Renaming Siemens Step 7 FBs Using Rewire Without Breaking Logic

Overview

Renumbering a Function Block (FB) in a Siemens SIMATIC Step 7 Classic (S7-300/S7-400) project is a routine but high-risk maintenance task. The block number is referenced in every call location (FCs, other FBs, OB1, multi-instance DBs, the symbol table, the System Data, and any CFC/SFC chart) and the underlying absolute address inside the block is referenced from the instance DB. If you simply delete the old FB and insert a new one at a different number you will orphan every call, scramble the instance DB mapping, and force a full rewire of multi-instance code by hand.

Step 7 Classic ships a dedicated tool for this exact situation: Options → Rewire... It rewrites every reference to the old block number with the new block number, retains the instance DB, retains the symbol assignment (if one exists), and produces a change log so you can verify what was touched before you save the offline project and download to the PLC.

Prerequisites

1. STEP 7 V5.4 SP5 or newer (V5.6 SP2 recommended). The Rewire dialog requires the SIMATIC Manager component installed; it is not present in the older STEP 7 Lite toolset.
2. An offline S7 project with the FB already compiled cleanly. Resolve every compiler warning or error before the rewire, otherwise the change log will be polluted with pre-existing problems that hide real new issues.
3. The Symbol Table (S7-Program → Symbols) editable and writable on disk. If the project lives on a read-only share, copy it locally first.
4. Sufficient free numbers in the FB number range. By default the user FBs occupy 0-255; extended FBs use 256-2047. If you are renumbering to FB1060, you must be working in an S7-400 station with the extended number range, or the S7-300 CPU firmware you are using must permit it (e.g. CPU 319 with firmware ≥ 2.4). Verify this against your CPU's manual before attempting the rename.
5. A clean, versioned backup of the project archive (.zip or .arj) so the change can be rolled back if Rewire reports an unexpected number of substitutions.

Understanding the Two Priority Modes

Step 7 Classic supports two mutually exclusive resolution strategies between symbolic names and absolute addresses. Every rewire decision depends on which one is active when the program is compiled. Always check Options → Customize → Block tab → Address Priority before the procedure and restore it afterwards.

Step-by-Step Rewire Procedure

The procedure below renames FB60 to FB1060 while keeping every parameter, instance, multi-instance, and symbol association intact. It assumes a single S7-400 station with a standard distributed I/O topology. Adapt station names to your project.

Step 1 — Archive the project

1. Close the project in SIMATIC Manager.
2. From Windows Explorer, right-click the project folder → 7-Zip → Add to archive (or use the STEP 7 built-in File → Archive). Save the archive outside the source directory.
3. Record the project version, station type, and CPU firmware in a release note. The rewire change log is meaningless without a before-state.

Step 2 — Compile the project to a clean state

1. Open the project, right-click the S7-Program node of the target station, choose Compile and Check Objects.
2. Open the Accessible Nodes or the local offline view and read the Compile Log window. Resolve every entry that is not informational.
3. Re-run Check Block Consistency (Options → Check Block Consistency). A green status here is the only safe starting point for a rewire.

Step 3 — Switch to Absolute Address Has Priority

1. Select Options → Customize... in SIMATIC Manager.
2. Open the Block tab (in some translations Bausteine).
3. Under Address priority, select Absolute Address Has Priority and click Apply. Do not close SIMATIC Manager.

Step 4 — Add the new symbol entry

1. Open S7-Program → Symbols.
2. Locate the row for the old symbol (commonly FB60 with a meaningful mnemonic such as Motor_Control_FB). Note the symbol name and data type (FB 60).
3. Right-click the row and choose Insert Row just above it, or scroll to an empty row.
4. Enter the new symbol name, the new address (FB 1060), and the data type FB 1060. Keep the old row temporarily; you will delete it after Rewire completes.
5. Save and close the symbol table. Ctrl+S writes the file to the project directory.

Step 5 — Run the Rewire

1. Select the Blocks folder of the S7-Program so that Rewire applies to the entire program, not just a single object.
2. Choose Options → Rewire...
3. In the Old Number field enter 60. In the New Number field enter 1060.
4. Leave All block types unchecked except FB, or you will accidentally rewrite FC, DB, OB, and SFB/SFC numbers that happen to match.
5. Click OK. A progress bar shows the number of files scanned; expect several hundred to several thousand substitutions on a medium-sized plant project.

Step 6 — Read the change log

Rewire writes a plain-text report. The default location is the project root with a name like Rewire.log; in some installations the report opens inline in the SIMATIC Manager message window. Read it cover to cover and look for:

• Number of FCs/FBs modified (this is your objective evidence the rename was applied).
• Number of OBs touched. OB1, OB35, OB82-87, OB100-102, OB121-122 are the common ones.
• Number of DBs touched. Each instance DB associated with FB60 will need its background block re-anchored — this is expected.
• Any line beginning with ERROR or WARN. These block the rename from completing; resolve and re-run.

Step 7 — Reconcile the symbol table

1. Open the symbol table again.
2. Delete the row for the old address (FB 60). Keep the row for the new address (FB 1060).
3. If any consumer of the project referenced the old symbol in operator faceplates (WinCC flexible, WinCC, HMI tags) update the tag pointer. WinCC stores the absolute block number in the tag address; Rewire does not modify the HMI project.

Step 8 — Return to Symbol-has-priority and recompile

1. Open Options → Customize → Block tab and select Symbol has priority.
2. Click Apply.
3. Run Check Block Consistency followed by Compile and Check Objects. Every block should compile without warnings.
4. Save the project (Ctrl+S on the S7-Program node).

Step 9 — Download to the PLC

1. Connect to the target CPU and place it in STOP if you are changing a block that runs in OB1, OB35, or any time-critical OB. For a pure renumber with no logic change, a download in RUN-P is also possible in newer CPUs (firmware ≥ 3.1 on S7-400 CPUs and CPU 319), but STOP is safer for a first deployment.
2. Perform a full download of the Blocks container. A delta download may skip the new FB if the comparison engine matches it against the cached offline file.
3. Perform a Cold Restart only if a multi-instance DB reset is required. For an in-place renumber, a Warm Restart is sufficient and preserves retain data.

Verification

Verification is the part of the rename that actually matters. Do not declare success after a clean compile alone.

1. Cross-reference check. Open the Reference Data → Display from the Blocks folder. Filter by the old block number. The result must be empty. Filter by the new block number. The list must contain every call site that referenced the old number, plus the multi-instance FB instantiations inside other FBs.
2. Symbol resolution check. In any FC that called the old FB, place the cursor on the call line. The status bar at the bottom of SIMATIC Manager must show the symbol name of the new FB, not ??.
3. Online block comparison. In RUN or STOP, right-click the new FB and choose Compare → Online/Offline. The result must be Identical. Repeat for every instance DB that was re-anchored.
4. Live I/O check. For a motor-control FB, force a single instance in the instance DB and observe the I/O. The block number on the B-Stack of the online block viewer must be 1060, not 60.
5. HMI tag check. Trigger a value change in a WinCC tag that points to a DB variable used inside the renamed FB. The faceplate should update within one configured acquisition cycle.
6. Cross-station consistency. If you have a redundant S7-400H pair or a software redundancy package, repeat the download on the standby CPU. The IM/IML sync must report In sync.

Multi-Instance DB Considerations

Every call to an FB creates an Instance DB (for single-instance calls) or a multi-instance slot inside the caller's instance DB. The instance DB records the FB number it was created from, the FB's static variable layout, and the start address of each VAR_TEMP/VAR_INPUT/VAR_OUTPUT/VAR_IN_OUT/VAR_STAT/VAR_TEMP. A renumber does not change the static layout, but it does change:

• The FB-Nr field in the DB header, which is what the CPU uses to verify the instance is bound to the right FB type.
• The AR2 register setup when the DB is opened, which is computed from the DB number and the FB number.

Rewire updates both transparently. You can confirm by opening the instance DB in Declaration View online; the header should now read FB 1060 and the View → Data View initial values should match the offline project exactly.

What Rewire Does Not Touch

Common Pitfalls and Field-Proven Caveats

1. Renumbering into a number that is in use. If FB 1059 already exists with a different signature, Rewire refuses to continue but leaves the project in a half-modified state. Always list the entire FB range in the Blocks folder before choosing the new number.
2. Renumbering past the CPU's maximum FB number. S7-300 CPUs without the extended FB number range (older CPU 312/314/315) limit user FBs to 0-255. Re-numbering to 1060 will compile offline but download will fail with STOP: load error (type 33 / W#16#4579). Check the CPU manual first; the latest CPU 319-3 PN/DP (6ES7 318-3EL01-0AB0, firmware V3.3) supports the extended range up to 2047.
3. Renumbering an SFB/SFC. Rewire will accept the input but the system function tables are read-only. The change log will show 0 modifications. Treat any SFB/SFC rename attempt as a configuration error.
4. Renumbering across stations. A multi-station project (e.g. AS01 + AS02) shares the symbol table at the project level but has independent Blocks folders. ReWire acts on the currently selected S7-Program. You must run it once per station, or use the project-wide Check Block Consistency to find every occurrence.
5. Forgetting the safety program. If the FB participates in a F-runtime group (S7-300F/S7-400F), the F-shared DB and F-call signature must be re-checked. A rewire does not change the F-signature, so the safety program will continue to pass the signature check, but you must still document the change in the safety log book.
6. Compiling in Absolute-Address-Has-Priority mode by accident. If the engineer on call switches back to Symbol priority and then makes any edit to a DB, the offsets are refreshed. This is the safe behavior — but if the edit was made while still in Absolute priority, the offsets are now wrong and the rewire's effect is partially undone. Always end the procedure with a recompile under Symbol priority.
7. Forgetting to delete the stale symbol row. Step 7 tolerates the duplicate, but the online viewer will show two symbols for the same address and the HMI download will warn. The duplicate is the single most common cause of "it works on my laptop but not at the plant" symptom patterns.

Alternative: Symbolic-Only Renumbering

If the goal of the rename is purely cosmetic (for example to make the FB number match a project-wide naming convention), the entire procedure can be reduced to a symbol table edit. The block number on disk stays the same, but every consumer that references the symbol follows on the next compile. The trade-off is that the on-disk number still says FB60, which can confuse a future engineer reading cross-references. Use symbolic renumbering only when:

• The CPU's number range forbids the new number.
• You do not have authorization to do a STOP-mode download.
• The project is shared with an OEM who will not accept a different block number.

Related Procedures

• Copying an FB to a new number uses the same Rewire mechanism but with a different intent. After the copy, the original FB is deleted and every call is rewired to the new number. Step 7 offers a one-shot Blocks → Copy/Paste followed by a manual Rewire, which is the workflow preferred by teams that audit every modification.
• Renaming the symbol but not the number is the inverse of this procedure. Edit the symbol table only, recompile. Useful when the engineering convention uses the block number for nothing more than an internal handle.
• Moving an FB to a library requires a separate Rewire with Library block enabled, plus an update of the master data library. The change log will list every block that imported the old FB.

Diagnostic Codes Encountered During the Procedure

FAQ