Resolving CFC BACnet Process Errors and Block Limit Warning 7325 in PCS 7

In SIMATIC PCS 7, encountering compiler warning 7325 followed by runtime "process error" status flags on BACnet input blocks when integrating building automation controllers (such as Desigo PXC00-E.D) occurs when the configured BACnet object count exceeds the communication polling capacity of the automation station. While CFC compiles successfully with warning 7325, instantiating over 100 BACnet objects in a single cyclic execution group exhausts controller communication memory buffers and request queues. Resolving this issue requires distributing BACnet polling blocks across multiple slower cyclic interrupt tasks (e.g., OB32 at 1000 ms), optimizing Change of Value (COV) polling increments, and observing maximum object limits.

BACnet Integration Architecture in SIMATIC PCS 7

In hybrid industrial and facility automation projects—such as cleanrooms, pharmaceutical HVAC monitoring, or central utility plants—SIMATIC PCS 7 interfaces with Building Management Systems (BMS) utilizing the international BACnet/IP protocol.

Controllers such as the Siemens Desigo PXC00-E.D act as BACnet communication servers, hosting Analog Input (AI), Analog Output (AO), Binary Input (BI), and Multistate objects.

In the Continuous Function Chart (CFC) environment, dedicated communication driver blocks query these BACnet objects over Industrial Ethernet. However, unlike native S7 communications that utilize optimized cyclic memory copies, BACnet communication relies on client-server request/response transactions and Change of Value (COV) subscriptions governed by finite controller communication buffers.

Understanding CFC Compiler Warning 7325

During CFC compilation, the engineering system evaluates resource allocation:

Warning 7325: "The maximum number of blocks per runtime group or task has been exceeded."
While the compiler treats this as a non-fatal warning and generates executable code, it indicates that the number of block calls assigned to a single organization block (e.g., OB35 at 100 ms) exceeds standard architectural sizing thresholds.
When 122 or more BACnet communication blocks are placed in the same fast interrupt group, the CPU attempts to trigger over a hundred network transaction requests within a single execution cycle.

Root Causes of Runtime Process Errors on BACnet Inputs

When the compiled program is downloaded to the controller, the program begins executing, but input blocks report a persistent Process Error:

Failure MechanismTechnical Root CauseDiagnostic ObservationResolutionBuffer StarvationController communication queue flooded with BACnet requestsWarning 7325 in compile; random blocks report Process ErrorMove BACnet blocks from OB35 to OB32 / OB31Network TimeoutDesigo PXC00-E.D cannot respond to 122 queries simultaneouslyCommunication timeout flag asserted on block outputIncrease BACnet polling intervalsCOV FloodSmall sensor fluctuations trigger continuous COV updatesEthernet network utilization spikes; packet dropsIncrease COV increment threshold in DesigoDevice Instance ConflictDuplicate BACnet Object IDs or Device IDs configuredS7 block cannot bind to target BACnet objectRe-verify BACnet Device ID and Object Instance

Because the PXC00-E.D microprocessor has finite socket handling capabilities, incoming requests exceed the receive buffer, resulting in dropped frames and random 'Process Error' status flags across the charts.

Optimization: Task Redistribution and COV Configuration

To eliminate warning 7325 and resolve runtime process errors:

Redistribute Blocks to Slower Cyclic Interrupts:
- Building environmental variables (HVAC temperature, relative humidity, damper status) change slowly and do not require 100 ms processing.
- In the CFC Runtime Editor (Edit -> Run Sequence), move the BACnet communication blocks from OB35 (100 ms) to OB32 (1000 ms) or OB31 (2000 ms).
- Partition the 122 blocks into multiple runtime groups (e.g., 4 groups of 30 blocks), executing them across staggered phases.
Optimize Change of Value (COV) Settings:
- In the Desigo configuration tool or block parameters, set a realistic COV Increment (e.g., set temperature COV to 0.5 °C rather than 0.01 °C). This prevents continuous telegram spamming caused by analog signal noise.
Recompile and Download:
- Compile CFC: verify that warning 7325 disappears.
- Download changes to the controller.

Verification

To verify BACnet communication health:
1. In CFC Editor, switch to online watch mode.
2. Inspect the BACnet driver blocks; confirm that the 'Process Error' flag clears (0) and that Quality Code indicates healthy status (ST = 16#80).
3. Verify that live analog values (temperature, airflow, damper position) match the physical values displayed on the Desigo tool.
4. Review the CPU diagnostic buffer to confirm zero communication error events.

Limitations and Operational Precautions

Never place high-volume third-party communication driver blocks into fast interrupt tasks (OB35, OB36). Doing so starves internal S7 communication buffers and can cause communication timeouts between the CPU and PCS 7 WinCC OS servers. Always consult controller technical specifications for maximum simultaneous BACnet object limits.

Engineer FAQ

Can CFC compiler warning 7325 cause CPU STOP?

Warning 7325 alone will not cause a STOP, but the underlying execution overload can cause OB execution times to exceed cycle boundaries, triggering OB80 time error interrupts.

How many BACnet blocks can a PXC00-E.D controller reliably handle?

Depending on firmware and network traffic, a PXC00-E.D typically supports up to 100–150 BACnet objects when polling intervals are relaxed to 1–2 seconds.

What does Change of Value (COV) mean in BACnet?

COV is a reporting mechanism where the BACnet server only transmits an update when the process value changes by more than the configured COV increment threshold.