WinCC flexible ChangeConnection for MP377 on S7-300/400H

Overview

A SIMATIC MP377 operator panel tied to a redundant S7 system — whether an S7-300 software-redundant pair or an S7-400H pair — must read process values from the active controller. WinCC flexible project tags are statically bound to a single configured connection, so a tag points to one MPI/PROFINET partner address. When the redundant system switches from CPU-A to CPU-B, the original tag still queries CPU-A, which is now standby. Without a mechanism to re-point the tag, the panel reads frozen, stale, or invalid values from the secondary CPU.

WinCC flexible ships the ChangeConnection system function. Triggered by a runtime event (PLC bit, scheduled task, or button press), it re-binds a tag (or a group of tags) to a different pre-configured connection. Combined with an H-system status bit read from the PLC, the MP377 detects which controller is active and switches its tag set to the correct connection in milliseconds — well within operator response time.

Prerequisites

Redundant S7 Topology Fundamentals

Three physical topologies are commonly seen with MP377 installations. Knowing which one applies determines how the active CPU is detected.

Scenario A — S7-300 Software Redundancy

Two standard S7-300 CPUs (no H hardware) linked through the SIMATIC S7-300 Software Redundancy library. One CPU runs as Master (FB 101 "SW_RED_M"), the other as Slave (FB 102 "SW_RED_S"). Both nodes are reachable on the MPI / PROFIBUS network at distinct MPI addresses. Switchover is software-driven and takes 100–500 ms. A status word stored in a shared DB reports the current role and can be read by the MP377.

Scenario B — S7-400H Hot Standby

Two S7-400H CPUs connected via fibre-optic sync modules (up to 10 km apart). Both are addressable on the plant bus. SFC 90 "H_CTRL" or the system status list (SSL, SZL-ID W#16#0171) reports the active role. Switchover is sub-100 ms. The CP 443-1 handles the H communication protocol and presents both CPUs under a single redundant IP pair to the operator panel when PROFINET is used.

Scenario C — S7-1500R/H

The newer S7-1500R/H family uses PROFINET redundancy. The MP377 does not directly support S7-1500R/H; a SIMATIC Comfort Panel (TP 1500 / TP 1900 / TP 2200) or an IPC with WinCC Runtime Professional under TIA Portal is required. Refer to the official Siemens documentation for the S7-1500R/H connection setup: Communication with the redundant system S7-1500R/H.

The WinCC flexible ChangeConnection System Function

ChangeConnection is a WinCC flexible system function that reassigns a tag to a different pre-existing connection at runtime. Its signature:

ChangeConnection ( Tag , NewConnectionName )

The function only accepts connection names that already exist in the WinCC flexible project. Calling it with an unknown name generates a runtime warning in the HMI diagnostic log ("Connection not found").

Configuring Two Connections in WinCC flexible

1. Open the WinCC flexible project.
2. Open Project tree → Communication → Connections.
3. Insert two connections, naming them exactly as referenced later in ChangeConnection:
   • Conn_CpuA → points to MPI address 2 (or PROFINET IP of CPU-A)
   • Conn_CpuB → points to MPI address 4 (or PROFINET IP of CPU-B)
4. Set the connection driver to SIMATIC S7 300/400 for both entries. For S7-400H with PROFINET redundancy, the driver must be SIMATIC S7 400H on the appropriate CPU slot.
5. For PROFINET, enter the CP 443-1 / CP 443-1 Advanced IP addresses; for MPI / PROFIBUS, enter the bus addresses (2 and 4).
6. Save and compile. Each connection must pass compilation without warnings.

Each connection consumes a connection resource on the MP377. The MP377 12"/15"/19" supports 6 concurrent S7 connections — two for the H pair leaves 4 free for other PLCs or HMI-PLC coordination area.

Step-by-Step: Automatic Connection Switching

The recommended architecture uses one Boolean HMI tag (the trigger) whose value indicates which CPU is active. The PLC writes the trigger from an H-system status flag. ChangeConnection is then called in the HMI scheduler on change of the trigger, re-pointing every process tag to the correct CPU.

Step 1 — Define the HMI Tags

Create a single set of process tags, but bind them initially to Conn_CpuA. They will be re-pointed at runtime.

Tag name      : TankLevel_PV
PLC tag       : DB100.DBD0   (REAL)
Initial conn  : Conn_CpuA

Tag name      : MotorSpeed_PV
PLC tag       : DB100.DBD4   (REAL)
Initial conn  : Conn_CpuA

Create the trigger tag:

Tag name      : ActivePlc_Trigger
PLC tag       : DB200.DBX0.0 (BOOL)   // 0 = CPU-A active, 1 = CPU-B active
Initial conn  : Conn_CpuA

Step 2 — Read the H-System Status into the Trigger

On an S7-400H, the active CPU bit is obtained by reading the SSL partial list "H system status" (SZL-ID W#16#0171) or by calling SFC 90 "H_CTRL". A typical pattern on the PLC side:

// ST (SCL) on CPU-A — write 0 to DB200.DBX0.0 when A is active
// Call in OB1 or OB35
IF "H_status".active_cpu_id = 0 THEN
    "DB_HMI".active_plc_bit := FALSE;   // 0 = CPU-A active
ELSE
    "DB_HMI".active_plc_bit := TRUE;    // 1 = CPU-B active
END_IF;

For S7-300 software redundancy, the slave CPU mirrors the active flag from the master via FB 102 "SW_RED_S" output SLAVE_STATE. The flag is broadcast by the master and stored in a shared DB read by both CPUs. The HMI consumes the same bit on whichever CPU is responding.

Step 3 — Create a Scheduled Task in WinCC flexible

1. Open Project tree → Scheduler.
2. Insert a new task, trigger type On tag change, select ActivePlc_Trigger.
3. Add a VB script as the action. WinCC flexible's scheduler does not natively support IF…THEN in function lists, so use a VB script that reads the trigger and calls ChangeConnection for each tag.

' VB script in WinCC flexible 2008 SP3+
Dim conn As String

If SmartTags("ActivePlc_Trigger") = False Then
    conn = "Conn_CpuA"
Else
    conn = "Conn_CpuB"
End If

' Apply to every process tag in the set
ChangeConnection SmartTags("TankLevel_PV"), conn
ChangeConnection SmartTags("MotorSpeed_PV"), conn
ChangeConnection SmartTags("ValvePos_PV"),   conn
' add one line per HMI tag

Step 4 — Tag Multiplex (Tag-Centric Alternative)

For projects with hundreds of tags, calling ChangeConnection on every one is expensive and slows the scheduler. Use WinCC flexible's multiplex tag (indirect addressing) instead:

1. Create two full sets of tags, one set bound to Conn_CpuA, one bound to Conn_CpuB.
2. Define a multiplex tag (e.g., TagMux_Index) that holds an index 0 or 1.
3. For each I/O field on the panel, use indirect addressing that reads either the A-tag or the B-tag based on the index.
4. The I/O field references the multiplexed tag; the multiplex swaps the pointer in a single update.

This eliminates the need for ChangeConnection, at the cost of doubling the tag count and configuration effort. It is the preferred pattern on large MP377 installations with 100+ tags.

PLC-Side Status Logic for H System Detection

The status bit the MP377 consumes must be deterministic. The following source flags are reliable on the listed systems:

Alternative: Using the WinCC flexible Connection Status Tags

WinCC flexible exposes built-in connection status tags. With a coordination area enabled, the tag @ConnectionState_<conn_name> reports 1 = OK, 0 = broken. This can be used in a poll-based loop to detect a switchover event and re-issue ChangeConnection.

' Polling loop in WinCC flexible scheduler, period = 500 ms
If SmartTags("@ConnectionState_Conn_CpuA") = 0 Then
    ' CPU-A went down or standby, force CPU-B
    ChangeConnection SmartTags("TankLevel_PV"), "Conn_CpuB"
    ChangeConnection SmartTags("MotorSpeed_PV"), "Conn_CpuB"
    ChangeConnection SmartTags("ValvePos_PV"),   "Conn_CpuB"
End If

This pattern is slower (500 ms latency) but works without any PLC-side status logic. It is the simplest implementation when modifying the PLC code is not feasible — for example, on a brownfield installation where the MP377 is added to a commissioned redundant system.

Connection Resource Limits and Planning

The MP377 has a hard cap on the number of active S7 connections. Exceeding the cap silently drops new connection requests, with the panel reporting Connection error 0x02 in the diagnostic log.

For an H-system, two connections (one per CPU) are the minimum. Add 1–2 spare for third-party PLCs or HMI-PLC coordination areas. If the design exceeds 6 connections, migrate the HMI to a Comfort Panel (TP 1500 / TP 1900 / TP 2200) under TIA Portal, which lifts the connection cap to 16.

Verification and Runtime Diagnostics

1. Compile and download the WinCC flexible project to the MP377 via ProSave or Ethernet.
2. Start Runtime. Open the panel's diagnostic page: Start → Settings → Control Panel → System → Info → Version. Confirm the image version is ≥ V1.3.2.
3. Open the diagnostic log: Start → Settings → System → Logging. Confirm no "Function not supported" warnings on ChangeConnection calls.
4. Force a switchover on the H system (STEP 7 → H-System → "Switch Master/Standby").
5. Observe the ActivePlc_Trigger tag on the MP377 (place it on a diagnostic screen for visibility).
6. Verify that process values update within one screen refresh cycle (≤ 1 s on MP377 with 250 ms acquisition cycle).
7. Disconnect the active CPU's MPI / PROFINET cable. The standby CPU should take over within 1 s, the trigger should flip, and the panel should continue reading correct values.
8. Reconnect the cable and force a switch back. Confirm bidirectional operation.

Troubleshooting Matrix

FAQ