Understanding OCM Attribute Constraints and Structure Tag Generation in SIMATIC PCS 7

In SIMATIC PCS 7 engineering, the Operator Control and Monitoring (OCM) checkbox appearing grayed out in CFC block properties—such as on analog channel drivers like PCS7AnIn or FbAnIn—is an intentional architectural design preventing peripheral driver blocks from generating WinCC structure tags. In standard PCS 7 hierarchy, channel drivers process raw hardware I/O and do not contain the S7_m_c block attribute. Instead, structure tags, alarms, and faceplate connections are generated exclusively by technological blocks (such as MonAnL, PIDConL, or MotL) interconnected to the driver outputs.

Driver Blocks vs Technological Processing Blocks in PCS 7

SIMATIC PCS 7 enforces a strict separation between physical I/O signal acquisition and operator supervisory control. This separation is reflected in two distinct block categories in the Advanced Process Library (APL):
- Channel Driver Blocks (PCS7AnIn, FbAnIn, FbDiIn): Reside on Sheet 1 of CFC charts. Their exclusive task is to read raw peripheral input words (PEW) from distributed I/O modules, evaluate hardware diagnostic bytes, apply signal smoothing, and output a structured variable (PV_Out) containing the value and high-resolution Quality Code (ST).
- Technological Blocks (MonAnL, PIDConL, MotL): Reside on Sheets 2–6. They execute process automation logic, evaluate high/low alarm limits, manage operating modes, and provide the human-machine interface for plant operators.

The S7_m_c Attribute and Automated WinCC Tag Generation

The generation of WinCC structure tags during OS compilation is governed by the Step 7 attribute S7_m_c (SIMATIC S7 Monitoring and Control):
1. Block-Level Attribute: For the OCM checkbox to be selectable in CFC, the block prototype definition must declare the block-level attribute { S7_m_c := 'true' }.
2. Parameter-Level Attribute: Within the block, individual variables requiring HMI exposure must also declare { S7_m_c := 'true' }.
3. Driver Block Restriction: To conserve AS-OS communication buffers and prevent gross inflation of WinCC tag counts, Siemens deliberately omits the S7_m_c attribute from standard channel drivers. Consequently, the OCM checkbox in CFC is permanently grayed out for PCS7AnIn and FbAnIn.

Interconnection Workflows for Process Monitoring

To display an analog measurement on the Operator Station:

Standard APL Method:
1. Place the channel driver FbAnIn (or legacy PCS7AnIn) on Sheet 1.
2. Place the technological monitoring block MonAnL on Sheet 2.
3. Interconnect FbAnIn.PV_Out to MonAnL.PV.
4. In MonAnL block properties, verify the OCM checkbox is enabled (checked by default).
5. Assign the technological tag name (e.g. TI_101) to the CFC chart or MonAnL block.
6. Compile the OS. The compiler automatically creates the WinCC structure tag TI_101 and instantiates the block icon in the assigned process picture.
Direct WinCC Manual Tag Definition (Non-Standard): If an engineer insists on reading a driver block directly without using a technological block, they must manually create an internal or external structure tag in WinCC Tag Management pointing to the specific instance data block offset (DBxxx.DBDxx), but this bypasses standard APL faceplates and alarm generation.

Comparison: Driver Block vs Technological Block Architecture

The following table contrasts driver blocks with technological monitoring blocks:

CharacteristicChannel Driver Block (FbAnIn / PCS7AnIn)Technological Block (MonAnL)Primary FunctionPeripheral signal conditioning & quality evaluationProcess monitoring, alarming, and operator controlStandard Chart LocationSheet 1 (I/O Margin)Sheet 2–6 (Process Area)S7_m_c Attribute PresentNo (Permanently omitted by design)Yes (Block and parameters configured)OCM Checkbox in CFCGrayed out (Disabled)Active (Checked by default)Automatic WinCC Structure TagNot GeneratedGenerated automatically during OS compileAssociated WinCC FaceplateNoneStandard APL Faceplate (@PG_MonAnL_Standard.pdl)Process Object (PO) ConsumptionZero AS POs (Driver blocks do not consume POs)Consumes 1 AS PO and 1 OS PO

Rule of Thumb: Never modify standard library driver blocks to enable S7_m_c. Always interconnect drivers to technological blocks.

Verification

Verify structure tag generation and signal processing:
1. In CFC, confirm that FbAnIn.PV_Out is interconnected to MonAnL.PV.
2. Open SIMATIC Manager and execute Compile OS > Changes only.
3. Open WinCC Explorer on the ES and navigate to Tag Management > SIMATIC S7 PROTOCOL SUITE > Named Connections.
4. Confirm that the structure tag representing MonAnL appears with all member tags (#Value, #QualityCode, PV_AH_Lim, etc.).
5. Launch WinCC Runtime and verify that the analog value updates smoothly on the operator screen.

Limitations and Operational Precautions

Essential rules regarding block attributes:
- Modifying standard Siemens library blocks in SCL to force S7_m_c = true creates maintenance liabilities; future library updates will overwrite custom changes and cause compilation collisions.
- Technological blocks like MonAnL consume licensed Process Objects; ensure sufficient AS and OS PO licenses are available in the project before mass instantiation.
- Do not connect multiple monitoring blocks to a single driver output if a single block can handle alarming and scaling requirements.

Engineer FAQ

Can I enable OCM on PCS7AnIn by editing the block properties dialog?

No. The OCM checkbox in the CFC block properties dialog is read-only if the underlying block definition does not contain the S7_m_c attribute in its compiled interface.

Why didn't Siemens put S7_m_c on driver blocks?

Driver blocks execute hardware-level data conversion. Exposing raw drivers would double WinCC tag counts and consume communication bandwidth with redundant data already handled by technological blocks.

Do channel driver blocks consume Process Objects (POs)?

No. Channel driver blocks (FbAnIn, FbDiIn) do not consume PCS 7 Process Objects; POs are counted only on technological blocks with OCM capabilities.