Troubleshooting WinCC V8 OMS+ Connection Drops to S7-1200 PLCs

Problem Description: Intermittent OMS+ Connection Loss in WinCC V8

WinCC V8 Update 5 runtime stations configured with mixed S7 channel loads — 10 native TCP/IP connections to S7-300/S7-400 controllers and 8 OMS+ (S7-Optimized Communication) connections to a heterogeneous fleet of S7-1200 CPUs — periodically report channel down events followed by automatic re-establishment. The legacy S7-1200 stations were commissioned in TIA Portal V11 and V12, while newer units were re-engineered in TIA V17 with active certificates for protected communication. Affected channels show status transitions between Connected, Disconnected, and Establishing in WinCC Channel Diagnosis, typically every 30 to 300 seconds.

The host is a Windows Server 2022 machine with a certified industrial NIC, SIMATIC NET V19, and a Softnet IE S7 license that nominally permits 64 S7 connections. Despite the apparent over-provisioning, the drops persist, indicating that the bottleneck is not at the host licensing tier but at the S7-1200 side or the OMS+ channel configuration itself.

Root Cause Analysis Overview

Four interacting factors typically cause the observed pattern. Each must be eliminated in turn before declaring the fault resolved.

Root Cause 1 — S7-1200 Communication Resource Exhaustion

S7-1200 CPUs (firmware V1.x through V4.x) allocate a fixed pool of S7 communication resources per CPU model. These resources are shared between PG, OP/HMI, S7 PUT/GET, and user-programmed TSEND/TRCV connections. The legacy S7-1200 firmware from TIA V11/V12 was not engineered for the simultaneous connection load that a WinCC V8 station with 8 OMS+ channels plus other HMI clients places on it.

S7-1200 Maximum Connection Counts by CPU Class

Verify exact resource values in TIA Portal under Device properties → Communication resources for the specific CPU and firmware version in use. The values above reflect Siemens documentation; check the precise CPU article for production deployments.

Reference: Siemens Support entry 109479986 — S7-1200/S7-1500 communication resources and number of possible connections.

Diagnostic Procedure

1. Connect TIA Portal to the affected S7-1200 online.
2. Open Online & Diagnostics → Diagnostics buffer and filter for entries containing connection or resource.
3. Look for diagnostic event ID 0x01C8 / W#16#0190 range messages indicating connection rejection due to resource limits.
4. Check Online → Accessible nodes to confirm how many partners the CPU currently reports.
5. In WinCC Explorer, open Tools → Channel Diagnosis, select the OMS+ channel, and capture the error code (e.g., 0xFFDF3840 for "connection establishment failed").

Resolution

• Disconnect any redundant Comfort Panels or secondary HMI clients from the affected S7-1200 CPUs.
• If the application requires the full OMS+ count on a single CPU, migrate the legacy TIA V11/V12 projects to TIA V17 with a CPU 1215C or 1217C (FW 4.4+) that supports the expanded resource pool.
• Consolidate WinCC tags so that the OMS+ connection count is reduced. One WinCC V8 channel can multiplex multiple tag groups; verify tag tree in the WinCC Tag Management editor.

Root Cause 2 — SIMATIC NET License Tier

SIMATIC NET ships with a graduated licensing model. Each tier imposes a hard ceiling on the number of S7 connections that can be simultaneously active on the host.

With 10 S7-300/400 TCP/IP channels plus 8 OMS+ channels the host needs at least 18 concurrent S7 connections. The Lean tier (8) is insufficient. If the customer believes Softnet IE 64 is installed but the drops still occur, validate the actual installed license through the SIMATIC NET License Tool rather than the apparent installation.

Verifying the Active License

1. Open Start → Siemens Automation → SIMATIC NET → License Manager (or SN_LicMgr.exe).
2. Confirm the active license key — not the presence of the license file on disk.
3. Check the S7DOS service: sc query S7DOS must show RUNNING and the description must reference Softnet IE rather than Lean.
4. In Windows Event Viewer → Applications and Services → Siemens Automation, look for the event S7 connection count exceeded.

Reference: Siemens Support — Which licenses can be used for SIMATIC NET PC software.

Root Cause 3 — Windows Server OS and SIMATIC NET Architecture

SIMATIC NET V19 is the first release with full certification on Windows Server 2022. Earlier SIMATIC NET versions reject Server SKUs during install or run in a degraded mode where the S7DOS service cannot bind to the network stack correctly, producing drop-and-recover behavior. Confirm the following:

Root Cause 4 — OMS+ Certificate and TIA V17 Protection

OMS+ is a Siemens-defined S7-Optimized channel that uses TLS 1.2/1.3 and X.509 certificates when the S7-1200 firmware is V4.4+ and the TIA project enables Connection mechanisms → Permit access with PUT/GET communication is disabled. Mixed fleets that include older CPUs without certificates force WinCC V8 to negotiate security parameters that the legacy firmware cannot match. The result is a TLS handshake failure that the WinCC OMS+ channel logs as a normal disconnect, retries, succeeds, fails again — the exact cyclic pattern observed.

Alignment Procedure

1. Open the TIA V17 project for the modern S7-1200 stations.
2. Navigate to Device properties → Protection & Security → Connection mechanisms.
3. Select Permit access with PUT/GET communication from a remote partner if WinCC V8 must interoperate without certificate exchange.
4. If certificates are required, export the WinCC station certificate from WinCC Explorer → Computer → Properties → OMS+ and import it into the S7-1200 project under Global security settings → Certificate manager.
5. Repetitively download the project to the S7-1200 to ensure the certificate store is updated.

Reference: Siemens Support — Using S7-1200/S7-1500 certificates in WinCC V8 (OMS+).

Step-by-Step Diagnostic Workflow

1. Capture baseline: In WinCC Channel Diagnosis, record the drop intervals and the specific S7-1200 partners that are blinking. Export the WinCC diagnostic file WinCC_S7_ChannelDiag.log from <WinCC Project>\Diagnosis.
2. Validate S7-1200 resources: Online each affected S7-1200 and read the diagnostic buffer. Apply Root Cause 1 fix if resource exhaustion is logged.
3. Validate SIMATIC NET license: Confirm Softnet IE 64 is the active license, not Lean. If a license upgrade is required, obtain the correct license from the Siemens license portal and activate via ALM.
4. Validate Server OS configuration: Confirm S7DOS service is bound to the physical NIC, firewall is open on TCP/102, and power management is disabled on the adapter.
5. Validate OMS+ certificates: Open the TIA V17 project, export/import certificates, and either enable PUT/GET or deploy the certificates correctly.
6. Restart sequence: Stop WinCC Runtime, restart S7DOS service, start WinCC Runtime. Verify all 18 channels show Connected with no drop events over a 10-minute window.

Verification Procedure

Once the suspected root cause has been remediated, validate stability through the following steps:

Preventive Measures and Best Practices

• Inventory the S7-1200 fleet by firmware and CPU model. Map each WinCC OMS+ connection to a CPU with sufficient resources.
• Standardize TIA Portal version. Mixing TIA V11/V12 with V17 is the primary driver of certificate/handshake drift. Plan a TIA V17+ migration for the legacy stations.
• Reserve a single Softnet IE license tier for the host that exceeds the worst-case connection load by at least 20%.
• Document the certificate strategy: PUT/GET enabled, or full X.509 chain. Mix-and-match is the most common cause of intermittent OMS+ drops.
• Configure a single physical NIC for S7DOS. Disable VMQ, RSS, and any virtual switch on that adapter.
• Add WinCC redundancy only after channel stability is achieved; redundant WinCC pairs double the connection load on the S7-1200 and will trigger Root Cause 1 if the CPU pool is already near saturation.

Extended Troubleshooting Matrix

Configuration Snippets

OMS+ Channel Timeout Parameter (WinCC V8)

In the WinCC Tag Management, the OMS+ channel exposes the connection establishment timeout in the channel unit properties. For mixed-fleet deployments increase the timeout from the default 5 s to 30 s:

Connection establishment timeout: 30000 ms
Connection break tolerance: 3 retries
Keep-alive interval: 15000 ms
Tag polling cycle (acquisition): 1000 ms

Windows Firewall Rule for S7DOS (PowerShell)

New-NetFirewallRule -DisplayName "SIMATIC NET S7DOS TCP 102" `
  -Direction Inbound -Action Allow -Protocol TCP `
  -LocalPort 102 -Profile Any `
  -Program "%ProgramFiles%\Siemens\Automation\SimaticNet\s7dos\sld.exe" `
  -InterfaceAlias "Intel I210 Ethernet" -Enabled True

Disabling NIC Power Management (PowerShell)

Disable-NetAdapterPowerManagement -Name "Ethernet*" -IncludeHidden
Set-NetAdapterAdvancedProperty -Name "Ethernet*" `
  -DisplayName "Energy Efficient Ethernet" -DisplayValue "Disabled"
Set-NetAdapterAdvancedProperty -Name "Ethernet*" `
  -DisplayName "TCP/UDP Checksum Offload (IPv4)" -DisplayValue "Disabled"

FAQ