C135 PCS 7 (PCS7)
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Understanding Readback Behavior for APL Operator Control Blocks in PCS 7

Understanding Readback Behavior for APL Operator Control Blocks in PCS 7

In SIMATIC PCS 7, the Chart Readback (Rücklesen) function behaves differently between analog and digital operator control blocks due to fundamental differences in state storage architecture. While analog operator blocks like OpAnL store operator setpoints in static, readback-capable parameters (SP_LiOp, SP_Int), digital operator control blocks like OpDi01 and OpDi03 operate on transient pulse commands rather than holding static setpoint variables. When an engineer executes Chart -> Readback in SIMATIC Manager, the compiler reads parameters marked with the S7_read_back system attribute. Because OpDi01 and OpDi03 process momentary operator toggles to command external technological logic, the current plant state resides in the downstream equipment block rather than the operator block itself.

Table of Contents

Architecture of the CFC Chart Readback (Rücklesen) Function

In continuous process automation, operators frequently fine-tune operational settings directly from WinCC runtime faceplates:
- Online Alterations: Tuning PID gains, modifying high/low alarm limits, adjusting analog dosing setpoints, or toggling operating modes.
- The Reconciliation Challenge: These adjustments are written directly into online CPU data blocks (instance DBs). If an engineer subsequently performs a full program compile and download from the offline project, the online adjustments would be completely overwritten by older offline initial values!
- The Solution (Rücklesen): The Readback utility queries the online controller, extracts live parameter values from active data blocks, and writes them back into the offline CFC charts in SIMATIC Manager.

The S7_read_back Attribute and Parameter Classification

The CFC Readback wizard does not blindly copy every single byte of controller memory. It evaluates system attributes declared in the block type interface:
- S7_read_back := 'true': Only block parameters stamped with this attribute are eligible for readback.
- Filter Categories in the Readback Dialog:
1. All Parameters: Reads all readback-flagged parameters.
2. OCM (Operator Control and Monitoring) Parameters: Reads only parameters tagged with S7_m_c := 'true' that are operated from WinCC.
3. Designated Parameters: Reads only specific engineering variables manually flagged for capture.

Comparing Analog Operator Blocks (OpAnL) vs. Digital Blocks (OpDi01/03)

The Advanced Process Library differentiates how analog values and digital commands are handled:

Block TypePrimary FunctionInternal Memory ModelReadback BehaviorOpAnL (FB1865)Analog operator setpoint generationStores persistent setpoint value in static parameter SP_Int with S7_read_back := 'true'.Fully Supported: The current online setpoint is successfully read back into the offline CFC chart.OpDi01 (FB1866)Digital 1-out-of-2 operator controlTransient command block. Evaluates momentary toggle commands (SetOp, RstOp) and generates pulses.Not Supported: Contains no static state parameter flagged for readback.OpDi03 (FB1867)Digital 1-out-of-3 operator selectionMulti-state selection block (e.g., Off / Speed 1 / Speed 2). Outputs momentary selection pulses.Not Supported: State is maintained dynamically or tracked via external feedback.

Why Digital Operator Blocks Lack Static Readback Parameters

Engineers frequently question why Siemens designed OpDi01 and OpDi03 without static readback parameters:
* The Pulse Command Philosophy: In process safety and DCS design, digital operator blocks are intended to command external equipment—such as a motor starter, a route diverter, or a sequence trigger.
* State Ownership Belongs to the Drive: The true state of the equipment resides in the physical contactor feedback and the technological block (MotL, VlvL). If OpDi01 held a static latched state that was read back and downloaded during a restart, the controller could unintentionally restart field machinery upon CPU reboot!
* Feedback Tracking: OpDi01 provides an input pin Feedback. If connected to the actual running confirmation of the downstream equipment, the faceplate dynamically reflects the true plant state without needing to store an internal static latch.

Method: Capturing and Synchronizing Operator Settings into the S7 Program

To ensure that valid operational tuning and operator setpoints are safely synchronized into your offline project before maintenance downloads:

Step 1: Open the Readback Wizard in SIMATIC Manager

  1. In SIMATIC Manager, select the Chart folder of the AS station.
  2. Select menu Chart -> Read Back....
  3. The Readback configuration dialog opens.

Step 2: Configure Readback Scope

  1. In the dialog, select Read back from: CPU.
  2. Under Scope, select:
    - Operator control and monitoring (OCM) possible: Captures operator-entered setpoints, alarm limits, and modes from APL blocks.
  3. Select whether to read back All charts or Selected charts.
  4. Click OK.
  5. STEP 7 connects online to the CPU, reads parameter values, and overwrites the initial values in the offline CFC database.

Step 3: Handle Digital Operator States via Feedback Interconnection

If your application requires a digital operator block to remember its active state across restarts and downloads:
1. Do not rely on readback for OpDi01.
2. Connect the output of OpDi01.Out to a memory latch block (e.g., FlipFlop or a technological block with persistent memory).
3. Wire the latched running state back into OpDi01.Feedback.
4. Result: Upon CPU download or restart, OpDi01 immediately interrogates Feedback and renders the correct active state on the WinCC faceplate.

Verification

To verify that parameter readback and digital states operate correctly:
- In WinCC Runtime, modify an analog setpoint on an OpAnL faceplate (e.g., change from $50.0$ to $75.0$).
- In SIMATIC Manager, execute Chart -> Read Back... (OCM possible).
- Open the offline CFC chart containing OpAnL: confirm that parameter SP_Int now displays 75.0.
- Perform a CFC Change Download to the AS CPU: confirm that the online setpoint remains rigidly at $75.0$ without reverting.
- Verify digital state tracking: toggle an OpDi01 command: verify that the faceplate status icon reflects the active state derived from the Feedback pin.

Limitations and Operational Precautions

  • Readback Overwrites Offline Engineering Edits: If an engineer made un-downloaded configuration edits in offline CFC charts, executing a Chart Readback will overwrite those values with whatever is currently running in the physical CPU. Always verify whether the online CPU or offline project holds the intended master data.
  • Readback During Plant Transients: Avoid executing Chart Readback during active chemical batch transitions or during equipment calibration, as transient setpoints may be captured permanently.
  • Instance DB Sizing: The Readback utility requires that the offline block type interfaces match the online instance DB structures exactly. If block types were modified offline without downloading, Readback will abort with interface mismatch errors.

Engineer FAQ

Can I manually add S7_read_back := 'true' to OpDi01?

You can technically edit the SCL source code in the system library and recompile, but this is strongly discouraged. Siemens library blocks are certified; modifying standard block attributes can cause unpredictable faceplate behavior and will be overwritten during future library updates.

Which block should I use if I need a digital parameter that supports readback?

Use the OpSt00 / OpSt01 (Operator State) blocks or use a standard integer selection block (OpDi03) where mode selection is tied to static parameters, or store the state in a standard data block variable with S7_read_back.

Why does Chart Readback take several minutes to complete?

The utility queries hundreds of individual data blocks over the Industrial Ethernet plant bus. For projects with thousands of Process Objects, reading back the entire CPU memory takes between 2 and 10 minutes depending on network bandwidth.

Does Readback capture tuning parameters inside PIDConL?

Yes. Proportional gain (Gain), integral action time (TI), derivative time (TD), and deadband values in PIDConL are marked with S7_read_back := 'true' and are fully captured during OCM readback.

Technical References

  • Siemens Industry Online Support: SIMATIC PCS 7 CFC for S7 Manual (Section: Reading Back Chart Parameters).
  • SIOS Entry 109742323: PCS 7 Compendium Part A - Configuration Guidelines (Section: Data Consistency and Readback).
  • SIOS Entry 109809264: SIMATIC Process Control System PCS 7 Advanced Process Library Manual.
  • Siemens Support Guide: Managing Parameters and Setpoints with CFC Readback in PCS 7.

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Related Tags

  • Readback
  • APL
  • OpAnL
  • OpDi01
  • CFC
  • Rücklesen
  • Industrial Automation
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