Troubleshooting CFC Test Mode Disconnection ('Beobachten' Turns Red) Due to S7-400 Communication Overload

When CFC Test Mode (online monitoring / 'Beobachten') in SIMATIC PCS 7 disconnects after 5-6 seconds and the test mode glasses indicator turns red, the root cause is communication resource exhaustion in the S7-400 CPU. S7-400 controllers allocate a strict cyclic communication time slice to process online diagnostic jobs. If an engineer monitors too many CFC blocks simultaneously, opens multiple online charts across a heavily loaded CPU, or if 'Scan cycle load from communication' in HW Config is configured too low (e.g. 20%), the CPU cannot respond to watch requests before the PG timeout expires. To resolve this: close inactive online charts, deselect 'Watch On' on unneeded blocks, increase 'Scan cycle load from communication' to 30-40% in CPU Properties, and verify PG communication quota allocations.

Executive Summary

During commissioning, troubleshooting, or plant optimization of SIMATIC PCS 7 systems, engineers rely extensively on CFC Test Mode ('Beobachten') to monitor real-time block execution and verify control loop responses. A frequent and frustrating failure occurs when an engineer activates test mode on a heavily utilized S7-400 or CPU 410 controller: the online values update for 5 to 6 seconds, and then the test mode glasses icon suddenly turns red, freezing all values and dropping the diagnostic session. This article examines S7-400 communication scheduling architecture, identifies the thresholds that trigger watch job dropouts, and provides proven configurations to maintain stable online monitoring.

Architecture and Fundamental Mechanics

S7-400 Operating System Communication Slice and Watch Job Architecture:

Cyclic Operating System Communication Quotas:
In SIMATIC S7-400 controllers, the operating system executes a round-robin schedule divided between process logic (OB1 and cyclic interrupts OB30-OB38) and background operating system services (I/O updates, diagnostics, and communication).
Communication with the Engineering Station (STEP 7 / CFC Test Mode) is processed during the cyclic slice designated by the parameter Scan cycle load from communication in CPU Properties.
By default, this parameter is set to 20%. This guarantees that process execution retains at least 80% of CPU scan bandwidth, capping communication processing time to 20% of total cycle execution time.
Watch Jobs (Beobachten) and PDU Limits:
When an engineer enables 'Watch On' on a CFC chart, the Engineering Station compiles a list of dynamic addresses and transmits cyclic read requests (Protocol Data Units - PDUs) to the CPU.
Each S7-400 CPU possesses a finite number of internal communication buffers and can process only a limited number of concurrent watch jobs per cycle.
When multiple large CFC charts are monitored simultaneously, or when large structural blocks (APL motors, valves, PIDs) have dozens of pins registered for dynamic monitoring, the CPU's communication job queue overflows.
The Red Glasses Dropout Mechanism:
When the communication queue overflows or when process execution consumes the scan time, the CPU postpones processing incoming PG watch requests.
If the Engineering Station does not receive an acknowledgment within the hardcoded watchdog timeout window (typically 5 seconds), CFC concludes that the AS connection was lost. The test mode indicator turns red, and online polling is terminated to protect the controller from communication lockup.

Step-by-Step Implementation Procedure

Reduce Monitored Block and Pin Volume in CFC:
* In the active CFC chart, avoid enabling 'Watch On' on entire charts containing dozens of complex blocks.
* Selectively monitor only the specific block or pins under active investigation: Right-click the specific block and select Watch On, or right-click individual pins and add them to the watch list.
* Close all background CFC charts that have test mode active.
Optimize Scan Cycle Load from Communication in HW Config:
* In SIMATIC Manager, open HW Config for the S7-400 station.
* Double-click the CPU (e.g. CPU 410-5H or CPU 417-4H) to open Properties.
* Navigate to the Cycle / Clock Memory tab.
* Locate the parameter Scan cycle load from communication (%).
* Increase the value from the default 20% to 30% (or 40% if the CPU is heavily loaded with high communication demands and process scan time has adequate headroom).
* Ensure that the configured scan cycle monitoring time (watchdog) provides sufficient margin for the increase.
Verify Reserved PG Communication Connections:
* In the CPU Properties dialog, select the Communication tab.
* Verify the number of reserved connections for PG communication.
* Ensure that at least 2 connections are reserved for PG engineering access so that background SCADA connections do not starve engineering test mode.
Check for High Cycle Time and OB Execution Overruns:
* Select PLC -> Module Information -> Performance Data.
* Check the current, maximum, and average scan times. If the CPU scan time is fluctuating wildly or near the maximum cycle limit, investigate whether cyclic interrupts (OB35, OB32) are overloading the processor.
Save, Compile, and Download HW Config:
* In HW Config, select Station -> Save and Compile.
* Download the updated hardware configuration to the CPU (using Configuration in RUN / CiR if supported).
* Re-activate CFC Test Mode; confirm that the glasses icon remains steadily green without disconnecting.

Verification

Verify Stable Online Monitoring in CFC:
1. Activate CFC Test Mode (Test -> Test Mode); verify that the glasses indicator stays green continuously for several minutes without turning red.
2. Open Module Information -> Diagnostic Buffer; verify that no communication timeout or connection abort events are logged.
3. Check CPU cycle time; verify that increasing communication load did not cause OB80 cycle time faults.

Limitations and Operational Precautions

Operational Precautions and CPU Resource Boundaries:
- Do not increase 'Scan cycle load from communication' above 50%; setting communication load excessively high can lengthen the main OB1 process cycle time and delay technological reaction times.
- Never leave CFC Test Mode running unattended on multiple engineering stations during active plant production; always switch test mode OFF when diagnostic monitoring is complete.

Engineer FAQ

Why does CFC test mode disconnect and turn red after 5 seconds?

The S7-400 CPU's communication queue overflowed due to excessive online watch requests, causing the CPU to miss the 5-second PG response watchdog window.

Where do I increase the CPU communication allowance in STEP 7?

In HW Config, open CPU Properties -> Cycle / Clock Memory tab, and increase 'Scan cycle load from communication (%)'.

What is the default communication load percentage on S7-400 CPUs?

The default value is 20%, which reserves 80% of CPU scan bandwidth for process logic execution.

Does monitoring multiple CFC charts simultaneously increase CPU load?

Yes. Every active online chart registers dynamic watch requests (PDUs), multiplying the communication burden on the CPU.

Can I adjust the CFC test mode refresh rate?

Yes. In the CFC editor under Options -> Customize -> Color/Format -> Test Mode, you can adjust the dynamic sampling interval (e.g. increasing from 1s to 2s to reduce communication load).