Resolving CFC Download Error: 'The Interface of Block FC/FB Has Been Changed, Please Re-Import'

In SIMATIC PCS 7 CFC, encountering the error 'The interface of block FC/FB has been changed, please re-import' during an AS download indicates that a block's input, output, or in/out parameter signature was modified in the block folder or library, invalidating the CFC chart's internal interface timestamp. S7-400 CPUs reject changes-only downloads (Delta Downloads) when block interfaces shift. To resolve this without project corruption, open any CFC chart, select Options -> Block Types, select the modified block, and execute 'Update Block Types'. If the modification was accidental, restore the original FB/FC from the Master Data Library or project backup and recompile.

Anatomy of the Block Interface Timestamp Mismatch in SIMATIC S7

In SIMATIC STEP 7 and PCS 7, every Function Block (FB) and Function (FC) possesses an internal binary signature known as the Interface Time Stamp. This timestamp encodes the exact count, order, data types, and byte offsets of all input (VAR_INPUT), output (VAR_OUTPUT), and in/out (VAR_IN_OUT) variables.

When a block is compiled into Continuous Function Charts, the CFC database caches this interface signature. If an engineer later opens the underlying SCL or LAD/STL source, modifies an interface parameter (even something as minor as renaming a pin or changing INT to DINT), the compiler generates a new interface timestamp. When the engineer initiates an AS download, the system detects that the chart's cached interface does not match the block folder, immediately aborting the transfer.

Why Delta Downloads in RUN Are Blocked by Interface Shifts

In continuous process automation, engineers rely on Download Changes in RUN to update logic without stopping the production plant:

When an interface changes, the underlying instance Data Block (DB) layout shifts in memory.
If the S7-400 CPU permitted a delta download of a shifted DB while user code executed, memory pointers would point to invalid offsets, causing immediate unrecoverable CPU crashes (Area Length Errors, OB121/OB122).
Consequently, the S7-400 operating system strictly forbids delta downloads whenever a block interface timestamp changes.

Accidental Edits vs Legitimate Interface Modifications

When this error occurs, engineers must determine whether the change was intentional:

Accidental Edit: An engineer opened a standard library block (e.g. an APL function or communication FC) and recompiled it, generating a new timestamp even though no code changed. In this case, reverting the block to match the original backup restores delta download capability.
Legitimate Enhancement: A custom technological block had new parameters added. In this case, the interface change must be formally re-imported across all charts via Update Block Types.

The Role of 'Options -> Block Types -> Update Block Types'

The CFC command Update Block Types is the official mechanism to resolve interface discrepancies:

Opening Options -> Block Types in CFC scans the project chart folder against the S7 offline block folder.
It highlights blocks whose interfaces have diverged.
Clicking Update Block Types re-imports the new interface into all instantiated charts, preserves existing wire connections, and updates all instance DBs.

Warning: Re-importing a modified interface will almost always invalidate delta download in RUN, requiring a full CPU STOP download.

Comparative Remediation Matrix: Interface Mismatch Recovery Scenarios

The following table compares recovery strategies depending on whether plant shutdown is permissible:

Operational ConditionAction TakenDownload CapabilityCPU Status RequiredPlant ImpactAccidental Edit (Keep CPU in RUN)Restore original block FB/FC from backup / Master Data LibraryDownload Changes in RUN preservedCPU remains in RUNZero plant interruptionLegitimate Change (Outage Permitted)Execute 'Update Block Types' in CFC -> Full AS CompileFull AS Download requiredCPU in STOPComplete plant halt during downloadLegitimate Change (CPU 410 TCiR)Use Type Change in RUN (TCiR) with CPU 410-5HDownload Changes in RUN supportedCPU remains in RUNSupported only on modern CPU 410Deleting Block Manually in CFCBreak wires, re-insert blockFull compile required; delta download blockedCPU in STOPSevere risk of lost logic

Step-by-Step Recovery Procedure: Re-Importing and Harmonizing Blocks

Follow this sequence to resolve the download block:

Scenario A: Reverting an Accidental Edit (Preserving RUN Download):
1. Do not re-import the block in CFC.
2. Open the project Master Data Library or retrieve a clean project backup from before the edit.
3. Copy the original, unmodified FB/FC into the target S7 Program Blocks folder, overwriting the edited block.
4. In SIMATIC Manager, run Options -> Check Block Consistency.
5. Open CFC; execute Compile -> Changes Only and download changes in RUN.

Scenario B: Committing a Legitimate Interface Change:
1. Open any CFC chart. Navigate to Options -> Block Types.
2. Select the modified block in the Chart folder list.
3. Click Update Block Types.... Confirm the prompt to update all chart instances.
4. Compile charts: select Compile Charts as Program -> Entire Program.
5. Schedule a maintenance window, switch the CPU to STOP, and perform a full AS Download.

Verification

Confirm resolution through these diagnostic checks:

Block Consistency Verification: In SIMATIC Manager, run Check Block Consistency. Verify green tree with zero timestamp errors.
CFC Compilation Log: Verify CFC compilation finishes with 0 errors and confirms timestamp alignment.
Download Execution: Perform download. Verify the AS download dialog accepts the transfer without throwing 'Interface changed' popups.
CFC Test Mode Monitoring: Go online in CFC; confirm the block updates dynamic values cleanly.

Limitations and Operational Precautions

Observe the following safeguards:

Never Edit Library Blocks in Offline Program: Standard library blocks (APL, CEMAT) should never be opened or edited in the user block folder; always maintain master copies in the Master Data Library.
Backup Discipline: Always archive the multiproject prior to updating block types so that accidental interface shifts can be reversed.
Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and hardware capacity prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Engineer FAQ

How do I avoid a CPU STOP if I accidentally edited a block interface?

Do not re-import in CFC; restore the original, unedited block FB/FC from a project backup into the Blocks folder and recompile changes only.

What is Type Change in RUN (TCiR)?

TCiR is an advanced feature available on CPU 410-5H controllers that allows certain block interface updates to be downloaded without switching the CPU to STOP.