How to Calculate CPU Load and Size Controllers for SIMATIC PCS 7 Projects

In SIMATIC PCS 7 engineering, the licensed Process Object (PO) capacity of a controller (e.g., 2,000 PO on a CPU 410-5H System Expansion Card) does not represent or determine the physical CPU execution load. Process Objects represent commercial software licensing units, whereas CPU load represents the cyclic computational utilization of the processor hardware. For a plant with approximately 2,800 physical I/O channels requiring 1,500 to 1,700 Process Objects, a single CPU 410-5H can technically host the entire configuration. However, to satisfy strict design specifications requiring CPU execution load to remain below 50%, engineers must calculate net execution times across organization blocks (OB35, OB34, OB32) or split the workload across two controller stations.

Process Object (PO) Licensing vs. Physical CPU Execution Load

A widespread engineering misconception confuses commercial licensing limits with physical processing performance:
- Process Objects (PO): A commercial accounting metric in SIMATIC PCS 7. One PO corresponds roughly to one technological block (a motor MotL, valve VlvL, analog monitor MonAnL, or PID controller PIDConL) that communicates with WinCC OS. The System Expansion Card (SEC) unlocks PO capacity up to 2,600+ POs.
- CPU Load (%): The actual percentage of time the controller's microprocessor spends executing instructions during each cyclic scan interval.
- Independence of the Metrics: A CPU configured with 1,700 POs running all blocks in a fast 100 ms interrupt (OB35) can easily reach $90\%$ CPU load. Conversely, the identical 1,700 POs distributed properly across 200 ms, 500 ms, and 1,000 ms interrupts may run at an effortless $35\%$ CPU load.

Converting Physical I/O Counts to Process Object (PO) Estimates

To estimate Process Objects from tender I/O schedules, industry-standard conversion ratios apply:

Signal TypePhysical Channel CountTypical Technological MappingEstimated Process Objects (POs)Analog Inputs (AI + AIR)$\sim 600$ channelsAnalog monitoring (MonAnL) and closed-loop control (PIDConL)$\sim 500 - 550$ POAnalog Outputs (AO + AOR)$\sim 60$ channelsModulating control valves (VlvAnL)$\sim 50 - 60$ PODigital Inputs (DI + DIR)$\sim 1,750$ channelsInputs to motor feedbacks, valve limits, electrical interlocks$\sim 700 - 800$ PODigital Outputs (DO + DOR)$\sim 340$ channelsMotor run contactors (MotL), solenoid valves (VlvL)$\sim 300 - 340$ POTotal Plant Footprint$\sim 2,750$ I/O channelsComplete plant unit operations$\approx 1,550 - 1,750$ Total POs

Why Sizing solely by PO Limits Risks Cycle Overload

Tenders frequently mandate that "CPU cyclic load must not exceed 50% under peak operating conditions":
* The Single-Controller Risk: A single CPU 410-5H with an SEC 2k card has sufficient load memory and PO license capacity to hold 1,700 POs.
* The Cycle Execution Bottleneck: In a continuous process plant, executing 1,700 Advanced Process Library (APL) blocks, channel drivers, diagnostic blocks, and complex interlocks generates substantial CPU interrupt load.
* H-System Synchronization Penalty: If a redundant S7-400H / CPU 410-5H pair is utilized, memory synchronization across optical links adds a continuous $10\%$ to $15\%$ overhead to cyclic scan times.
* Conclusion: Packing 1,700 POs into a single controller will push typical CPU utilization to $60\% - 75\%$, violating the client's $50\%$ load ceiling.

Method: CPU Load Estimation and Multi-Controller Sizing Workflow

To design a high-reliability system conforming to the $<50\%$ CPU load requirement, execute this engineering sizing process:

Step 1: Use the Official Siemens SIMATIC PCS 7 CPU Sizer Tool

Download the official SIMATIC PCS 7 Tool for Estimating AS Process Objects and CPU Load.
Enter the exact count of specific block types:
- Number of PID controllers (PIDConL).
- Number of motors (MotL) and modulating valves (VlvAnL).
- Number of analog inputs (MonAnL) and digital inputs (Pcs7DiIn).
Define the execution cycles:
- Fast closed loops (pressure, flow): OB35 (100 ms).
- Standard drives and discrete interlocks: OB34 (200 ms).
- Slow thermal loops and monitoring tanks: OB32 (1000 ms).
The tool computes the predicted execution time in milliseconds and calculates net CPU load.

Step 2: Determine Controller Station Architecture

If the projected load for 1,700 POs exceeds $50\%$:
* Option A: Split into Two Controller Stations (Recommended):
- AS 1 (CPU 410-5H): Units A & B (Raw material handling, reaction unit), ~850 POs. Estimated CPU Load: $\approx 30\% - 35\%$.
- AS 2 (CPU 410-5H): Units C & D (Distillation, product storage, utilities), ~850 POs. Estimated CPU Load: $\approx 30\% - 35\%$.
- Benefits: Easily satisfies the $<50\%$ specification, leaves $65\%$ expansion headroom for future plant phases, and isolates process units during maintenance turnarounds.
* Option B: Aggressive Execution Cycle Optimization on a Single AS:
- Shift $60\%$ of technological blocks from OB35 to OB33/OB32.
- This keeps a single CPU 410-5H near the $45\% - 50\%$ threshold, but leaves zero margin for future plant expansions.

Verification

To verify CPU performance and load compliance during commissioning:
- Open SIMATIC Manager and connect online to the controller: select PLC -> Module Information -> Performance Data.
- Check the Scan cycle time: verify that the current cycle time and maximum cycle time remain well below configured watchdogs.
- Instantiate the standard PCS 7 block CPU_RT (FB70) in OB100 and OB1:
- Monitor parameter NET_TIME and CPU_LOAD: confirm that live execution load remains strictly below $50\%$ under full production running conditions.
- Test peak load behavior: simulate high-alarm storm conditions (e.g., trip simulation) to confirm that CPU utilization does not spike past safety margins.

Limitations and Operational Precautions

Tender Compliance Obligations: If contract specifications explicitly state that "CPU load shall not exceed 50%", submitting a design utilizing a single controller loaded to 70% risks contractual rejection during Factory Acceptance Testing (FAT).
Communication Resource Limits: Sizing must evaluate communication connections (S7 named connections to OS servers, BATCH servers, and peer controllers). A single CPU 410-5H supports up to 120 communication connections.
System Expansion Card (SEC) Sizing: Sizing two CPUs with SEC 1000 cards is often commercially comparable to sizing one CPU with an SEC 2000 card while providing vastly superior physical reliability.

Engineer FAQ

If an SEC card has 2,000 PO capacity, does that mean 1,400 PO will run at 70% load?

No. There is no direct mathematical relationship between the SEC PO license limit and processor load. Load depends entirely on which organization blocks execute the code, how many analog calculations are performed, and communication telegram volume.

What is the maximum Process Object capacity of a CPU 410-5H?

The CPU 410-5H can be expanded up to 2,600+ Process Objects (or an unlimited PO SEC card for specialized large applications), supported by its large 48 MB load/work memory.

What is the recommended target CPU load for continuous process DCS applications?

Industry engineering guidelines (NAMUR, IEEE) recommend designing DCS controllers for a nominal running load between $30\%$ and $40\%$, ensuring ample headroom for burst alarm storms and future plant extensions.

How do S7-400H fiber-optic sync links impact CPU load?

Fiber-optic synchronization continuously copies modified memory data between the primary and standby CPUs. High data turnover in fast OBs increases sync traffic, consuming approximately $10\%$ to $15\%$ of overall processing throughput.

Technical References

Siemens Industry Online Support: SIMATIC PCS 7 CPU 410-5H Process Automation Manual (109815344).
SIOS Entry 109742323: PCS 7 Compendium Part A - Configuration Guidelines (Section: CPU Sizing and Performance).
SIOS Entry 109809264: SIMATIC Process Control System PCS 7 Released Modules.
Siemens Engineering Whitepaper: Performance and Dimensioning in SIMATIC PCS 7.

Related Tags

CPU 410-5H
CPU Sizing
Process Objects
CPU Load
Hardware Selection
S7-400
Industrial Automation

How to Calculate CPU Load and Size Controllers for SIMATIC PCS 7 Projects