Impact of Modifying S7 Block System Attributes (S7_link, S7_m_c) on PLC and OS Downloads in SIMATIC PCS 7

In SIMATIC PCS 7, modifying the system attribute 'S7_link' on an existing block parameter (to enable or disable CFC chart interconnections) does not alter the physical memory footprint or interface structure of the compiled instance data block. Consequently, updating 'S7_link' requires only a standard CFC compilation and a 'Download Changes in RUN' to the controller—it never requires a complete CPU download (CPU STOP) nor an OS compilation/download. In contrast, modifying the 'S7_m_c' attribute (Operator Communication and Monitoring) alters WinCC tag generation, which additionally requires an OS Changes Compile and an OS Online Changes Download to runtime servers without interrupting plant operations.

The Role of System Attributes in SIMATIC PCS 7 Blocks

In SIMATIC STEP 7 and PCS 7, function blocks (FBs) utilize System Attributes declared in block headers and variable declaration lines to instruct engineering compilers how to handle parameters:
- S7_link (Interconnection Capability):
* Syntax: { S7_link := 'true' } or { S7_link := 'false' }.
* Governs whether an input or output pin can be interconnected to another block output or peripheral address within the CFC editor.
* Setting S7_link := 'false' hides the pin from chart interconnection, forcing it to be parameterized as a static value or invisible in CFC.
- S7_m_c (Operator Monitoring and Controlling):
* Syntax: { S7_m_c := 'true' } or { S7_m_c := 'false' }.
* Instructs the OS compiler (s7tosx.exe) to create a corresponding WinCC structure tag for that parameter in WinCC Tag Management.
- S7_visible (CFC Pin Visibility):
* Determines whether a parameter is displayed by default when a block is placed into a chart.
- S7_server / S7_a_type:
* Configures alarm server attributes for ALARM_8P message evaluation.

Downstream Compilation and Download Scope for S7_link vs S7_m_c

In continuous process manufacturing plants, an unscheduled controller STOP can cause millions of dollars in production losses. Understanding download boundaries is vital:

Modifying S7_link Only:
- Affects exclusively the CFC graphical editor's connection validator.
- The compiled block's inputs, outputs, memory size, and instance DB offsets remain 100% identical in the S7 CPU.
- PLC Scope: Requires compiling the chart container (Compile > Changes only) and downloading changes (Download > Changes in RUN). The CPU remains in RUN continuously.
- OS Scope: Zero impact. OS compilation and download are completely unnecessary.
Modifying S7_m_c:
- Tells the system whether the parameter should be exposed to WinCC HMI.
- PLC Scope: If only the attribute changes without altering parameter data types, a Download Changes in RUN is supported.
- OS Scope: The OS must be compiled using Compile OS > Changes, followed by Download to OS > Changes (Online Changes Download). WinCC runtime continues operating without interruption.

Step-by-Step Procedure for Modifying System Attributes Safely in Production

To modify attributes on an active production system:

Modify the Block Definition:
- For SCL Source Blocks: Open the SCL source file in the SCL Editor. Locate the parameter (e.g. SimOn), modify the attribute string { S7_link := 'true' }, and recompile the SCL source.
- For Compiled Library Blocks (LAD/FBD/STL): Open the block in the LAD/STL/FBD Editor. In the variable declaration table at the top, select the parameter, right-click, and select Object Properties > System Attributes. Check or uncheck S7_link. Save the block.
Update Block Types in CFC:
- In SIMATIC Manager, open any CFC chart.
- Select Options > Block Types.
- Select the modified block in the chart container and click Update Block Types.
- Review the log: verify that the update does not report interface structural changes (no added/deleted variables).
Compile and Download Changes to Controller:
- In CFC, select Chart > Compile > Changes Only.
- Select PLC > Download > Changes Only.
- Confirm that the controller accepts the delta download while remaining in RUN mode.
Verify Interconnection:
- The parameter in the CFC chart now displays an active connection tab, allowing drag-and-drop wiring to other blocks.

System Attribute Modification Impact and Download Matrix Table

The following table details the compilation and download scope across different block modifications:

Modification Made to Block ParameterPLC Compilation RequiredPLC Download ScopeOS Compilation RequiredOS Download ScopeCauses CPU STOP?Toggle S7_link (true $\leftrightarrow$ false)CFC Changes OnlyChanges in RUNNoneNoneNOToggle S7_visible (true $\leftrightarrow$ false)CFC Changes OnlyChanges in RUNNoneNoneNOToggle S7_m_c (true $\leftrightarrow$ false)CFC Changes OnlyChanges in RUNOS Changes OnlyOS Changes in RUNNOModify Parameter Default ValueCFC Changes OnlyChanges in RUNNone (unless OCM)NoneNOAdd New Parameter to Block InterfaceCFC Complete ProgramCPU STOP (Full Download)Entire OSFull OS DownloadYESChange Parameter Data Type (INT $\rightarrow$ REAL)CFC Complete ProgramCPU STOP (Full Download)Entire OSFull OS DownloadYES

Verification

To confirm that attribute changes executed safely without plant downtime:
1. In the CFC chart, verify that the modified parameter now permits interconnections: connect a digital signal to the pin and verify that the line renders cleanly.
2. Monitor the S7-400 CPU front panel: verify that the RUN LED remains solid green and no STOP transitions or diagnostic buffer faults occurred.
3. In SIMATIC Manager, check the CPU Diagnostic Buffer (Ctrl + D): verify that event 16#4300 (Download changes successfully completed) is recorded.
4. If S7_m_c was toggled, open WinCC Tag Management on the OS and verify that the tag was created or removed according to the attribute setting.

Limitations and Operational Precautions

When modifying block attributes on running plants:
1. Never Reorder Variables in the Block Declaration: Adding or reordering variables shifts the memory offsets within the instance DB. Shifting offsets forces a complete CPU download requiring a CPU STOP. Always keep attribute edits restricted strictly to the attribute column.
2. Master Data Library Consistency: Always execute attribute edits inside the Master Data Library first, and then propagate the changes to target AS projects via 'Update Block Types'. Editing local AS block folders directly causes library synchronization mismatches during future maintenance.
3. Verify 'Download Changes in RUN' Capability Beforehand: Before compiling, check CFC Chart > Compile > Compilation/Download to ensure the reserve memory for Changes Download is not exhausted.

Engineer FAQ

Will changing S7_link ever force a complete download of the PLC?

No. S7_link is a metadata attribute evaluated strictly by the CFC compiler during chart interconnection. It does not alter the machine code or data block structures downloaded to the CPU.

Why did 'Download Changes' get disabled after I updated block types?

This happens if you accidentally modified the variable name, data type, or added/removed a pin in addition to changing the attribute. Any structural change invalidates the instance DB footprint, forcing a full download.

Can I change S7_link on standard Siemens library blocks like MotL?

While technically possible by saving the block to a project library, modifying standard Siemens APL library block attributes is strongly discouraged because future PCS 7 library updates will overwrite your changes.