Resolving WinCC Alarm Logging Freezes and KVM Switch Communication Dropouts in PCS 7 V8.1

In SIMATIC PCS 7 V8.1, WinCC OS alarm and message display freezing—requiring periodic PC reboots—coupled with 'Loss of connection with PLC' alarms when toggling a KVM switch, is caused by SQL Server transaction log saturation, display driver thread deadlocks, and hardware KVM signal disruption. Toggling non-industrial KVM switches breaks USB Human Interface Device (HID) or Display Data Channel (DDC) handshakes, causing the WinCC Graphics Runtime (pdlrt.exe) UI thread to lock and CP 1623 network packets to momentarily drop. To resolve, deploy an industrial KVM switch with continuous DDC/EDID emulation, isolate industrial Ethernet cables from KVM chassis grounding, install WinCC V8.1 SP1 Update 5 (or higher), and configure WinCC Alarm Logging circular archives with automated segment switching to prevent SQL database lockups.

Executive Summary

In control room environments operating PCS 7 V8.1 redundant OS servers, operators occasionally experience severe visualization stability issues: the WinCC Alarm Control table freezes and stops displaying incoming process alarms until the server is rebooted, while underlying SQL databases continue accumulating historical data. Concurrently, pressing a KVM (Keyboard, Video, Mouse) switch to toggle operator screens triggers false 'Loss of connection with PLC' alarms. This guide explains the technical interactions between Windows hardware abstraction, KVM signal bus behavior, CP 1623 communication drivers, and WinCC Alarm Logging database locks, providing practical engineering solutions to eliminate both symptoms.

Architecture and Fundamental Mechanics

The instability stems from the confluence of three distinct architectural phenomena:
1. KVM Switch DDC/EDID Disruption and Thread Blocking:
- Commercial consumer KVM switches physically disconnect the monitor's Display Data Channel (DDC/EDID) and USB bus when switching channels.
- When Windows detects display disconnection, the graphics driver fires display reconfiguration events. In WinCC V8.1, pdlrt.exe (Graphics Runtime) and the alarm ActiveX control (WinCC AlarmControl) attempt to redraw UI elements while display surfaces are invalid, causing the main message pump thread to deadlock.
2. Grounding Noise and Ethernet Link Glitches:
- Many low-cost KVM switches share a common chassis ground across USB, video, and integrated network ports.
- Switching introduces capacitive ground bounce or electromagnetic interference (EMI) across adjacent network cables, causing the CP 1623 / CP 1613 Industrial Ethernet adapter to momentarily reset its link state. PCS 7 drivers immediately interpret this 50-millisecond drop as a controller connection fault, logging 'Loss of connection with PLC'.
3. WinCC Alarm Logging Database and Buffer Saturation:
- The WinCC Alarm Logging service (CCAlgRtServer.exe) receives messages from AS controllers and buffers them in RAM before flushing to Microsoft SQL Server.
- If the UI thread deadlocks or if SQL Server transaction logs reach file growth limits without segment detachment, the internal queue fills. Once saturated, incoming alarms are logged to the database but cannot be rendered to the active screen, giving the illusion of a frozen system until reboot.

Step-by-Step Implementation Procedure

Replace Consumer KVM with Industrial DDC/EDID Emulating KVM:
a. Discontinue using passive or consumer-grade mechanical/electronic KVM switches.
b. Install an industrial-grade KVM switch (e.g., Siemens SIMATIC Industrial Flat Panel or certified Adder/Raritan/ATEN KVM) that features Continuous True DDC/EDID Emulation and Active USB HID Emulation on all ports.
c. Ensure the KVM keeps monitor and USB states persistently active to both PCS 7 OS stations, regardless of which channel is currently selected on the console.
Physically Isolate Industrial Ethernet and Plant Bus Cabling:
a. Ensure CP 1623 Industrial Ethernet cables (Green Industrial Ethernet FC cables) do not pass through the KVM switch chassis.
b. Maintain minimum 20 cm physical clearance between network cables, monitor cables, and power supplies.
c. Verify that the OS Server and operator console share an equipotential grounding bus meeting Siemens EMC guidelines.
Apply Latest WinCC V8.1 Stability Updates:
a. Check installed software version on both OS servers via SIMATIC Manager Help -> About -> Installed Software.
b. If running base PCS 7 V8.1, install PCS 7 V8.1 SP1 and the latest cumulative update (Update 5 or higher) for WinCC V7.3.
c. Updates resolve known race conditions in CCAlgRtServer.exe and ActiveX alarm controls during display resizing.
Optimize WinCC Alarm Logging Circular Archive Configuration:
a. Open WinCC Explorer on the Engineering Station.
b. Right-click 'Alarm Logging' and select Properties.
c. In Archive Configuration, navigate to 'Message Archive'.
d. Configure segment limits: set single segment size to a manageable capacity (e.g., 500 MB) and total archive size to 10 GB.
e. Set time-based switching (e.g., 1 day or 1 week) to force automated segmentation and prevent massive transaction logs.
Verify SQL Server Health and Database Cleanup:
a. In Windows Services, verify SQL Server (WINCC) service is running under the LocalSystem or designated administrative account.
b. Confirm that OS server hard drives have at least 25% free disk space on all partitions holding SQL data files.

Verification

Repeatedly toggle the industrial KVM switch back and forth between PC channels: verify that no 'Loss of connection with PLC' alarms appear in the alarm banner.
Trigger a live process alarm in CFC: confirm the alarm appears instantly on the WinCC Alarm Control screen without delay.
Monitor OS Server stability over a continuous 72-hour period: verify memory consumption of CCAlgRtServer.exe and pdlrt.exe remains stable without freezing.

Limitations and Operational Precautions

Software updates cannot compensate for physically defective KVM switches that break network physical layer links or introduce heavy ground loops. Hardware isolation and certified DDC emulation are mandatory.

Engineer FAQ

Why does pressing a KVM switch trigger 'Loss of connection with PLC'?

Electrical ground bounce or physical link disruption caused by non-isolated KVM switches momentarily drops the Industrial Ethernet link on the CP 1623 adapter.

What is DDC/EDID emulation in industrial KVM switches?

It is hardware circuitry that continuously presents monitor resolution and identity signals to all connected computers, preventing Windows from reconfiguring displays during switching.

Why do WinCC alarms continue logging to the database but fail to appear on screen?

The background SQL logging service is operational, but the foreground UI rendering thread (CCAlgRtServer / pdlrt) has deadlocked due to display surface recreation errors.

How do WinCC Alarm Logging segment settings prevent system hangs?

Configuring smaller, time-based circular segments prevents single database files from growing uncontrollably, avoiding SQL Server locks and query timeouts.

Can standard office KVM switches be used in PCS 7 control rooms?

No. Standard office KVM switches lack DDC emulation, industrial EMI shielding, and galvanic isolation, leading to frequent UI crashes and communication alarms.