Siemens KTP600 HMI Buttons Not Writing to S7-1200: IM Folder Fix

Problem Description

On a Siemens KTP600 Basic color PN (6AV6647-0AD11-3AX0 or current catalog variant) configured with TIA Portal V12 SP1, the following symptom presents after a partial project change and download cycle:

• HMI runtime boots normally, screen graphics load, button visuals are correct.
• Pressing a configured button (configured as Set bit, Set bit while key is pressed, or Toggle) generates an on-screen error indicating the tag could not be written to the controller.
• The HMI's Diagnostics > System Events view shows entries such as "Tag write protected", "Connection interrupted", or "Tag value could not be written" (alarm IDs 400002, 400003, 400006, 400020 depending on the configured alarm class).
• The same event is configured in the button's Events tab of the screen editor, but the PLC tag never toggles.
• Cross-check with Project tree > Devices & Networks > Connections shows the HMI connection is in Online & Diagnostics state Established and the S7-1215C DC/DC/DC CPU is reachable through the configured PROFINET interface (X1).

The configuration error masks the underlying runtime artifact: the IM (Image) folder in the TIA project directory holds the generated WinCC Runtime files, and a partial rebuild has left a corrupted cache that is out of sync with the freshly downloaded HMI image. The button event table is functionally correct, but the compiled Runtime expects different tag offsets than what the S7-1200 firmware 4.x currently exposes.

Root Cause Analysis

When TIA Portal V12 compiles an HMI project for a KTP600 panel, WinCC writes a series of intermediate binaries into a folder named IM located at:

\<ProjectName>\IM\

Inside this folder you will find the generated *.bin and *.db runtime containers, compiled tag lists, text list indices, alarm buffers, and the Active runtime flag. Under normal conditions, every Compile > Software (rebuild all) action regenerates this folder atomically.

The failure mode occurs when:

1. The TIA Portal session is terminated abnormally (system crash, power loss, Force close of the TIA Portal V12 process).
2. Multiple incremental compiles occur in quick succession after a tag rename or DB restructuring.
3. The TIA Portal project is opened from a synchronized cloud share (OneDrive, Dropbox, Synology Drive) where an external sync agent locks or partially overwrites a file inside IM while the TIA Portal V12 compiler holds a write handle.
4. The HMI download is interrupted between transfer of the IM container and the panel's flash commit, leaving the panel booting with a hybrid runtime that mixes two compile states.

Symptoms of an IM-folder mismatch specifically include tag writes that compile cleanly, pass the HMI's internal connection test, but are rejected by the panel's runtime dispatcher with a "tag could not be written" status. The PLC tag may still be readable (because reads use the same path but a different code branch), so the panel appears to be talking to the CPU, while writes silently fail.

Prerequisites

Step-by-Step Resolution

1. Close the project in TIA Portal V12. If TIA Portal holds the project open, you cannot reliably delete the IM folder on Windows level. Use Project > Close and wait until the editor surface clears.
2. Open Windows Explorer and navigate to the project root. The default location for a TIA Portal V12 project is Documents\Automation\<ProjectName>\ or any user-defined path selected in the TIA Portal project save dialog.
3. Verify the folder structure. You should see the following subfolders and files alongside the project archive:
   • AdditionalFiles\
   • IM\ <-- target folder
   • Logs\
   • System\
   • tmp\
   • UserFiles\
   • <ProjectName>.ap12 (TIA Portal V12 project database)
4. Create a safety copy of the IM folder. Right-click IM, Copy, then Paste. Rename the new folder to IM_original. This gives you a fallback if the rebuild produces a worse state, and lets you diff text list contents for forensic analysis if the failure recurs.
5. Delete the original IM folder. Select IM and press Shift+Delete to bypass the Recycle Bin. Do not use TIA Portal's internal Project cleanup for this step — direct deletion guarantees no stray binaries remain.
6. Reopen the project in TIA Portal V12. Project > Open > ... and select <ProjectName>.ap12. Allow the project to fully load; you will notice that the HMI device view may take a few seconds longer to populate because the IM cache is empty.
7. Trigger a full HMI rebuild. In the project tree, right-click the HMI device (for example KTP600_1 [KTP600 Basic color PN]) and select Compile > Software (rebuild all). Equivalently, you can right-click the project root and select Compile > All > Software (rebuild all) if you also want to recompile the S7-1215C station. The compiler writes a fresh IM folder containing the regenerated *.bin containers.
8. Watch the compile output. In the Inspector > Info pane, confirm that the build completes with zero errors and zero warnings. Warnings about unused tags or missing text list assignments indicate other configuration gaps that may surface as the same button error in a different shape.
9. Download to the panel. Select the HMI in the project tree, click Online > Download to device, choose the configured PG/PC interface (PROFINET or Ethernet), and confirm the transfer. The KTP600 will reboot into the freshly compiled runtime.
10. Test the previously failing button. Press the button in runtime. The tag should now write to the S7-1215C without error. Cross-verify in TIA Portal using Go Online on the PLC and forcing or monitoring the affected tag in the watch table.
11. Clean up the safety copy. Once you have validated the fix in production, you may delete the IM_original folder. Keep the project-level backup of the entire \<ProjectName> tree until the next change cycle completes.

Why This Works

The IM folder is the canonical compile target for the WinCC Runtime that runs on the KTP600. When TIA Portal detects that the IM folder is missing, the HMI compiler performs a clean rebuild that re-resolves every tag, alarm, text list, and screen reference against the live project database. This eliminates the class of bugs that arises when the runtime cache points to a tag address that no longer exists in the current S7-1200 data block layout.

Tag writes that compile correctly but fail at runtime are particularly insidious because the HMI connection itself is healthy. The PLC returns no error — the panel itself is rejecting the write based on a stale internal pointer. A full rebuild re-establishes the correct pointer map.

Related Configuration Check: Text Lists

Button errors that look like write failures can also originate from a text list that references a tag whose value range no longer matches the configured list. According to the official TIA Portal documentation on text lists in TIA Portal, every text list is assigned a value range and the runtime indexes the list by the current value of the associated tag. If the tag was renamed, the text list silently points to a non-existent variable and the button event referencing that list can fail validation.

When the IM folder is regenerated, the text list indices are recompiled against the current tag database, which usually resolves any drift introduced by tag renames. If the error persists after the rebuild, open each text list referenced by the failing button and confirm that the configured tag is still present and the value range covers all valid tag states.

Verification Matrix

Additional Diagnostic Steps If the Error Persists

1. Check the HMI connection on the PLC side. In TIA Portal, open Devices & Networks, select the connection between the HMI and the S7-1215C, and verify the Connection path and Active connection establishment settings. A misconfigured partner is a common cause of write rejection even when reads succeed.
2. Inspect the access protection of the target tag. If the DB is configured as Know-how protected with a password, and the HMI connection does not provide the password, all read and write requests are denied.
3. Confirm that the tag is writable on the HMI side. In the tag properties, switch the Access mode between Absolute access and Symbolic access. S7-1200 symbolic access requires firmware V4.0+; older firmware silently rejects the write.
4. Check the PG/PC interface assignment. On the engineering station, open Control Panel > Set PG/PC interface and confirm that S7ONLINE (STEP7) -> TCP/IP is bound to the correct NIC. The wrong interface causes the download to succeed but leaves the connection pointing to an unreachable subnet.
5. Verify the HMI image version. The KTP600 image version must be compatible with the TIA Portal version used to compile the project. TIA Portal V12 requires image version 12.x. A mismatch can cause the panel to accept the transfer but reject specific tag operations.
6. Recreate the button from scratch. As a last resort, delete the failing screen object and re-add it. Sometimes a single broken pointer persists in the IM folder cache even after a rebuild; recreating the object forces a fresh pointer allocation.

Preventive Practices

• Save and close the project cleanly at the end of every engineering session. Do not rely on Windows shutdown timers to close TIA Portal.
• Store TIA Portal projects on a local SSD rather than on a synchronized cloud share. If a cloud share is required, exclude the IM and tmp folders from the sync agent.
• Perform a full rebuild after any change to DB structure, tag names, or HMI connection configuration. Incremental rebuilds are faster but can leave cross-references inconsistent.
• Tag clearly with comments. Use the tag's Comment field to record the last project version in which the tag was modified. This makes post-incident diffs much faster.
• Maintain a project archive policy. Export the TIA Portal project as a *.ap12 zip archive at the end of every approved change. The archive bundles the IM folder and protects against corruption of the live project files.
• Validate text lists after every tag rename. As noted in the TIA Portal text list documentation, text lists are indexed by the current value of a tag, and renaming a tag without updating the list creates a silent pointer mismatch.

Edge Cases and Platform Notes

The IM folder procedure is also applicable to larger panels in the SIMATIC HMI family — KTP700, KTP900, KTP1200, TP700 Comfort, TP900 Comfort, and TP1200 Comfort — when used with TIA Portal V12. On Comfort panels, the IM folder contains additional *.fwx container files for the more complex runtime. On Basic panels like the KTP600, only the basic *.bin containers are generated.

For projects built in TIA Portal V13 or later, the same procedure applies, but the project file extension changes (.ap13, .ap14, .ap15, .ap16, .ap17, .ap18, .ap19, .ap20). The IM folder is the canonical container across all versions and serves the same role.

If you are migrating an HMI configuration from WinCC flexible 2008 SP3 to TIA Portal V12, the IM folder does not exist in the source project. The migration wizard creates it during the first compile, and the same corrupt-cache class of errors can occur if the migration is interrupted. In that scenario, deleting the IM folder after migration and before the first download is a recommended best practice.