CFC editor, the engineer must select the appropriate parameter scope from three distinct options:

Option 1: OCM-Capable Parameters

This is the standard and recommended choice for general plant maintenance:
- Reads back all block inputs possessing the system attribute s7_m_c = 'true' (Operator Control and Monitoring).
- Captures all faceplate-adjustable setpoints, alarm limits, manual/auto states, and tuning parameters.

Option 2: Parameters Designated for Read Back

Reads back parameters possessing the system attribute s7_read_back = 'true'.
Used by advanced block library authors to designate internal algorithmic calibration variables that should be backed up even if they are not exposed to WinCC faceplates.

Option 3: All Parameters

Reads back every non-interconnected block input in the entire S7 program.
Use caution: Selecting 'All parameters' can overwrite engineering defaults on thousands of internal algorithmic pins with transient runtime states.

Why Parameters Can Revert to Defaults Following Download

Engineers frequently report that despite executing a Read Back, downloading the program caused values to revert to defaults. This occurs due to three specific engineering traps:

Interconnected Parameters Cannot Be Overwritten by Read Back: If a block parameter (e.g., Gain or SP_Ext) is connected to another block's output in CFC, the Read Back utility reads the online value, but cannot decouple the physical graphical connection. When the CPU starts, the upstream block evaluates its logic and immediately overwrites the input with its computed output!
Failure to Compile After Read Back: A Read Back updates the CFC graphical database, but does NOT automatically update the compiled S7 block container. If an engineer performs a Read Back and immediately clicks Download without compiling, the compiler downloads the pre-existing compiled blocks!
Reading Back from the Wrong Controller / Offline Project Mismatch: If block types were added or deleted offline prior to reading back, the instance DB addresses in the offline database may not match the online DB numbering, leading to misaligned data reads.

CFC Read Back Configuration and Parameter Matrix

The following matrix details the three Read Back operational modes:

Read Back Mode SelectionTarget Block Parameter AttributesTypical Parameters IncludedRisk / Operational ImpactOCM-capable parameterss7_m_c := 'true'Faceplate setpoints, alarm limits, PID Gain/TI/TDRecommended: Safe, captures all operator tuningDesignated for read backs7_read_back := 'true'Specialized calibration coefficients, tare valuesSafe; targeted at library-defined tuning valuesAll parametersAll inputs without signal interconnectionsStatic configuration bits, sample times, internal filtersModerate risk: Overwrites structural block defaultsInterconnected ParametersAny attribute with active signal linkParameters driven by upstream calculation blocksIgnored: Upstream block overwrites value upon execution

Verification

To confirm that parameter read back executed successfully:
1. Identify a test block in CFC (e.g., PIDConL). Note its offline Gain value (e.g., 1.0).
2. Online via WinCC faceplate, adjust the Gain to 2.5.
3. In CFC Editor, select Chart > Read Back.... Select OCM-capable parameters and click OK.
4. Open the CFC chart offline: Confirm that PIDConL.Gain now displays 2.5 in white offline text.
5. Execute Compile Charts as Program with 'Changes only'.
6. Download changes to the AS controller: Confirm that the online value remains 2.5 without interruption.

Limitations and Operational Precautions

Observe the following operational constraints:
- Never Download Entire Program in RUN Without Pre-Checks: An entire program download requires stopping non-redundant CPUs. Even on fault-tolerant systems, full downloads should only occur during planned maintenance windows.
- Save Multiproject Before Reading Back: Always create a clean archive of the multiproject before executing a Read Back. If an accidental Read Back captures erroneous transient values (e.g., during sensor calibration), you can revert to the archived baseline.
- SFC Chart Read Back: SFC step timers and transition flags do not participate in standard CFC parameter read back; only block instance parameters are captured.

Engineer FAQ

Why did my setpoint revert to default after reading back and downloading?

This happens if the setpoint parameter has an active signal interconnection from another block in CFC, or if you downloaded without first recompiling the charts following the Read Back operation.

Can I read back parameters while the plant is actively running?

Yes. Read Back is a non-invasive, read-only communication request that reads data blocks over Industrial Ethernet without affecting CPU execution or process control.

What is the difference between s7_m_c and s7_read_back?

s7_m_c flags a parameter for Operator Control and Monitoring (creating WinCC tags and faceplates). s7_read_back specifically flags a parameter for the CFC Read Back utility, even if it is not exposed to WinCC.