1. PCS 7 Network Architecture Overview
SIMATIC PCS 7 separates process-control networking into two logical domains: the Plant Bus (the field-level backbone that connects AS stations, engineering stations, and OS servers to S7-400H / S7-410H controllers) and the Terminal Bus (the OS-level backbone that connects operator stations, OS servers, OS clients, and engineering stations). Understanding the difference is the prerequisite to any correct CP choice.
| Segment | Primary Carriers | Default Protocol | Typical Devices Connected |
|---|---|---|---|
| Plant Bus | CP 443-1 / CP 443-1 EX30 in AS, CP 1623 / CP 1613 in PC stations | ISO-on-TCP, TCP/IP, PROFINET IO | AS 410 (S7-400H), OS Server, ES |
| Terminal Bus | Standard Ethernet NIC, IE General, Terminal Bus Adapter (IE PB) | TCP/IP (S7 communication, OPC, WinCC) | OS Server, OS Client/Multi-client, ES, Web server |
The reference architecture is documented in the Siemens manual SIMATIC Process Control System PCS 7 Fault-tolerant Process Control Systems, which describes redundant OS server pairs communicating over both a redundant plant bus (to the S7-400H) and a redundant terminal bus (to the OS clients). See the official PCS 7 V7.0 Fault-tolerant Process Control Systems PDF (SIOS document 24450730).
2. Problem Statement: Redundant Client-to-Server Communication
A typical customer requirement is:
- Two redundant PCS 7 OS Servers, each equipped with two CP 1623 modules for redundant plant-bus communication to an AS 410 (S7-400H) station.
- Five OS Client stations that must remain online even when one OS Server fails.
- Where possible, avoid CP 1623 in the clients to reduce cost.
Three questions drive the design:
- Which CP types belong in the OS Clients when plant bus and terminal bus are common?
- Which CP types belong in the redundant OS Servers when plant bus and terminal bus are physically separated?
- Does REDCONNECT (S7-REDCONNECT) software need to be licensed on every client?
3. CP Hardware Comparison: CP 1613, CP 1623, and IE General
| Module | Siemens Order Number (MLFB) | Bus Type | Max Per PC Station | Driver / Firmware | Typical Use |
|---|---|---|---|---|---|
| CP 1613 A2 | 6GK1 161-3AA01 | PCI / PCIe 10/100 Mbit | 4 | SIMATIC NET CP 1613/1623 driver, S7-REDCONNECT capable | Redundant plant-bus communication, time stamping (4 ms) |
| CP 1623 | 6GK1 162-3AA00 | PCIe 10/100 Mbit | 4 | SIMATIC NET CP 1613/1623 driver, S7-REDCONNECT capable | Identical to CP 1613 A2; current successor part |
| CP 1628 | 6GK1 162-8AA00 | PCIe, ERTEC 400, IRT capable | 2 | SIMATIC NET driver, PROFINET IRT, S7-REDCONNECT capable | Newer PCS 7 V9+ deployments, IRT plant bus |
| IE General (SOFTNET-S7) | Software-only, tied to on-board NIC | TCP/IP | 1 | SOFTNET-S7 / IE-AGENT in STEP 7 / PCS 7 | Non-redundant S7 communication from PC station (ES, OS client) |
| Standard on-board Ethernet NIC | Manufacturer specific | TCP/IP | Multiple (one per Windows adapter) | Windows NDIS driver | Terminal bus only; no S7-REDCONNECT, no time stamping |
4. Server-Side CP Layout for Redundant Plant Bus + Separated Terminal Bus
When the plant bus and terminal bus must be physically separated (recommended for cybersecurity zoning, OT/IT separation, and clean fault isolation), the redundant OS server pair receives the following CP allocation:
| OS Server Pair | Plant Bus Slot 1 | Plant Bus Slot 2 | Terminal Bus |
|---|---|---|---|
| Server 1 | CP 1623 #1 (primary) | CP 1623 #2 (standby) | On-board / standard GbE NIC |
| Server 2 | CP 1623 #1 (primary) | CP 1623 #2 (standby) | On-board / standard GbE NIC |
The redundant CP 1623 pair on each server uses the SIMATIC NET S7-REDCONNECT software layer, which is part of the standard PCS 7 OS Server delivery and is enabled by a separately licensed S7-REDCONNECT authorization on the server. The two CPs operate in MAC address-based or IP-based redundancy modes (configurable in the CP properties dialog of STEP 7 HW Config). Failover time is < 200 ms typical, transparent to WinCC.
The terminal bus on each server uses a normal commercial GbE NIC, configured in Windows and reachable from clients. No SIMATIC NET driver or REDCONNECT license is required for the terminal-bus side of the server because the OS server — OS client relationship is a normal WinCC server/client channel.
5. Client-Side CP Layout: Avoiding CP 1623
OS clients only need to talk to the OS servers; they do not need to talk to the S7-400H directly. Therefore, OS clients should be placed on the terminal bus only, and a CP 1613/1623 is unnecessary. The hardware-config constraint that limits each PC station to one IE General is not a problem because a client only needs one S7 logical endpoint.
5.1 Recommended Client Configuration (Single Common Backbone)
| OS Client | Connection to Plant Bus + Terminal Bus (combined) | Connection to Terminal Bus Only |
|---|---|---|
| CP / NIC choice | 1 x IE General (SOFTNET) on-board NIC | 1 x Standard GbE NIC, no IE General required |
| REDCONNECT license | No | No |
| STEP 7 HW Config entries | 1 IE General, 1 S7 connection to OS server pair | No IE General needed; only WinCC client configuration |
5.2 When IE General Is Still Required
Insert one IE General in the OS client only if the client must act as a STEP 7 engineering station on the plant bus, or if the client must run S7-OPC with its own S7 connection to the AS. Pure operator HMI clients do not require IE General at all.
6. REDCONNECT Licensing and Software Requirements
REDCONNECT is the software layer that turns a CP 1613 / CP 1623 pair into a fault-tolerant S7 connection. Its scope is strictly the S7 connection to the S7-400H or S7-410H. It does not make WinCC client/server links redundant.
| Station Role | Need S7-REDCONNECT Software? | Reason |
|---|---|---|
| OS Server (redundant) | Yes | Required for redundant S7 connection to AS 410H |
| OS Server (single) | No | Single CP path is enough |
| OS Client (operator only) | No | WinCC handles server failover in the client channel |
| OS Client (engineering + operator) | No (for WinCC); only STEP 7 / S7-OPC | ES communicates through IE General or through server |
| Engineering Station | Only if directly connecting to AS 410H | Otherwise routes through OS server |
7. Terminal Bus Adapter: Practical Redundancy for OS Clients
Where the customer genuinely requires redundant client links, the cleanest approach is the SIMATIC NET Terminal Bus Adapter. This Y-cable-style adapter splits a single PCI / PCIe backplane feed into two physical Ethernet ports that share one MAC and one IP address, presenting itself as one logical network interface but physically patched to two independent switches.
Key characteristics:
- Acts as a single NIC from the Windows / PCS 7 viewpoint.
- Provides media-level redundancy: if one switch, patch cable, or port fails, the second path continues to forward.
- Does not provide S7-REDCONNECT level redundancy (no S7 connection failover).
- WinCC server/client failover uses the partner server's IP address; the Terminal Bus Adapter simply keeps the local NIC alive on the surviving switch.
- Catalog reference: Siemens SIMATIC NET catalog section IE PB / Terminal Bus Adapter.
8. Step-by-Step Configuration Procedure
The following procedure builds a redundant OS server pair with five OS clients, using a separated plant bus and terminal bus, no CP 1623 in clients, and a Terminal Bus Adapter for media redundancy.
8.1 Prerequisites
- PCS 7 V8.2 or later (V9.x recommended for current support).
- STEP 7 / SIMATIC Manager or TIA Portal with PCS 7 plug-in, depending on the PCS 7 version.
- Licenses: PCS 7 OS Server, PCS 7 OS Client (B or C), S7-REDCONNECT (server only).
- Hardware: 2 x CP 1623 per server, 1 x Terminal Bus Adapter per client, managed switches with MRP or STP for the plant bus.
8.2 Server-Side Hardware Configuration
- Open the PC station in STEP 7 HW Config.
- Insert a CP 1623 in slot 1 of the PC station, set Plant Bus, assign IP address
192.168.1.11. - Insert a second CP 1623 in slot 2, same subnet, IP
192.168.1.12. - In the CP properties, enable S7-REDCONNECT and select MAC-address-based redundancy (recommended for deterministic failover).
- Insert the standard on-board Ethernet as the terminal-bus adapter; assign IP
192.168.2.11. - Download the PC station configuration. The SIMATIC NET configuration console will show both CP 1623 modules as Active / Standby.
8.3 Client-Side Hardware Configuration
- Insert one IE General into the client PC station in STEP 7 HW Config.
- Configure one S7 connection from the IE General to the OS server pair using the partner server — server name
OS_SERVER_PAIRin NetPro. - Patch the on-board NIC into the Terminal Bus Adapter.
- Connect the two adapter pigtails to two independent managed terminal-bus switches.
- In Windows, bind the Terminal Bus Adapter as the default gateway for the
192.168.2.0/24segment. - Configure the WinCC client to use Preferred Server = Server1, Alternate Server = Server2 with a failover timeout of 30 s.
8.4 Verification
- Pull the S7 cable on CP 1623 #1 of Server1: WinCC must continue without operator-visible interruption within < 5 s.
- Power off Server1 entirely: WinCC clients must switch to Server2 within 30 s (configured failover timeout).
- Pull one of the two Terminal Bus Adapter pigtails on any client: the OS must remain connected.
- Check SIMATIC NET Diagnostics on the server: CP 1623 status must report Active + Standby OK; event log must be free of S7 connection aborted (0x031E) errors.
9. Hardware-Configuration Error Patterns
| Symptom | Likely Cause | Resolution |
|---|---|---|
| Cannot insert a second IE General in PC station | SOFTNET driver limitation (one IE General per PC station) | Use CP 1613 / CP 1623 if more S7 endpoints are needed, or route through OS server |
| S7 connection to AS 410H flaps every few minutes | Plant-bus switch not configured for MRP / redundancy; broadcast storm | Enable MRP master on a managed switch; enable STP/RSTP as fallback |
| WinCC client shows "Server not available" on first attempt after switchover | Failover timeout too aggressive, or DNS stale | Increase WinCC client failover timeout to 30 s; verify DNS A-records point to both servers |
| CP 1623 reports "module not active" after PLC download | PC station configuration not downloaded after AS changes | Re-download the PC station from HW Config to refresh CP parameters |
| Only one CP 1623 is recognized | SIMATIC NET driver not updated for CP 1623 variant | Install latest SIMATIC NET PC software; verify in Device Manager that both CPs show without yellow bang |
| Terminal Bus Adapter only forwards on one port | Spanning Tree blocking the backup port | Enable port-fast / BPDU filter on client-edge ports, or use MRP instead of STP |
10. Cybersecurity and Segmentation Considerations
Separating the plant bus and terminal bus into different VLANs (or physically isolated switches) is a PCS 7 recommended practice and a prerequisite for many zone-and-conduit models. The Terminal Bus Adapter must not bridge between the plant and terminal VLANs; it is purely an Ethernet media device. Firewall rules must enforce:
- Plant bus ↔ terminal bus: allow only specific service ports (WinCC, OPC UA, S7 communication).
- Terminal bus ↔ office / IT: restricted to web (HTTPS) if a WinCC WebNavigator is deployed.
- Plant bus ↔ IT: blocked by default.
11. Sizing and License Counts
For a five-OS-client configuration with a two-server pair:
| License Item | Quantity | Notes |
|---|---|---|
| PCS 7 OS Server Runtime | 2 | One per server, includes S7-REDCONNECT base functionality on plant bus |
| PCS 7 OS Client Runtime (Type B) | 5 | Operator-only, no project editing |
| PCS 7 Engineering Station (optional) | 1 | Only if a dedicated ES exists |
| S7-REDCONNECT Authorization | 2 (server only) | One per server, optional license |
| SIMATIC NET CP 1613/1623 driver | 4 (server only) | Per installed CP, included with PCS 7 OS Server media |
12. Frequently Asked Questions
Do OS clients in PCS 7 require a CP 1623 to communicate with redundant OS servers?
No. OS clients communicate with the OS servers over the terminal bus using WinCC client channels, which require only a standard Ethernet NIC or IE General. CP 1623 and S7-REDCONNECT are required only on the OS servers for redundant S7 connections to the S7-400H / S7-410H.
Can I install more than one IE General in a single PC station?
No. PCS 7 / STEP 7 hardware configuration restricts each PC station to a single IE General because the SOFTNET-S7 driver cannot manage more than one logical S7 connection endpoint. If more S7 endpoints are required, use CP 1613 / CP 1623 modules instead (up to four per PC station).
Is REDCONNECT software required on PCS 7 OS clients?
No. REDCONNECT is licensed only on the OS servers (or ES) that maintain redundant S7 connections to the S7-400H / S7-410H. OS clients inherit WinCC server failover through the OS client configuration; they do not need a separate REDCONNECT license.
What is a Terminal Bus Adapter, and does it deliver S7-REDCONNECT level redundancy?
A SIMATIC NET Terminal Bus Adapter is a Y-style network adapter that splits one NIC into two physical Ethernet connections sharing one MAC / IP. It provides media-level redundancy only. It does not replace S7-REDCONNECT, which handles S7-connection level failover between CP 1623 modules on the plant bus.
How do I verify redundant OS server communication after commissioning?
Disconnect the plant-bus cable on CP 1623 #1 of the primary server; the OS must continue without operator interruption and WinCC diagnostics must report the second CP as Active. Then power off the primary server entirely and confirm OS clients fail over to the standby server within the configured WinCC client failover timeout (typically 30 seconds).