Download S7-1200 S7-1500 Tag Comments in RUN Without DB Re-Init

Problem Definition: Why DB Re-Initialization Is a Concern on a Running Machine

On a live S7-1200 or S7-1500 CPU, an unintended transition from RUN to STOP during a download causes the process to halt, outputs to drop, and any data block values that are not retentive to be re-initialized to the initial values declared in the offline project. For a packaging line, a bottling skid, a furnace temperature controller, or any continuous process, that translates directly into scrapped product, lost throughput, and a process engineer standing at the panel asking why the system just went down.

The most common trigger is a documentation-only change — adding, editing, or deleting a comment on a tag in a PLC tag table, on a tag inside a global DB, on a network title, or on a block header. In the offline engineering tool, these look like harmless edits. In the CPU's change-tracking logic, they are compiled into a project delta that is compared against the online project. If the change check determines that the delta affects anything other than pure comments, the CPU is forced to STOP to re-initialize the affected blocks and reload them.

The good news: TIA Portal and the S7-1200/S7-1500 firmware explicitly support comment-only downloads in RUN. The bad news: there is one major exception — comments inside the device and network configuration of the hardware (HW Config) — and several edge cases that engineers trip over in the field. The remainder of this article walks through what is safe, what is not, and the exact procedure to perform a comment-only download in RUN without re-initializing any data blocks.

How TIA Portal Decides Between RUN and STOP

When you click Download to device, TIA Portal performs a consistency check against the CPU's current online content. The result of this check determines the operating mode transition. The decision tree is summarized in the SVG flowchart below.

The classification table that the S7-1200/S7-1500 firmware uses internally is shown below.

The S7-1200 supports downloads in RUN from firmware V4.0 onward (article group 6ES721x-1xxxxx-xxxx). The S7-1500 supports it from the first shipped firmware version (V1.0+). The official Siemens support FAQ titled "Downloading blocks in the 'RUN' operating mode to the device" documents the supported change classes and is the authoritative reference for the rules above. For TIA Portal project engineering context, refer to the TIA Portal V18/V19 documentation on the Siemens Industry Online Support portal.

The Hardware Configuration Exception

The reason is that the hardware configuration is treated as a single compiled object. The online/offline diff does not look inside the HW Config object to determine whether a sub-element is a comment or a topology change. From the CPU's perspective, an HW Config download is an atomic event that requires a controlled re-initialization of the module addressing tables and PROFINET/PROFIBUS device lists.

Practical implication: if you are working on a live machine and want to add documentation to a tag, do not add that documentation inside the Devices & Networks editor. Add the comment in the PLC tag table, the DB tag declaration, or the watch table — these are the safe locations for RUN-mode downloads.

Retentive vs. Non-Retentive: What Survives a STOP Even When You Get It Wrong

Even if a download does force a STOP transition, tags marked with the Retain attribute retain their last-written values across the stop/start cycle. The at-risk tags are those without the Retain attribute — they are reset to the Initial value declared in the offline DB declaration.

The default setting in TIA Portal for new tags is Retain = false. Engineers frequently rely on this default without realizing that a STOP transition will reset their counters, setpoint registers, and state-machine flags. Verify the retentivity assignments of the tags most critical to your process before performing any online download to a live machine.

Firmware and Software Prerequisites

Confirm the following prerequisites before attempting a comment-only download in RUN:

• CPU firmware: S7-1200 must be at firmware V4.0 or higher. The order number group 6ES721x-1xxxxx-xxxx covers all variants. Check the firmware version in Online & Diagnostics → Diagnostics → CPU information or on the CPU's display. S7-1500 supports RUN-mode downloads from all shipped firmware versions (V1.0+).
• Failsafe CPU consideration: On a failsafe CPU (S7-1200F, S7-1500F), comment-only downloads are still permitted, but the failsafe program signature is recomputed. Safety mode is not affected as long as no program logic changes. The safety-related change log inside the CPU's diagnostic buffer records every signature change for audit purposes.
• TIA Portal version: TIA Portal V13 SP1 or higher is required for the full RUN-mode download feature set on S7-1200. TIA Portal V14 or higher is recommended for S7-1500. The current engineering tool is TIA Portal V18 / V19. Older TIA Portal versions can read the project but the Download in RUN option may be greyed out.
• Online connection: PROFINET, PROFIBUS, or Ethernet — any online path supported by TIA Portal works. The CPU does not need to be put into a special state; the active RUN mode is sufficient.
• Project consistency: The offline project must compile without errors. The download will be refused if the project is dirty or has unresolved references.
• Project ↔ CPU binding: Verify the offline project is bound to the correct CPU by right-clicking the device node and selecting Go online → .... A common field error is downloading a project compiled against one CPU's serial number to a different physical CPU. TIA Portal will warn, but the warning can be overridden in the field.

Pre-Download Checklist

1. Open the project in TIA Portal and perform Project → Compile → All (rebuild). Resolve any compiler errors. Comments are not validated as code, so a comment typo will not cause a compile error — but a typo in a tag name that you intended to add as a new tag will.
2. Open Project tree → PLC → PLC tags and add the new comment to the relevant tag row. The comment column is the rightmost column in the tag table; if it is hidden, right-click the column header and select Show/Hide columns and enable Comment.
3. For DBs, open the DB in the project tree, double-click the Static section header, and edit the comment column on the target tag.
4. Save the project. TIA Portal will update the comment in the offline project. The CPU still shows the old comment until you download.
5. Click Go online in the toolbar. Confirm the online connection to the target CPU. Do not use the Online → Accessible nodes shortcut for the first connection — use the project tree right-click → Go online → Secure online connection (or plain online) to ensure the right CPU is selected.
6. Sanity-check the change set by going Online → Compare offline/online. The comparison must show comment changes only, with no interface or code deltas flagged.

Step-by-Step: Performing the Comment-Only Download in RUN

1. Right-click the PLC node in the project tree (not the device) and select Download to device → Software (only changes). The phrase only changes is the critical selection — this tells TIA Portal to compute a delta and download only the changed blocks.
   If you choose Download to device → Hardware and software, TIA Portal will attempt to re-flash the device configuration and force a STOP transition. Use this only for full configurations, not for documentation updates.
2. TIA Portal performs the consistency check and displays a preview dialog listing the blocks that will be downloaded. Verify that the listed blocks are exactly the ones you edited comments in. A typical comment-only download list looks like:

   DB_Recipe (DB100) - comment header updated
   PLC_Tags - default tag table, comments modified
   No code change, no interface change

3. Confirm the dialog. TIA Portal asks for the operating mode fallback. The default option is Download in RUN if possible, otherwise STOP. Keep this default. The CPU will stay in RUN because the change class is comment-only.
4. Click Download. The transfer takes 1–3 seconds per block depending on block size. The CPU does not stop; the RUN LED remains solid green.
5. After download, the load preview dialog closes and a status message confirms Download (RUN) finished, no errors. The online view of the block now shows the new comment.
6. Perform an Online → Compare offline/online check to confirm the offline project matches the online CPU.
7. Open Online → Online & Diagnostics → Diagnostic buffer and confirm a Download of changes in RUN entry with the current timestamp and no STOP entry with the same timestamp.

Symbolic Comments, Network Comments, and Block Comments — All Safe in RUN

The following comment locations are all classified as comment-only changes by the S7-1200/S7-1500 change-check logic and will not force a STOP:

• PLC tag table comments: the rightmost column on the default tag table, on user-defined tag tables, and on the system tag tables for the CPU's own special functions (high-speed counters, pulse outputs, etc.).
• DB tag comments: the comment column on the Input, Output, InOut, Static, and Temp sections of a global DB or instance DB.
• UDT (user-defined data type) comments: the header comment and per-element comments inside a UDT are comment-only. Renaming a UDT element, however, propagates to every DB that uses the UDT and forces a STOP.
• Block header / title / version / author / family comments: the block properties dialog's General tab and the Information tab.
• Network title and network comment: in FBD and LAD editors, the small comment box on the right of each network.
• Function (FC) and function block (FB) interface comments: the comment column on the Input, Output, InOut, Static, and Temp sections of the block interface declaration.
• Watch table comments: the comment column in a watch table is stored in the offline project only, not the CPU, and is therefore a local edit with no download required.

Verifying That the CPU Did Not Re-Initialize

After a comment-only download in RUN, perform the following checks to confirm the system state is preserved:

1. Operating mode LED: The RUN LED on the CPU remains solid green. If the LED drops to STOP (yellow) or blinks (stop with error), the change was not comment-only and the CPU re-initialized.
2. Diagnostic buffer: Open Online → Online & Diagnostics → Diagnostic buffer. Look for an entry of class Download with text "Download of changes in RUN". There should be no entry of class STOP or Re-initialization with a timestamp matching the download.
3. DB values: Open the affected DB in the project tree, right-click → Monitor/Modify, and verify that the actual values match the values you expect from the running process. Non-retentive tags should retain their last-written values because no re-initialization occurred.
4. Retentive tag sanity: If your DB uses the Retain attribute on specific tags, those tags will survive a STOP transition anyway. The non-retentive tags are the ones at risk — check those first.
5. Process status: Confirm with the operator that the controlled process did not drop out. If the process is automated and you cannot query the operator, check the HMI trend logs for the timestamp of the download; a one-cycle or several-cycle gap indicates a process interruption.
6. Online/offline block signature: In the project tree, the block's online/offline comparison icon should be a green check after a successful comment download. A yellow triangle or red cross indicates a deeper mismatch.

Troubleshooting Matrix: When the Download Drops to STOP

Best Practices for Live-Machine Documentation Updates

• Always perform a change-only download: from the project tree's PLC node, choose Download to device → Software (only changes). Avoid the hardware-and-software option for any work performed on a live CPU.
• Document in the tag table, not in the watch table: a watch-table comment lives only in your offline project; a tag-table comment lives in the CPU after download. If you want the comment visible to anyone else who later opens the project, put it in the tag table.
• Avoid bundling comments with structural changes: if you need to rename a tag and also fix its comment, do it in two separate downloads. Add the comment in RUN first; then schedule a maintenance window to perform the rename in a controlled STOP.
• Use block comments for cross-reference: a single block-level comment in the DB header explaining the purpose of the block is more valuable to the next maintainer than per-tag comments and is also less likely to require future edits.
• Keep a comment-only change log: in the project tree, use the Project history feature to record the date, author, and purpose of every comment-only change. This is invaluable during audits and during post-incident analysis if a process stop does occur.
• Verify after every download: always inspect the diagnostic buffer immediately after a RUN-mode download. The presence of a Download of changes in RUN entry with the current timestamp, and the absence of any STOP entry with the same timestamp, confirms that the change was comment-only.
• Use secure online connections: when downloading to a CPU on a production network, use the secure online connection option in TIA Portal. The secure connection does not affect whether the comment downloads in RUN; it only protects the integrity of the transfer.
• Backup before the first download: take an online snapshot of the CPU (Online → Snapshot of the online data) before the first comment download. If anything goes wrong, the snapshot lets you restore the CPU to its exact pre-download state.

Related RUN-Mode Considerations

Comment-only downloads are the simplest case of in-process online editing. The same change-check framework also supports the following operations in RUN:

• Function block instance initial values: changes to the Initial value column of a static tag inside an FB can be downloaded in RUN as long as the tag type is unchanged. The actual instance values are not affected.
• Code-block online editing: TIA Portal supports Edit online for FCs and FBs. The change-check rules for online edits are stricter than for comments, but they are also documented in the Siemens RUN-mode download FAQ.
• Recipe DB regeneration: regenerating a recipe DB from its UDT requires STOP. Plan the recipe schema change for a maintenance window. Recipe data values inside an unchanged UDT can be edited and downloaded in RUN as a comment-only change to the DB.
• Watch table: a watch table is a local engineering artifact, not stored on the CPU. Comments in a watch table are personal notes and require no download.

FAQ