S7 Data Block Initial Values Not Updating: STEP 7 Troubleshooting

Problem Description

An engineer downloads a STEP 7 v5.x project containing a shared data block (DB) with a STRUCT of ten INT values used as process time parameters. The engineer edits the initial values in the data block declaration, performs a CPU memory reset on the S7-300/S7-400 (or in PLCSIM), and re-downloads only the DB block. In the online view of the DB, the actual values still display the previous (old) parameter set, even though the offline declaration now contains the new initial values. The user has confirmed that no other logic in the program overwrites the DB variables, and the issue reproduces consistently on every download cycle.

This symptom is a classic STEP 7 v5.x misunderstanding. The root cause is not a defect in the controller, PLCSIM, the project, or the download. It is a normal behavior of how SIMATIC Manager separates initial values from actual values for instance data blocks. The fix requires a single additional action — re-initializing the data block — that is easy to miss because it is not part of the standard download flow.

Symptoms Checklist

• Initial values were edited in the DB declaration table (offline).
• Project was saved, compiled, and downloaded to the CPU or to PLCSIM.
• A CPU memory reset (MRES) was performed to clear retain/non-retain data.
• In online view, the DB still shows the previous (pre-edit) values.
• Cross-reference search confirms the DB is not being overwritten by user logic in OB1, OB35, FB, or FC blocks.
• Issue reproduces after deleting the DB from the S7 program and re-downloading only the DB.

Root Cause: Initial Values vs. Actual Values

STEP 7 v5.x data blocks carry two separate value tables for every declared variable:

When the user changes the initial values in the declaration table and performs a normal block download, STEP 7 updates the offline image and the load image on the CPU, but it does not push the new initial values into the work-memory actual values. The actual values from the previous run remain in place. This is by design, because the runtime system cannot assume that an online modification to a declaration is meant to wipe the current process state. Per the Siemens Support entry 837001 — "What is the significance of the initial and actual values of a data block in STEP 7 v5.x?":

The initial value is only configured once for a data block or for a newly declared variable data block. If the variable already exists, the actual value is retained after a download until an explicit initialization of the data block is performed.

Consequently, the DB on the CPU continues to expose the previous actual values to user logic, while the new initial values sit unused in the load image waiting for the next initialization event.

Affected Versions and Platforms

The issue applies to the classic STEP 7 v5.x line and is independent of the CPU family or PLCSIM version:

Solution: Re-Initialize the Data Block

There are three supported methods to push the new initial values into the actual values. Pick the one that fits your workflow.

Method 1 — "Initialize Data Block" from SIMATIC Manager (Recommended)

1. Open the offline project in SIMATIC Manager.
2. Open the S7 program and expand the "Blocks" container.
3. Right-click the affected DB (e.g., DB100).
4. Select Edit > Initialize Data Block (German UI: Bearbeiten > Datenbaustein initialisieren).
5. SIMATIC Manager prompts: "Do you want to initialize the data block?" — confirm with Yes.
6. The actual values of the DB are now overwritten with the initial values from the offline declaration.
7. Download the (now initialized) DB to the CPU or to PLCSIM using PLC > Download or by dragging the block onto the online target.

This is the workflow called out in the Siemens support entry. It is the simplest method when the change is a deliberate one-time reset, and it is safe because the offline initial values are already the new values you typed in.

Method 2 — Download the Entire S7 Program with Initialization

1. In SIMATIC Manager, select the S7 program or the entire Station.
2. Choose PLC > Download.
3. When prompted "Do you want to initialize the data blocks?", select Yes.
4. STEP 7 will copy all DB initial values into the actual values during the download.

This is useful when the change to the DB is part of a broader software revision (e.g., version bump) and you want every DB re-initialized consistently.

Method 3 — CPU Memory Reset (MRES) Followed by a Cold Restart

1. Place the CPU in STOP mode.
2. Perform a memory reset (MRES) using the mode selector: turn to MRES, hold until the "STOP" LED flashes, return to MRES, release.
3. Switch the CPU back to RUN/PROG and perform a cold restart (OB102). On a cold restart, all DBs are re-initialized from the load image.

Method 4 — Use the "Initialize on Download" Option (Per-DB Persistent Setting)

For projects that routinely re-initialize a specific DB on every download, STEP 7 v5.x exposes a per-block attribute:

1. Right-click the DB in the Blocks container.
2. Choose Object Properties > Download tab (German: Eigenschaften > Download).
3. Check the option "Initialize data block on download" (German: Datenbaustein bei Download initialisieren).
4. Confirm with OK and save the project.

From this point on, every download of the DB to the CPU will overwrite the actual values with the initial values. This is convenient for commissioning but dangerous for production, because an engineer who edits an initial value by accident and downloads will lose the running process state. Best practice is to enable this attribute only for commissioning blocks and to disable it before handing the project over to operations.

Verification

After applying any of the four methods, verify that the new values are now active in the running CPU or PLCSIM instance.

Verification Procedure

1. Open the DB in online view: right-click the DB > Open Online (or Monitor/Modify).
2. Switch to the Data View tab (German: Datenansicht).
3. Confirm that the Actual Value column now matches the Initial Value column for every variable in the STRUCT.
4. Cross-check the S7 program with Options > Reference Data > Display and search for any cross-reference that writes to the DB. If logic is intentionally writing to the DB at runtime (e.g., operator setpoints, recipe load), the DB will only reflect the new initial values for one cycle before the program overrides them — that is expected.
5. Trigger the production sequence and confirm that the process times now match the new values.

Diagnostic Table

Edge Cases and Field-Proven Caveats

1. DBs Generated by an FB (Instance DBs)

An instance DB generated from an FB (Function Block) does not have its own independent initial-value table. The initial values are inherited from the FB's declaration (the STAT section). Re-initializing the instance DB pushes the FB declaration defaults into the actual values. If the FB contains code that pre-processes the STATs in its startup section, the pre-processed values will overwrite the defaults after the first cycle.

2. UDT-Based Data Blocks

Data blocks built from a UDT (User-Defined Data Type) inherit their initial values from the UDT source. If the UDT is edited centrally, every DB based on that UDT shows the new initial values after a project-wide recompile, but — as before — only after a download-with-initialization does the CPU work memory pick them up. The UDT must be compiled and downloaded before the dependent DB, otherwise the DB retains the old UDT structure.

3. DBs with the "Unlinked" Attribute

A DB marked as unlinked in its object properties exists only in load memory and is never copied to work memory. The "Initialize Data Block" function is meaningless for an unlinked DB because the work-memory image is not generated. To use an unlinked DB at runtime, you must change the attribute to non-unlinked and re-download.

4. Retain Behavior

If a DB is declared as retain in the CPU's retentivity configuration, the actual values survive a warm restart and a power cycle. In that case, neither MRES nor a warm restart will bring the new initial values into play. You must either remove the retain attribute for the affected DB, perform a full memory reset (which wipes retain), or use Method 1 or 4 to push the new initial values into the retain area explicitly.

5. PLCSIM-Specific Behavior

PLCSIM v5.x mirrors the S7-300/400 CPU behavior, including the initial-vs.-actual value separation. When using PLCSIM for unit tests of a DB-heavy function (e.g., a recipe loader that reads ten process times from a STRUCT), the test harness must explicitly initialize the DB before exercising the FB. The Runmode S7 DB Initializer utility automates this for batch test rigs: it copies the offline initial values into the PLCSIM actual values at the start of every test case, eliminating the side effect.

6. Block Consistency After Edit

If the DB's STRUCT signature has changed (e.g., a new field was added, a type was changed from INT to REAL), STEP 7 may refuse to download the block as a simple download and instead demand a block-consistency download or a full program download. In that case, the download dialog will offer the initialization option automatically.

Cross-Reference Diagnostics: Confirm Logic Does Not Overwrite the DB

Before assuming the cause is the download mechanism, always check the user program. A common reason that "initial values do not take effect" is that an OB, FC, or FB is copying values into the DB at every cycle or at startup.

Procedure

1. In SIMATIC Manager, choose Options > Reference Data > Display.
2. When prompted, generate fresh reference data (this requires a successful compile).
3. Open the Cross-Reference view and filter by the DB number (e.g., DB100).
4. Inspect every Write access: the source block, the network, and the operand. Any of the following constructs will overwrite the actual value:

         L     500              // 500 ms new process time
         T     DB100.DBD0       // overwrites DB100.processing_time_1
   

5. If a write is found, decide whether the write is intended (recipe load, setpoint from HMI, etc.) or accidental. If it is intended, the DB will only hold the new initial value for one scan before the program re-writes it — that is normal and not a bug.

Best Practices for Commissioning and Operations

Frequently Asked Questions