Architectural Comparison of KVM Extenders vs Remote Desktop (RDP) for PCS 7 OS Stations

In SIMATIC PCS 7 architectures, connecting operator consoles to centralized computers requires distinguishing between Hardware KVM extenders and Microsoft Remote Desktop Protocol (RDP). Siemens officially tests and releases RDP exclusively for WinCC OS Clients, strictly prohibiting RDP connections to WinCC OS Servers and Engineering Stations. Terminating or disconnecting an RDP session to an OS Server can suspend Microsoft SQL Server background threads and disrupt real-time communication. For OS Servers and Engineering Stations, hardware KVM extenders over fiber-optic or CAT6 cabling are mandatory.

Official Siemens Release Policy: KVM vs RDP

When designing centralized control room architectures where IPC servers and client towers are housed in a secure Central Equipment Room (CER / CPD) away from operator consoles, engineers evaluate connection options:

Hardware KVM Extenders (Keyboard, Video, Mouse): Transmit uncompressed raw video (DisplayPort/DVI/HDMI) and USB peripheral signals over dedicated physical cabling (CAT6 twisted pair or optical fiber). KVM operates below the operating system layer, remaining completely transparent to Windows, WinCC, and SQL Server.
Remote Desktop Protocol (RDP): Creates a virtual remote Windows session across standard Ethernet using TCP Port 3389.

The Strict Siemens Release Policy:
* WinCC OS Clients: RDP is tested and officially released for remote access to dedicated OS Clients.
* WinCC OS Servers & Single Stations: RDP is strictly NOT released by Siemens.
* PCS 7 Engineering Stations (ES): RDP is strictly NOT released for direct engineering access.

Technical Hazards of RDP on WinCC OS Servers and ES Nodes

The prohibition against using RDP on WinCC OS Servers is rooted in the internal architecture of Windows Terminal Services and Microsoft SQL Server:

Session Disconnection Thread Suspension: When an RDP window is closed by clicking the "X" banner or when network latency causes an RDP drop, Windows moves the session into a "Disconnected" state. In this state, Windows optimizes resources by suspending or de-prioritizing GUI threads and user-space COM interfaces.
WinCC Runtime Process Disruption: WinCC Server background daemons (CCProjectMgr.exe, pdlrt.exe, CCEmergencyStore.exe) that share hooks with the interactive desktop can experience thread lockups or communication dropouts when the interactive desktop is virtualized or disconnected.
Database Lockups and Redundancy Failovers: In redundant OS Server pairs, unexpected thread pauses caused by RDP session termination frequently trigger false heartbeat loss, prompting redundant servers to declare partner failure and execute unwanted failovers.
Graphics Hardware Acceleration Bypass: RDP replaces the physical graphics card driver with the Microsoft Remote Display Adapter, which lacks hardware DirectX/OpenGL acceleration required for smooth APL faceplate animation.

When RDP Is Permitted: Dedicated OS Client Deployments

RDP is supported on dedicated OS Clients under specific engineering guidelines:

Client Architecture: An OS Client does not host a local SQL Server process database; it queries tag data from the OS Servers across the Terminal Bus.
Session Persistence: If an RDP connection to an OS Client drops, plant monitoring on that individual console is temporarily lost, but the central plant OS Servers continue logging alarms and archiving process values without disruption.
Console Session Command: When connecting to an OS Client via RDP, always launch the session in administrative console mode:
cmd mstsc /v:<Client_IP> /admin

Hardware KVM Extenders: CAT6 vs Fiber-Optic Architectures

For control rooms housing OS Servers and primary operator stations:

CAT6 Point-to-Point KVM Extenders:
Distance limit: Up to 100 meters (330 feet).
Video quality: High, but susceptible to electromagnetic interference (EMI) if routed near variable frequency drives (VFD) or high-voltage power conduits.
Fiber-Optic KVM Extenders (Multi-Mode / Single-Mode):
Distance limit: 500 meters (multi-mode) up to 10 kilometers (single-mode).
Complete galvanic isolation, zero latency, uncompressed 4K video resolution, and total immunity to industrial electrical noise.
Essential for installations where central control rooms are physically distant from the computer equipment rooms.

Verification

Confirm remote connection reliability and system health:

WinCC Server Redundancy Monitoring:
* Inspect internal redundancy tags @RedundancyState and @RM_Master.
* Verify that server redundancy synchronization remains uninterrupted during operator interactions.
KVM Video and Input Responsiveness:
* Test operator mouse movement and faceplate opening speeds across the KVM link.
* Verify zero input lag or screen tearing during alarm flood conditions.
Event Log Audit:
* Inspect Windows Event Viewer System logs on the OS Server; verify zero Terminal Services session termination errors.

Limitations and Operational Precautions

BIOS and Boot Access: KVM extenders provide hardware-level BIOS access during initial boot and power-on self-tests; RDP is completely non-functional until Windows fully boots and starts network services.
Multiple Monitors: Industrial multi-monitor operator stations (2, 3, or 4 screens per desk) require multi-head KVM extenders with matching video outputs.
Anti-Virus and Remote Tools: Third-party remote software (e.g., TeamViewer, VNC) is also subject to strict Siemens validation guidelines; unapproved remote software must not run on production servers.

Engineer FAQ

Why is RDP not released by Siemens for WinCC OS Servers?
Disconnecting an RDP session can suspend user-session GUI and database threads, risking WinCC runtime freeze or false redundancy failover.
Is RDP supported on PCS 7 OS Clients?
Yes, Siemens officially tests and supports RDP connections to dedicated OS Clients.
What type of KVM extender is recommended for long distances in industrial plants?
Fiber-optic KVM extenders are strongly recommended because they provide complete galvanic isolation, zero latency, and total immunity to electrical noise over thousands of meters.
Can I access the BIOS over an RDP connection?
No, RDP operates within Windows; hardware KVM extenders allow full access to BIOS settings, RAID utilities, and boot menus.
What command opens an RDP session directly into the physical console session?
Running mstsc /admin connects to Session 0 (the physical console session) on supported Windows operating systems.
Can KVM extenders transmit audio for alarm horns and bells?
Yes, industrial KVM extenders feature bidirectional stereo audio channels to drive external audible horn sounders and operator microphones.
Does using a KVM extender require any software installation on the PC?
No, hardware KVM extenders are pure plug-and-play hardware devices that require zero device drivers or software agents on the host computer.
Can multiple operator desks access the same server through a KVM matrix?
Yes, digital KVM matrix switches allow multiple operator consoles to dynamically switch between different servers and clients.
Does RDP support multi-monitor operator consoles?
Yes, modern RDP clients support multi-monitor spanning (mstsc /multimon), though network bandwidth requirements increase significantly.
What happens to WinCC graphics if an RDP session drops?
The interactive desktop is moved to a disconnected state, which can pause graphic refresh scripts and delay faceplate updates.

Technical References

Siemens Industry Online Support (SIOS) Portal: Remote access to SIMATIC PCS 7 systems via RDP and KVM (Entry ID: 109477028).
SIMATIC PCS 7 PC Configuration and Installation Manual, Siemens AG.
SIMATIC WinCC V7.5 Information System - Remote Access Guidelines.

Related Tags

PCS 7, WinCC, KVM, RDP, Remote Desktop, OS Server, OS Client, Control Room Architecture

Architectural Comparison of KVM Extenders vs Remote Desktop (RDP) for PCS 7 OS Stations