Troubleshoot BSEND Failure After Renumbering a PCS 7 Communication FB
After moving a PCS 7 communication FB to another number, verify the block references and instance relationships, then diagnose BSEND using its actual result and status. STEP 7's documented Rewire function provides a controlled way to replace block addresses, but a successful compile does not establish that sending works. For a V8.1-to-V9.1 migration with FB100 moved to FB103, the exact repair depends on the real call structure and communication diagnostic.
Table of Contents
- Separate the Number Conflict from the Transfer Failure
- Method 1 — Correct and Rebuild Available Source
- Method 2 — Use the Documented STEP 7 Rewire Function
- Diagnose BSEND Independently
- Verification
- Limitations and Operational Precautions
- Engineer FAQ
- Technical References
- Related Articles
- Related Tags
Separate the Number Conflict from the Transfer Failure
The reported migration is from PCS 7 V8.1 to V9.1. A user communication FB occupied FB100; compilation requested FB100 with the header OB_BEGIN. Renaming the user block to FB103 allowed compilation, but sending from the S7-400 to an S7-300 no longer worked. Receiving reportedly continued.
The report establishes a number conflict and a later communication failure. It does not prove that an internal call remained at FB100 or that renumbering alone will repair sending. Preserve the original project and migrated project for comparison.
Method 1 — Correct and Rebuild Available Source
When maintainable source is available, review the user block declaration and all explicit references affected by the new block number. Rebuild the user block from the corrected source and update the engineering objects through the applicable CFC block-type workflow.
This is an engineering approach requiring the actual source and project structure. Do not assume that changing an object's displayed name changes every embedded absolute call. Before deployment, compare the resulting call structure and instance relationships with the intended program.
Method 2 — Use the Documented STEP 7 Rewire Function
Siemens documents the following procedure in Programming with STEP 7, section 9.3.5:
- In a project copy, select the relevant Blocks folder.
- Choose Options > Rewire.
- Enter the intended old and new block addresses, such as FB100 and FB103 only if that mapping is correct for the user block.
- Confirm the operation and inspect the resulting information file, including changed blocks and replacement counts.
The destination block must not already exist. The manual also states that an instance DB is assigned to the rewired FB while retaining its DB number. Resolve the current project state before applying an example mapping; do not rewire a generated OB_BEGIN block as though it were the original user block.
Diagnose BSEND Independently
The manufacturer reference provides ERROR and STATUS diagnostics for SFB/FB12 BSEND. Capture the actual send-job result rather than inferring it from a successful compilation.
As an engineering comparison, inspect the intended connection ID, corresponding partner R_ID, send area and length against the working baseline and receiving configuration. Interpret the observed status using the applicable CPU/function documentation. No status value was supplied here, so no specific communication error is diagnosed.
Verification
Verify the renumbered user-block calls and instance associations offline before planning a download. Then, under approved test conditions, initiate an intended transfer, record the send result and confirm the corresponding received data changes at the partner.
Check both directions separately. Continued reception does not demonstrate that the send job is correct. Retain the rewire report and before-and-after communication observations with the change record.
Limitations and Operational Precautions
The actual user source, internal calls, instance DB relationships and BSEND status are unavailable. The exact cause and final repair therefore remain unresolved.
The opened rewiring procedure is STEP 7 documentation, not a complete PCS 7 V9.1 migration recipe. Follow the applicable migration and CFC update instructions before compilation or download. Do not assume that a full download is avoidable or required; determine that from the actual proposed changes and operational plan.
Engineer FAQ
Does renaming FB100 to FB103 prove that all calls changed?
No. Inspect the resulting references and call structure.
Can FB103 already exist when Rewire starts?
The documented function requires an unoccupied destination.
Does a successful compile prove BSEND communication?
No. Observe the send result and partner data.
Technical References
- Siemens: Programming with STEP 7, April 2017, A5E41552389-AA — section 9.3.5, page 210, Rewiring.
- Siemens: System Software for S7-300/400 — System and Standard Functions, May 2010 — Volume 2, section 22.6, BSEND and its ERROR/STATUS diagnostics. CPU-specific availability must be confirmed.