How to Measure and Analyze Network and Communication Bus Load in SIMATIC PCS 7

In SIMATIC PCS 7, evaluating communication and network load requires measuring two distinct architectural metrics: controller communication cycle load and physical Industrial Ethernet bus utilization. Controller communication load (the percentage of CPU cycle time handling S7 communication, alarms, and WinCC polling) is monitored online in SIMATIC Manager via Module Information and configured in HW Config CPU Properties (default 20%). Physical network bus load is measured using SCALANCE switch Web-Based Management (WBM) port statistics, dedicated diagnostic tools such as Siemens BanyNet / BANYmon, or through SNMP monitoring integrated into the PCS 7 Maintenance Station.

Controller Communication Load vs Physical Network Bus Load

When assessing system stability or planning expansions in a running SIMATIC PCS 7 installation (e.g., an ES/OS Single Station, an OS Server pair, and redundant AS controllers), engineers frequently ask: 'What is the current communication and network load in percentage?'

To answer accurately, one must separate the concept of Communication Load into two independent operational domains:
1. AS Controller Communication Load (Cycle Load %): The portion of the automation CPU's processing power dedicated to servicing S7 communications (cyclic polling from WinCC OS, block alarm transmissions via ALARM_8P, cross-AS communication via BSEND/BRCV, and engineering downloads).
2. Physical Ethernet Network Bus Load (Bandwidth Utilization %): The actual physical traffic volume passing through Industrial Ethernet cables and network switches (Terminal Bus and Plant Bus) relative to the maximum channel bandwidth (100 Mbit/s or 1000 Mbit/s).

Measuring AS CPU Communication Cycle Load in SIMATIC Manager

In S7-400 and CPU 410-5H controllers, the operating system cyclically executes user automation logic (OB1, cyclic interrupt OBs) and allocates a configurable slice of processing time to communication services:

Configuring and Viewing Communication Load

HW Config Setting: In HW Config, double-click the CPU, open the Cycle/Clock Memory tab, and locate 'Cycle load due to communication [%]'. The default value is 20% (configurable from 10% to 50%). This parameter sets an upper limit on how much cycle time communication processing can consume before yielding back to user logic.
Online Diagnostic Buffer / Module Information:
1. In SIMATIC Manager, select the AS CPU and press Ctrl+D (or PLC -> Module Information).
2. Switch to the Performance Data tab.
3. Inspect the current, minimum, and maximum cycle times alongside communication response metrics.
4. If user scan times lengthen during heavy operator faceplate usage, the communication slice is fully saturated.

Measuring Industrial Ethernet Network Utilization via SCALANCE and BanyNet

Physical network bandwidth cannot be determined directly from a single internal WinCC tag without external infrastructure monitoring.

Method 1: SCALANCE Web-Based Management (WBM)

Managed industrial switches (e.g., SCALANCE X-200, X-300, X-400 such as 6GK5308-2FL00-2AA2):
1. Connect a web browser to the SCALANCE switch IP address (e.g., https://192.168.1.1).
2. Log in with administrative credentials.
3. Navigate to Statistics -> Packet Rates / Bandwidth Utilization.
4. View real-time port load percentages, packet counters, and broadcast frame rates for each connected AS, OS, and ES port. In a healthy PCS 7 plant bus, average load typically remains below 10-15%.

Method 2: Siemens BanyNet / BANYmon Bus Analyzer

Siemens BanyNet (from IT4Industry) is a specialized diagnostic hardware/software suite designed specifically for PCS 7 networks:
* Bus Load Display: Provides continuous graphic and numeric percentage displays of bus load per port.
* Telegram Interpretation: Dissects S7 protocol frames, identifying high-frequency ALARM_8P bursts, redundant frame replication, and cyclical polling overhead.
* SNMP Integration: Gathers diagnostic traps from all configured SCALANCE switches across the plant.

Method 3: PCS 7 Maintenance Station (Asset Management)

The PCS 7 Maintenance Station integrates network switch monitoring into the OS runtime via the SNMP OPC Server. It continuously monitors port states and triggers OS alarms if port bandwidth exceeds user-defined thresholds (e.g., >70% utilization).

Network Monitoring Tools and Diagnostic Capability Matrix

The following table outlines the diagnostic tools available for measuring communication and network load in SIMATIC PCS 7:

Diagnostic Tool / UtilityScope of AnalysisMeasured MetricsPrimary Use CaseEngineering EffortSIMATIC Manager Module InfoS7-400 / CPU 410-5HCPU Communication Cycle Load (%)Detecting CPU scan time starvationMinimal (Built into Step 7)SCALANCE WBMSCALANCE SwitchesPhysical Port Bandwidth (%), Packet RatesIdentifying physical cable saturationLow (Web browser access)Siemens BanyNet / BANYmonEntire Industrial EthernetTelegram decoding, S7 frame analysis, Bus LoadIn-depth troubleshooting & network auditSpecialized hardware/softwareWireshark with SPAN PortSwitched Ethernet segmentRaw packet capture, Broadcast storm analysisDiagnosing ARP storms or protocol faultsModerate (Requires mirror port)PCS 7 Maintenance StationPC Stations, AS, SwitchesSNMP port status, Link states, Bandwidth alarmsContinuous plant asset managementIntegrated with PCS 7 MS license

Combining CPU cycle metrics with switch port statistics yields complete transparency across both logical and physical layers.

Verification

To verify network and communication load health across the system:
1. In SIMATIC Manager, check the AS cycle time under heavy operator control; confirm that maximum cycle time remains well below the configured watchdog timeout (e.g., <150 ms in OB1).
2. Open the SCALANCE WBM for Plant Bus switches; confirm that all operational ports report link status at 100 Mbit/s Full Duplex (or 1000 Mbit/s) with bus load below 15%.
3. In WinCC Explorer, verify that the @RM_MASTER and @RM_SERVER_STATE tags are stable with zero communication drop alarms.
4. Verify that broadcast frames (multicast/broadcast packet counters) on the switches represent less than 1% of total traffic.

Limitations and Operational Precautions

Increasing 'Cycle load due to communication [%]' above 30% improves WinCC screen refresh rates but directly increases OB1 cycle times, potentially slowing down background non-critical logic.
Connecting an unmanaged office switch to the PCS 7 Plant Bus or Terminal Bus is strictly prohibited; unmanaged switches do not support RSTP/HRP redundancy and drop diagnostic SNMP packets.
When capturing packets with Wireshark on a production network, never run captures from an active OS Server NIC without dedicated port mirroring (SPAN), as packet capturing drivers can introduce latency.
Ensure all SCALANCE switches have broadcast storm rate limiting enabled in their firmware to protect controllers against network flooding.

Engineer FAQ

What is the typical network bus load in a healthy PCS 7 Plant Bus?

In a well-designed 100 Mbit/s PCS 7 Plant Bus with redundant servers and 4 to 8 controllers, average physical network load rarely exceeds 5% to 15%. Peak bursts during full downloads or alarm storms may reach 30% to 40%.

Why did increasing tag update cycles slow down my controller?

Setting WinCC tags to fast update cycles (e.g., 250 ms or 500 ms) forces the AS CPU to spend more of its allocated communication cycle time packaging S7 data telegrams, reaching the configured cycle load limit.

Can I view switch bus load directly on standard WinCC process pictures?

Yes, by using the PCS 7 Maintenance Station with SNMP integration, or by configuring an SNMP OPC client to read the ifInOctets and ifOutOctets MIB variables from SCALANCE switches.