Resolving DOTS_SWP Swap File Errors in Siemens PCS 7 CFC

Overview of the DOTS_SWP Behavior in STEP 7 and PCS 7

The DOTS_SWP directory and its resident DOTS00.SWP file are an internal working-storage mechanism used by the Siemens SIMATIC Manager environment, most visibly by the CFC (Continuous Function Chart) editor in PCS 7. The directory is created at runtime, used as a temporary swap area while projects are loaded and compiled, and removed automatically when SIMATIC Manager closes. The behavior is documented in Siemens technical support material covering the general conditions for SIMOTION and CFC: "CFC creates the 'DOTS_SWP' directory in the root directory of the partition that has the most free memory space" (Siemens Support, Simotion Corrected General Conditions since V4.1.2 to V4.4).

Although the file appears in the same manner as a Microsoft Word shadow copy, it is not user data and is not recoverable. Engineers observing the folder for the first time typically believe the workstation has been compromised by malware. The behavior is benign by design but becomes problematic when the partition selected by the algorithm is small, slow, write-restricted, or shared with another application that fills the disk during compile operations.

When and Why the DOTS_SWP Folder is Created

Three preconditions must all be true before DOTS_SWP appears on a drive:

1. SIMATIC Manager (or PCS 7) must be running. The folder is never written by background services.
2. A S7 project (S7P) or multi-project must be open. Opening SIMATIC Manager with no project loaded does not create the folder.
3. The CFC editor must be invoked at least once during the session, or a CFC chart must be present in the active project. The swap area is allocated when CFC first touches the chart database.

The companion executable that allocates and frees the directory is S7tgtopx.exe, located by default in the STEP 7 install path:

Because the binary resolves the target volume relative to its own load drive, installations on D:\ will preferentially allocate on D:\. The explicit selection logic, however, is "drive with most free space at allocation time," so an engineer with multiple partitions will see the folder migrate between sessions depending on which volume has the largest free-byte count.

File Contents and Lifecycle

The single artifact inside the folder is named DOTS00.SWP. Its size is dynamic:

The directory is removed when SIMATIC Manager closes cleanly. An unclean termination (kill of S7tgtopx.exe, BSOD, power loss) leaves the folder orphaned. Orphaned folders can be deleted manually; they will be re-created on the next session.

Common Symptoms and Reported Errors

While the folder is functionally a scratch area, several failure modes appear repeatedly in field deployments:

The error code 1739:310 originates from the CFC chart cache layer when the runtime cannot extend its swap segment. The literal translation from the German message catalog is "no space left on device" (errno 28 on POSIX-style runtimes). On Windows it is mapped to HRESULT 0x80070027 / Win32 39.

Root Cause Analysis

The selection algorithm in S7tgtopx.exe is a single-pass scan over fixed and removable drives. It does not weigh:

• Drive rotational latency or SSD endurance
• Drive write policy (some USB enclosures report read-only after a CRC mismatch)
• Whether the volume is a network share (UNC mapping returns valid free-space data and is selected)
• Whether the user has write permission (the allocation fails silently and reverts to the install drive)

Combined with Windows file-system reporting quirks (e.g., the read-only attribute displayed for files inside the folder is purely a Windows Explorer presentation artifact and is not the DOS read-only bit; see Microsoft Q&A on read-only folders for the underlying behavior), the result is that the algorithm picks a partition that is technically largest but operationally unsuitable.

Root cause for the disk-full class of failures is therefore not a corrupt file but a wrong drive selection compounded by a small or write-restricted volume.

Solution: Configure the Swap File Folder on PCS 7 V8.2 / V9.0

Starting with PCS 7 V8.2.x and V9.0.x, when the latest CFC hotfix updates are installed, the swap-file destination can be assigned explicitly through the SIMATIC Manager menu tree. This is the supported remediation path.

Prerequisites

• SIMATIC Manager closed on all nodes of the multi-project
• User logged in with administrator privileges on the engineering station
• PCS 7 V8.2 SP1 or later, or PCS 7 V9.0 SPx with current CFC update package installed (verify via SIMATIC Manager > Help > About > Installed Software)
• Target partition formatted NTFS, write-enabled, with at least 1 GB of sustained free space

Step-by-Step Configuration

1. Launch SIMATIC Manager.
2. In the project tree, select the root of the project or multi-project (do not select an individual chart or station).
3. Open the menu: Options → Charts → Settings for swap file…
4. In the dialog, the current path field displays the algorithm-detected volume. Click Browse and select the target directory (for example C:\Siemens\CFC_SWAP\).
5. Confirm with OK. The setting is written to the project registry under HKCU\Software\Siemens\Automation\SWP_Path and persists per Windows user.
6. Re-open the project to force S7tgtopx.exe to allocate DOTS_SWP in the new location.

Verification

1. After opening the project, watch the target partition for the DOTS_SWP directory creation event (use Sysinternals Process Monitor filtered to S7tgtopx.exe and file create operations).
2. Trigger a full compile (CFC → Compile → Charts or Charts → Übersetzen) and confirm DOTS00.SWP growth on the selected drive.
3. Confirm no DOTS_SWP directory appears on previously selected drives within 30 seconds of opening the project.

Workarounds for STEP 7 V5.x and PCS 7 V6 – V8.1

The configuration menu was introduced only with PCS 7 V8.2. Older installations require one of the following workarounds. Each addresses a specific failure mode.

Workaround A – Constrain Free Space on Unwanted Volumes

Because the algorithm selects "drive with most free space," fill the unwanted volumes with non-erasable files until the target drive becomes the largest. This is reliable but operationally awkward.

1. Identify unwanted drives (for example, a USB backup drive that mounts as E:\).
2. Create a 1 GB sparse file on each unwanted drive: fsutil file createnew E:\placeholder.dat 1073741824
3. Mark the file as system + hidden so it is not accidentally deleted: attrib +H +S E:\placeholder.dat
4. Open the project. Verify DOTS_SWP appears on the engineering drive instead.

Workaround B – Remap the Install Drive

The algorithm falls back to the install drive when no other candidate has more free space. If STEP 7 is installed on a small SSD but the data volume is large, moving the install path via junction is unreliable because of service dependencies. The simpler approach is to install STEP 7 on the desired drive (for example, an SSD with 50 GB free) so the fallback path is correct.

Workaround C – RAM Disk

For very large projects, point the largest free-space candidate at a RAM disk. This requires a third-party RAM disk driver (ImDisk, AMD Radeon RAMDisk, etc.) sized at 2 GB or larger. The swap area is highly I/O intensive during compile, and RAM-backed storage can cut CFC compile time by 30 – 60 percent on multi-project builds.

1. Install and start the RAM disk driver.
2. Create a 4 GB NTFS-formatted RAM disk, mount as R:\.
3. Pre-fill it with a 1 GB placeholder file so it remains the largest free-space candidate across reboots (the placeholder survives only if the driver supports persistent images).
4. Open the project and confirm DOTS_SWP lands on R:\.

Workaround D – Migrate to PCS 7 V9.0

If the deployment footprint allows, upgrading to PCS 7 V9.0 with the latest CFC update gives the engineer the supported configuration path described above and eliminates the algorithmic drive selection entirely.

Disk Space Sizing Calculation

To pre-size the target partition before commissioning, estimate the worst-case swap requirement:

Where:

• Charts_total = total CFC chart count across the multi-project
• FB_instances = total function-block instance count (CTRL, MOT, VAL, etc.)
• SFC_instances = total SFC chart instances

Worked example for a 1500-chart PCS 7 plant with 8000 FB instances and 200 SFCs:

SWP_max ≈ 1500 × 0.08 + 8000 × 0.012 + 200 × 0.005
≈ 120 + 96 + 1
≈ 217 MB

Apply a 3× safety factor and round up to the next whole-GB partition size: 1 GB free on the target drive is the engineering baseline. Plants exceeding 10,000 charts should target 2 GB free minimum.

Error Code Reference Table

Troubleshooting Matrix

Process Monitor Capture Procedure for Diagnostics

When escalating to Siemens Technical Support, capture a ProcMon trace of the DOTS_SWP creation event:

1. Launch Sysinternals Process Monitor as administrator.
2. Set filter: Process Name is S7tgtopx.exe then Add.
3. Set filter: Operation is CreateFile then Add.
4. Clear the display, open the S7 project, and stop the capture after 60 seconds.
5. Export the resulting log as PML and attach to the support request along with the version output from SIMATIC Manager → Help → About.

Safety and Operational Notes

Verification Checklist Before Closing a Ticket

1. DOTS_SWP directory lands on the configured drive, not the drive with the most free space.
2. Full CFC compile completes without 1739:31x error codes.
3. Directory is removed automatically within 5 seconds of SIMATIC Manager closing.
4. Reopening the project re-creates the directory in the configured path.
5. Multi-project builds compile in expected wall-clock time (within ±10 % of baseline).
6. Disk free space on the configured drive is > 1 GB after compile.