Siemens PCS 7 FC Online Parameter Change: Complete Guide

Overview

In Siemens SIMATIC PCS 7 and STEP 7 environments, the ability to modify block parameters while the CPU is in RUN (online) depends entirely on the block type, parameter interface declaration, and block attribute set. The most common field complaint is: "I built a custom FC (Function / Function Chart), wired constants to its inputs, downloaded the program, and now I cannot change those constants in Test Mode without going offline, editing, and re-downloading." The same user notes that PCS 7 APL (Advanced Process Library) blocks such as Add04, Or04, And04, and Mul04 clearly do expose their In01..In04 inputs to online modification. This behavior is not magic — it is the result of three deliberate design choices in the STEP 7 block interface and the PCS 7 master data library.

This reference documents the underlying mechanism (the S7_dynamic system attribute), the difference between INPUT, IN_OUT, and OUTPUT parameter classes, the memory model difference between FC and FB blocks, the PCS 7 V6.1 SP1 / V7.0+ online-to-offline writeback feature, and the field-proven workarounds that allow online parameter edits on user-defined FCs.

Block Type Memory Model: FC vs FB

STEP 7 differentiates FCs (Functions) from FBs (Function Blocks) at the memory-allocation layer, not just at the syntax layer. The PCS 7 engineering toolset (CFC, SFC, or the SIMATIC Manager / TIA Portal) treats them differently:

Because the FC has no permanent memory, the online monitor buffer cannot "see" an INPUT parameter the way it can see an FB's STAT in an Instance DB. STEP 7 only retains online-moddable values for parameters whose physical backing store is allocated in the CPU's load memory — that means Instance DBs, bit memory (M), and I/O (P/Q). Plain FC inputs sit in the local stack frame and disappear when the call ends.

The S7_dynamic System Attribute

The S7_dynamic attribute is the cornerstone of online parameter editability for PCS 7 APL blocks. The attribute is set on each parameter declaration inside the block interface (VAR_INPUT, VAR_IN_OUT, VAR_OUTPUT, VAR_STAT sections). It is a Boolean system attribute that STEP 7 / CFC interprets during the Chart compilation pass.

To set the attribute in the STEP 7 / SIMATIC Manager block editor:

1. Open the FC in the LAD/FBD/ST editor.
2. Right-click on the parameter name in the Interface column.
3. Select Object Properties → Attributes.
4. In the System Attributes column, type S7_dynamic.
5. In the value column, type true (with or without quotes — both are accepted by the STEP 7 parser).
6. Save and re-install the block (it must be downloaded to the CPU's offline program).

Why APL Blocks Permit Online Changes on INPUT Parameters

The PCS 7 APL (Advanced Process Library) blocks supplied by Siemens — for example FB1869 Add04, FB1865 Or04, FB1863 And04, FB1870 Mul04, and the family of CTxx, CTRL_PID, OP_A blocks — have all of their input parameters declared with both S7_dynamic := 'true' and a specific interface ordering that exposes them to the CFC runtime. The reason you can right-click an In01 input on Add04 and see the online value (highlighted yellow) is that the PCS 7 master data library generates a block wrapper DB for each chart instance. That wrapper DB contains a copy of every dynamic input value, and the CFC Test Mode is allowed to write to that DB while the CPU is in RUN.

User-created FCs do not automatically receive a wrapper DB unless:

• The block is registered in the PCS 7 master data library (XML import via Import/Export → Master Data Library), and
• The block is compiled with the PCS 7 block container templates (the source STL/SCL contains the proper KNOW_HOW_PROTECT / AUTHOR / FAMILY / NAME block attributes).

If either condition is missing, the CFC will treat the FC as a "custom block" and apply the more restrictive default behavior — only IN_OUT and OUTPUT parameters are modifiable.

Parameter Classes and Test Mode Visibility

Field-Proven Workarounds for Online FC Parameter Changes

Workaround 1 — Declare the parameter as IN_OUT

The simplest, most widely used technique. Change every constant-bearing parameter from VAR_INPUT to VAR_IN_OUT in the FC interface:

FUNCTION FC100 : VOID
VAR_IN_OUT
    i_SetPoint : REAL;   // was VAR_INPUT
    i_Gain     : REAL;   // was VAR_INPUT
    b_Enable   : BOOL;   // was VAR_INPUT
END_VAR
BEGIN
    // ... logic ...
END_FUNCTION

Then in the CFC chart, wire each IN_OUT pin to a process tag (DB bit, M bit, or input I). In Test Mode, click the tag value (it appears yellow) and type the new constant. The change takes effect on the next OB1 cycle without a download.

Workaround 2 — Connect the INPUT to an FB Instance

If you cannot change the parameter class (e.g., the FC is part of a Siemens-supplied library you are not permitted to modify), wrap the input source with an FB instance that exposes a modifiable interface:

1. Create a "value holder" FB with the same data type as the FC input.
2. Place the FB in a separate chart.
3. Wire the FB's OUT to the FC's INPUT.
4. The FB's output value lives in its Instance DB and is fully modifiable online.

This is the canonical PCS 7 pattern for any constant the operator may need to retune at runtime (controller setpoints, alarm limits, scaling factors).

Workaround 3 — Use a Watch Table / VAT for FC Inputs

STEP 7's VAT (Variable Table) is bound to the CPU's symbol table, not to FC local variables. Therefore you cannot use a VAT to monitor or modify an FC INPUT directly — the FC input is not a global symbol. This approach is a dead-end and is documented here only to forestall confusion.

Workaround 4 — Move the FC Body into an FB

When the FC is large, frequently retuned, or shared across many charts, the cleanest architectural fix is to convert it to an FB. The FB has a permanent Instance DB, supports STAT variables, and exposes every parameter (including INPUT) to online modification by default. Converting FC→FB requires:

1. Copy the FC code into a new FB.
2. Convert every local VAR_TEMP that must persist to VAR_STAT.
3. Generate or assign an Instance DB per chart instance.
4. Update the call site in the CFC chart (CFC auto-converts the block type on re-compile).

PCS 7 V6.1 SP1 / V7.0 Online-to-Offline Writeback

Before PCS 7 V6.1 SP1, any value changed in CFC Test Mode was lost the next time the project was compiled, because the offline chart was not updated. From PCS 7 V6.1 SP1 onward (and stable in V7.0 through V9.0 SP8), the parameter value is automatically written back to the offline chart when you click Apply in Test Mode. This means:

• The chart's offline representation is kept in sync with the live value.
• A subsequent Compile → Download will not overwrite your online edits.
• The online and offline projects can be diff-checked via Chart → Compare.

Step-by-Step Procedure to Add S7_dynamic to an Existing FC

Use this procedure when you have authority to modify the FC source and need the parameter to be modifiable online without converting to an FB.

1. In SIMATIC Manager (or TIA Portal with PCS 7 V9+), open the project that contains the FC.
2. Right-click the FC in the S7 Program → Blocks folder and select Open.
3. Switch to the Interface view (LAD/FBD/ST editor → View → Interface).
4. For each parameter that must be online-modifiable, right-click the parameter name and choose Object Properties.
5. Click the Attributes tab.
6. In the System Attribute column, enter S7_dynamic.
7. In the same row, in the unnamed value column, enter true.
8. Click OK. Repeat for every parameter.
9. Save and close the editor.
10. Compile the S7 program (Program → Compile All) to refresh the chart's runtime DB references.
11. Download the FC to the CPU (online → Download to Target System).
12. Open the CFC chart that calls the FC and re-compile the chart so the new dynamic-attribute metadata is propagated into the chart's block I/O icon.
13. In Test Mode, the parameters should now appear with a yellow highlight and be modifiable.

Verification Steps

1. Switch the CPU to RUN (or RUN-P) mode.
2. Open the CFC chart, right-click the chart, and choose Test Mode.
3. Click on the desired INPUT / IN_OUT / OUTPUT pin. A small input box appears.
4. Type a new value, press Enter.
5. Observe the tag's actual value at the destination (e.g., a WinCC faceplate or a Trend) update within one OB1 cycle.
6. Stop Test Mode. Verify that Chart → Compare Online/Offline shows no value drift.

Troubleshooting Matrix

Best Practices for Online-Tunable FCs in PCS 7

• Default to FB, not FC, for any logic with retunable parameters. The memory model aligns with the operator's mental model.
• Reserve FCs for stateless utility functions — calculations, format conversions, and bit-shuffling — where every input is wired to a tag the operator already tunes elsewhere.
• Centralize constants in DB999 "CFG_Constants" (or project-named equivalent). Avoid magic numbers in chart wirings.
• Document the S7_dynamic attribute in the block header (KNOW_HOW_PROTECT aside, the source STL must have a comment block explaining which inputs are operator-tunable).
• Use the PCS 7 Master Data Library importer if the FC is shared across projects. This unlocks the full set of online test features.
• Test online modification as part of the FAT (Factory Acceptance Test). Many issues only surface when an operator is actually running the plant.

Edge Cases and Field Notes

• S7-mEC / S7-1500 controller migration: In TIA Portal with S7-1500 CPUs (PCS 7 V9 onward), the FC INPUT online-edit restriction has been relaxed. With a properly declared S7_dynamic attribute and a PLC tag of the matching data type, INPUT parameters are modifiable directly in the chart's online view. The PCS 7 V7.0 limitations described above apply primarily to S7-400 CPUs with the SIMATIC Manager toolchain.
• Optimized vs non-optimized block access: For S7-1500, Instance DBs are optimized by default; absolute VAT access is restricted. Use symbolic tags or the watch table with symbolic name resolution.
• Operator Station authentication: Online value changes are logged to the PCS 7 OS audit trail. Confirm that the operator's WinCC user account has the "CFC Online Change" right enabled in the User Administrator.
• Fault-tolerant S7-400H: Online changes are synchronized to the standby CPU automatically. Do not attempt simultaneous edits from two ES (Engineering Station) clients.
• KNOW_HOW_PROTECT blocks: Source-protected blocks display yellow pin values but require the original SCL source to change the S7_dynamic attribute. Plan accordingly.

Standards and Reference Documents

The behavior described here is governed by the STEP 7 / PCS 7 block attribute specifications and the Siemens function manual set. Verify version-specific behavior against the official PCS 7 function manuals:

• Siemens PCS 7 V9.0 SP8 — Function Manual: CFC (entry ID 109782440 on Siemens Industry Online Support)
• Siemens PCS 7 V7.0 — Service Manual: CFC Online Test
• STEP 7 V5.7 — Programming and Operating Manual: S7-300/400 Block Concepts
• SIMATIC S7-1500 / TIA Portal V18 — Programming Guideline: Block Attribute S7_dynamic