Resolving WinCC Runtime V15.1 PLC Reconnection Failures

Problem Overview

WinCC Runtime Professional V15.1 fails to re-establish S7 connections to SIMATIC S7 PLCs after the PLCs are power-cycled. The runtime initially connects to the configured PLCs at startup, but once a PLC loses power (typical in crane installations where drives and controllers are switched off during maintenance or end-of-shift), the HMI/SCADA station refuses to bring the connection back up when the PLC returns to the network. Operators must restart the WinCC Runtime service to recover visibility. In large systems (10+ PLCs), the runtime silently caps active connections and refuses to bring a replacement PLC online without manual intervention.

This is a licensing and communication-stack issue, not a TIA Portal project configuration error. The configuration is correct — the missing piece is the SIMATIC Net Softnet S7 runtime license that WinCC RT Professional does not bundle by default. Without that license, the WinCC communication DLLs only allow the built-in number of S7 connections, and connection re-establishment logic relies on the SIMATIC Net OPC/DCOM layer to redial the partner.

Root Cause Analysis

WinCC Runtime Professional V15.1 ships with a fixed number of S7 connection slots that the runtime can use out of the box. When the configured tag count exceeds that number, the project refuses to compile or run, and when live PLCs are power-cycled, the runtime cannot open new S7 connections to replace the lost ones without a licensed SIMATIC Net S7 channel.

S7 Connection Quota in WinCC RT Professional

The default WinCC RT Professional install enables only 8 S7 connections through the bundled SIMATIC Net runtime. If the project contains more than 8 HMI connections to S7-300/400/1200/1500 PLCs, the additional channels are only usable when a Softnet S7 license (e.g. 6AV6371-1CA07-5AX0) is installed and active on the WinCC station. This explains the field observation that the system "only connects to 10 PLCs" — the extra two slots are filled by implicit system connections (alarm logging, time sync, redundancy), leaving the project-defined 8 S7 connections fully consumed.

When a PLC drops off the network, the SIMATIC Net layer is supposed to re-establish the channel on the next polling cycle. Without a Softnet license, the channel is held in a "not licensed" state, and the WinCC connection remains broken until the runtime is restarted. With a Softnet license, the channel goes through CONNECTING → CONNECTED automatically after the partner returns to the network.

Why the Initial Startup Works

On first launch, WinCC RT Professional opens all configured S7 channels in sequence. The first N channels (where N is the licensed count) succeed, and the rest are deferred or marked as "not licensed." When a working channel is dropped, the slot is freed, but the runtime does not automatically re-issue a connection request to a different PLC for that slot — that behavior requires the licensed SIMATIC Net S7 redial path.

Symptoms Checklist

• Runtime starts; first 8–10 S7 connections go online, remainder stay Disconnected.
• Power-cycling a connected PLC: tag values freeze; connection state shows Disconnected, never returns to Connected automatically.
• Manual restart of WinCC Runtime restores all originally connected PLCs.
• Tag simulation, alarm routes, and scripts continue to function — only the S7 channels are affected.
• License viewer (Automation License Manager) shows Softnet S7 license as not in use (no green arrow) or absent entirely.

Diagnostic Procedure

1. Verify the project connection count. In TIA Portal V15.1, open the WinCC RT project, expand HMI Tags → Show All Tags and count the number of connections under HMI Connections. Compare against the licensed S7 channel count.
2. Check the Automation License Manager. Launch ALM on the WinCC server and inspect installed licenses. Look for entries containing Softnet-S7 or SIMATIC NET S7. A licensed-but-unused entry shows the license is present but no process has claimed it — the SIMATIC Net service is not running or not registered.
3. Inspect the S7DOS channel diagnosis. Open Start → SIMATIC → SIMATIC Net → Commissioning → Communication Settings and verify the S7DOS configuration. The Station Configuration Editor must contain at least one S7ONLINE access point pointing at the correct Ethernet interface (Intel, Broadcom, or CP card).
4. Examine WinCC channel diagnosis. In the running WinCC RT, open Channel Diagnosis (Control Panel → Diagnosis) to view per-connection state, last error code, and redial counter. Repeated 0xFFFE0100 (no license) entries confirm the Softnet path is missing.
5. Confirm Server OS requirement. For more than 8 S7 connections, Siemens requires a Windows Server OS (2012 R2 / 2016 / 2019) on the runtime station. Windows 10/11 may technically install but is not approved for the higher connection counts.

Solution: Install and Activate SIMATIC Net V15.1

The fix is to install the SIMATIC Net V15.1 communication stack on the WinCC server and confirm a Softnet S7 license is bound to the runtime PC. The SIMATIC Net installer is part of the TIA Portal V15.1 media in the Optional Components section and may be omitted during a default TIA install — that omission is the most common cause of the symptoms described.

Prerequisites

• Administrator account on the WinCC server.
• SIMATIC Net V15.1 installation media (DVD or downloaded HSP from Siemens SIOS).
• Softnet S7 License (e.g. 6AV6371-1CA07-5AX0) for the number of S7 connections required (sold per channel, typically 64/128/256 max per license).
• Windows Server 2012 R2, 2016, or 2019 (or Windows 10 64-bit for ≤ 8 connections).
• TIA Portal V15.1 Update 8 or later applied (cumulative fix for OPC/connection redial issues).

Step-by-Step Installation

1. Close WinCC Runtime. Stop the WinCC RT Professional service and any TIA Portal instance connected to the project.
2. Mount the SIMATIC Net V15.1 DVD and run Setup.exe as administrator.
3. Select Install SIMATIC Net PC Software. Choose Complete install — this registers the S7DOS, OPC, and S7ONLINE services required by WinCC.
4. When prompted for the S7ONLINE access point, bind it to the correct Ethernet adapter used for PLC traffic. The default CP1613/CP1623 is reserved for IE/PB gateways; for standard Ethernet, select <Network adapter> → ISO-on-TCP.
5. Complete the installation and reboot the server. The SIMATIC Net Configuration console and Station Configuration Editor appear in the Start menu.
6. Transfer the Softnet S7 License. In Automation License Manager, drag the Softnet license file (.lic) from the USB dongle or local folder onto the server's host entry. ALM copies the license to %ProgramData%\Siemens\Automation\Licenses and binds it to the host ID.
7. Verify activation. In ALM, the Softnet S7 row should now show a green arrow (in use) once WinCC RT is started. If the arrow stays gray, the SIMATIC Net service has not claimed the license — restart the SIMATIC NET S7DOS service manually.
8. Restart WinCC Runtime. Boot the project. In the WinCC channel diagnosis, confirm all configured S7 connections transition to Connected and the Softnet license shows in use.

Power-Cycle Verification

1. With the runtime active and all PLCs online, power off one PLC at the field distribution panel.
2. Observe the WinCC channel diagnosis: the affected connection should transition to Disconnected within 1–2 polling cycles (default cycle = 1 s for fast tags, configurable per connection).
3. Power the PLC back on. The connection should automatically return to Connected within 5–15 s without operator intervention. The Softnet redial path is now active.
4. Repeat for each crane PLC to confirm full coverage.

Configuration Reference

Related Siemens Documentation and Updates

• Siemens Industry Online Support — primary portal for TIA Portal V15.1 updates and SIMATIC Net downloads.
• Procedure if there is no connection (RT Unified) — Siemens TIA documentation covering connection troubleshooting for S7-1200/1500 in WinCC Unified, applicable as a reference framework for legacy WinCC RT Professional installations.
• SIMATIC Net V15.1 Commissioning Manual (A5E45036827) — describes S7DOS configuration and S7ONLINE access point binding.
• WinCC Professional V15.1 System Manual — documents the 8-connection base limit and Softnet upgrade path.
• TIA Portal V15.1 Update 8 Release Notes — contains the connection redial fix referenced in the field report.

Edge Cases and Field-Proven Caveats

1. WinCC RT Advanced vs. Professional

WinCC Runtime Advanced uses a different connection model (limited to 4 S7 connections on a single panel). The symptoms described — 8–10 PLCs connecting with the remainder stuck — are unique to Professional. Confirm the installed license type before applying the Softnet fix; Softnet S7 licenses do not apply to RT Advanced panels.

2. Windows 10 vs. Windows Server

Siemens officially limits Windows 10 to ≤ 8 S7 connections in WinCC RT Professional. For > 8 connections, only Windows Server 2012 R2 / 2016 / 2019 (Standard or Datacenter) is approved. A working install on Windows 10 with 27 PLCs may boot but will exhibit exactly the silent-cap behavior described in the source.

3. Softnet License Not In Use

A Softnet license installed but shown without the green "in use" arrow indicates that the SIMATIC NET S7DOS service has not claimed it. Causes include: (a) the S7ONLINE access point is misconfigured, (b) the SIMATIC Net install was a minimal install missing the S7 channel, or (c) the S7DOS service is disabled. Resolve by reconfiguring the access point and restarting the service via services.msc.

4. 10-PLC Cap on Initial Boot

The observed "10 PLCs connect, 11th waits for 10th to be turned off" behavior is consistent with the 8-connection quota + 2 implicit system channels. The 11th PLC has no free slot. Once Softnet is active, the slot pool expands to the licensed count and the 11th connection establishes immediately.

5. PC Station Name Mismatch

When the WinCC server is added to the TIA project as a PC station, its name must match the Windows hostname exactly. A mismatch prevents the S7-1500 partner from completing the S7 connection handshake, and the redial cycle never succeeds. Verify in TIA Portal under Devices & Networks → WinCC station → Properties → General.

6. Network Infrastructure

Industrial Ethernet switches without IGMP snooping or with managed broadcast storm control can silently drop S7 connection establishment packets. Confirm the VLAN carrying PLC traffic has no ACLs blocking ISO-on-TCP (TCP port 102) or PN-DCP discovery.

Verification Checklist

• SIMATIC Net V15.1 installed and listed in Programs and Features.
• Softnet S7 license installed and showing green "in use" arrow in ALM.
• S7ONLINE access point bound to the correct Ethernet adapter.
• PC station name in TIA matches the Windows hostname.
• Windows Server OS confirmed (not Windows 10/11) for > 8 connections.
• TIA Portal V15.1 Update 8 (or later) installed.
• Channel diagnosis shows all S7 connections in Connected state on startup.
• Power-cycle test passes for at least 3 PLCs (in / out, observe automatic reconnection within 15 s).
• No manual runtime restart required for the test period (minimum 24 h burn-in recommended).

FAQ