Resolving WinCC flexible 2008 64kB Source Limit Error

Problem Overview

The error "No connection for PLC '...' found! The symbol is disabled for WinCC flexible or it exceeds the 64kB limit for a source!" appears in SIMATIC WinCC flexible 2008 Advanced when you try to point a tag (variable) on a PLC address inside an integrated STEP 7 project. The tag symbol on the HMI side becomes disabled (greyed out) and the connection is no longer available for selection. The text is misleading: most occurrences are not triggered by an actual symbol-table or DB exceeding 64 kB, but by a missing or mismatched S7 connection descriptor on the HMI station.

The symptom has been observed on multiple panels and runtime configurations, including the SIMATIC HMI TP 177B 6" color PN/DP, the SIMATIC HMI OP 170A integrated into a STEP 7 (SIMATIC Manager) project, and PC-based WinCC flexible Runtime stations. In one documented case the panel project worked normally for weeks before the error appeared "out of nowhere" after a configuration change. In another case the panel project continued to compile cleanly, and only the newly added PC Runtime station triggered the warning.

Working build (confirmed by an end user that resolved the issue):

• SIMATIC WinCC flexible 2008
• Version V1.3.0.0_1.83.0.1 build V1.3.0.0
• Operating system: Windows XP Professional SP2/SP3 or Windows 7 (32-bit) in the project context

Root Cause Analysis

The 64 kB source limit warning is generated by the AS-OS engineering interface (the integrated data exchange between STEP 7 and WinCC flexible) when it cannot resolve the PLC reference of a tag. There are four distinct root causes, ordered by frequency in the field:

The 64 kB threshold is hard-coded inside the WinCC flexible 2008 AS-OS transfer buffer. When the connection to the S7 CPU cannot be reconstructed by the parser, the engineering falls back to a raw block dump and trips this limit. The 64 kB text is therefore best read as "the parser could not link the symbol to its PLC source", not as a true size violation.

Affected Software Versions

The error has been reproduced on the following WinCC flexible 2008 versions. Newer versions are less affected because Siemens corrected the AS-OS parser in SP2 and consolidated the fixes in Hotfix 5 / Update 12.

Always check Help → About in WinCC flexible and confirm the exact internal version string before applying any patch. Siemens delivered the corrective updates through the Siemens Industry Online Support portal under entry ID 34062971 and the related Hotfix/Update read-me pages.

Solution 1 - Add the CP / IE Interface to the HMI Station

This is the most common field fix and the one that works for the OP 170A / TP 177B family. The HMI station needs an explicit network interface in the STEP 7 hardware catalog so the AS-OS engine can resolve the S7 route.

1. Open SIMATIC Manager and load the integrated STEP 7 + WinCC flexible project.
2. In the project tree, double-click the HMI station (for example OP 170A or TP 177B). This opens the HMI hardware configuration (HW Config) of the device.
3. Open the hardware catalog (View → Catalog) and navigate to:
   • SIMATIC PC Station → HMI for PC-based Runtime, or
   • SIMATIC HMI Station → [your panel family] for panel targets
4. Drag the appropriate communications processor onto the HMI station rack:
   • CP 5611 / CP 5613 / CP 5621 for PROFIBUS
   • CP 1616 / CP 1623 / CP 1628 for PROFINET / Industrial Ethernet
   • IE General for a soft PLC connection over TCP/IP
5. Assign the same PROFINET subnet / PROFIBUS network that the S7 CPU is connected to. The Properties → Interface dialog must show the same subnet and the same IP / MAC as the CPU rack.
6. Save and compile the HW Config. STEP 7 generates an S7 connection (S7-Verbindung) that the WinCC flexible tag browser can follow.
7. Switch back to WinCC flexible, recompile, and confirm that the tag is no longer greyed out.

Solution 2 - Install WinCC flexible 2008 Service Pack 2 and Update 12

If Solution 1 does not clear the error, or if the same project compiles fine from a second engineering station, the WinCC flexible 2008 parser is the suspect. Apply the cumulative patches in the documented order:

1. Uninstall the existing WinCC flexible 2008 installation through Control Panel → Add/Remove Programs. Reboot.
2. Install the base DVD image SIMATIC WinCC flexible 2008 Advanced.
3. Install Service Pack 1 for WinCC flexible 2008 (entry ID 34062971 at Siemens Industry Online Support).
4. Install Service Pack 2 for WinCC flexible 2008. SP2 reworks the AS-OS connection handling and is the structural fix for the false-positive 64 kB warning.
5. Install Hotfix 5 for SP1 if you cannot move to SP2 for license reasons. The hotfix alone has been observed to fail on PC Runtime stations; SP2 is the more reliable level.
6. Install the latest Update on top of SP2 (Update 12 / UD12 is the level reported as working: V1.3.0.0_1.83.0.1).
7. Reboot and re-open the project. Re-compile the HMI station.

Patch order matters. Installing Hotfix 5 on top of SP2 without the prerequisite SP2 re-installation can leave the AS-OS DLLs in an inconsistent state and the warning will persist. Always read the Readme file inside each patch before executing the setup.

Solution 3 - Change the Device Type from PC Runtime to HMI Panel

This workaround applies when the error appears only after a Device → Change Device operation that switched the target from a panel to a PC Runtime. The panel firmware image and the Runtime image share the editor, but the connection descriptors and the AS-OS engine paths differ between the two targets.

1. In WinCC flexible, right-click the HMI device in the project tree and select Change Device → Change Device Type.
2. Pick the panel model that matches the firmware loaded on the physical unit (for example TP 177B 6" color PN/DP).
3. Confirm the migration. WinCC flexible will translate the Runtime-specific properties; some unsupported objects (for example certain ActiveX controls) are dropped and must be re-added.
4. Save, compile, and re-test. The 64 kB warning typically disappears once the project aligns with a real panel target.

Conversely, if the project is meant to run on a PC and the device is set to a panel target, switch it to WinCC flexible Runtime through the same menu. The compiler chooses a different tag-resolution path for each device class.

Solution 4 - Genuine 64 kB Source Exceeded

If Solutions 1-3 do not apply (the S7 connection is correct, the patch level is current, and the device type is correct) the 64 kB warning is literal. Verify the actual size of the source the tag points to:

1. In STEP 7, open the symbol table (S7-Program → Symbols) and look at the bottom-right size indicator. The WinCC flexible 2008 AS-OS interface loads the entire symbol table; if it exceeds 64 kB, symbols beyond the limit are not visible to WinCC flexible.
2. Check the data blocks (DBs) referenced by tags. Open each DB in STEP 7 and confirm the block size. Any DB larger than 64 kB triggers the same warning on tags that map to addresses inside it.
3. To reduce the symbol table, split it across multiple Symbol Tables in STEP 7 and assign only the relevant table to the HMI's AS-OS interface (Options → AS-OS Assignment).
4. To reduce a single DB, split it: copy the upper half into a new DB, update the PLC program to use both, and remap the WinCC flexible tags to the new DB.
5. Recompile. The greyed-out symbols become available again.

For very large symbol tables, modern TIA Portal / WinCC (TIA) handles the limit differently and can be the better long-term platform. WinCC flexible 2008 is end-of-life and the 64 kB limit is a structural artifact of that product line.

Verification Procedure

After applying any of the four solutions, validate the fix with the following checks:

1. Tag dialog: Double-click a tag in the WinCC flexible project tree. The Address field must accept the PLC symbol and must not display the disabled icon. The tooltip should no longer show the 64 kB warning.
2. Connection list: Open Project → Connections. The S7 connection between the HMI and the CPU must show status OK, not Disconnected or Not configured.
3. Compile log: Trigger a full rebuild (Project → Compiler → Rebuild All). The output window must contain zero Warnings related to 64kB, symbol disabled, or no connection.
4. Live tag test: Start WinCC flexible Runtime (or transfer to the panel) and trigger a tag update on the HMI. The value must change when the PLC value changes; right-click the tag and select Update in the simulator if no live value is present.
5. Cross-check on a second PC: Copy the integrated STEP 7 / WinCC flexible project to a second engineering station with the same patch level. If the error reproduces on the second PC only, the engineering PC is at fault; if it reproduces on both, the project itself is at fault.

Troubleshooting Matrix

Prevention and Long-Term Recommendations

• Pin a patch level. Standardize the engineering team on WinCC flexible 2008 SP2 + Update 12 (V1.3.0.0_1.83.0.1) or later. Document the version in the project header so that all PC installations are identical.
• Keep the HW Config in sync. Any change to the S7 rack (CPU, CP, subnet) must trigger a review of the HMI station's HW Config. The most common trigger of the 64 kB warning is a CPU migration that left the HMI's CP reference pointing at the old subnet.
• Cap symbol-table growth. Avoid stuffing all PLC symbols into a single global table. Split per unit or per area; this also helps TIA Portal migration later.
• Backup before device exchange. Always take a full project backup before Change Device → Change Device Type. The migration can drop device-specific objects that the HMI needs.
• Plan migration to TIA Portal. WinCC flexible 2008 is end-of-life. TIA Portal / WinCC (TIA) removes the 64 kB AS-OS limit and provides a modern connection editor. New projects should start in TIA Portal; legacy projects should be migrated when the engineering effort allows.

FAQ