WinCC Advanced Modbus RTU Not Resuming After Cable Reconnection

Problem Overview

When a WinCC Advanced (TIA Portal V16, Update 1 and earlier) runtime station communicates with a third-party controller using the Modicon Modbus RTU driver — for example a Delta DVP series PLC acting as an RTU slave on a serial COM port — the HMI establishes the link and polls correctly during normal operation. After the serial cable is unplugged and reconnected (or after a transient RS-485 bus fault), the runtime does not automatically reopen the Modbus channel. Tags stay at their last quality="bad" value indefinitely, even though the controller is again reachable on the bus. The same PLC, cable and baud rate combination works without issues on competing HMI products that perform their own link supervision.

This article documents the underlying driver behavior, the diagnostic steps that isolate the failure to the HMI side (not the PLC), and the four layered workarounds that have proven reliable in the field. It also covers the TIA Portal V17 / V18 / V19 / V20 changes that touch the Modicon Modbus RTU channel and the WinCC runtime side handling of physical link state changes.

Affected Products and Versions

For the official Siemens documentation set for the current WinCC Basic / Advanced / Professional release, see the TIA Portal V20 WinCC Communication readme.

Symptoms

• HMI shows the last good process values; no live updates after the cable is plugged back in.
• WinCC channel diagnosis reports connection state Disconnected (or Not connected) and never returns to Connected on its own.
• Tag quality stays Quality = BAD, sub-status Device not available.
• Pressing an "Online" / "Connect" button that programmatically puts the connection in offline and back to online restores communication immediately.
• The same hardware and protocol on a different HMI vendor (e.g. Weintek, Pro-face, Delta DOP) re-establishes the link within 1–3 polling cycles without operator action.
• Disconnecting only the cable (not the PLC power) is sufficient to trigger the fault — a hot-pull of the DB-9 is enough.

Root Cause Analysis

The Modicon Modbus RTU channel in WinCC Runtime Advanced is a master-side driver that owns the COM port exclusively. Its internal state machine transitions from Open → Open (Error) on the first failed transaction, but the path back to Open (OK) depends on a successful open-port syscall on the underlying Windows COM handle. Two distinct failure modes have been observed in the field.

Failure Mode A — COM Handle Not Re-Opened

When the cable is removed, Windows delivers a EV_RXCHAR with an overrun, the driver's CRC check fails, and the channel raises an error. On reconnection, the next request is sent into a freshly re-attached cable, but the driver does not re-issue a CreateFile() on the COM port unless the connection object itself is recycled. The result is a permanently half-open channel that never recovers. This behavior is documented in the Siemens TIA Portal V20 WinCC Communication readme under the section on serial driver stability.

Failure Mode B — NDIS Link State Not Propagated

On panels with a combined Ethernet / serial multiplexer (e.g. SIMATIC IPC227G, IPC277G, or third-party industrial PCs running WinCC Runtime Advanced), the physical layer status of the COM port is exposed through a virtual NDIS miniport. When the cable is hot-unplugged, the miniport reports MediaState = MediaDisconnected. The Modbus RTU driver in older WinCC builds does not subscribe to this state and therefore cannot reset the channel. This is the same class of issue described in the Microsoft Q&A on Ethernet adapter reporting "media disconnected" every 2–6 hours — the OS surfaces the state, but the user-mode driver ignores it.

Why Competing HMIs Recover

Most non-Siemens HMI stacks (Weintek EasyBuilder Pro, Pro-face GP-Pro EX, Delta DOPSoft) use a polling supervisor that CloseHandle()s the COM port on any persistent error and reopens it on the next cycle. The Siemens Modicon Modbus RTU channel lacks this self-healing loop, which is why the symptom is invisible on other vendors' hardware.

Diagnostic Procedure

1. Open the WinCC Advanced project in TIA Portal and navigate to Devices & Networks → Connections. Confirm the Modicon Modbus RTU connection points to the correct COM port (e.g. COM1, COM3) and that the baud rate, parity, data bits and stop bits match the Delta PLC exactly (default Delta DVP: 9600 / 8 / E / 1, station 1).
2. Compile the project and start WinCC Runtime Advanced on the HMI panel. Open the channel diagnosis screen (HMI tag @ConnectionState or the system diagnostics view).
3. Unplug the RS-485 cable at the panel end. Within one polling cycle (default 1 s) the tag quality for every Modbus address drops to BAD.
4. Plug the cable back in. Observe the connection state: if it remains Disconnected for more than 30 s while the same cable on a Weintek cMT series re-establishes automatically, you have confirmed the WinCC defect.
5. Open Windows Device Manager → Ports (COM & LPT) → your COM port → Properties → Power Management. Uncheck Allow the computer to turn off this device to save power. Some USB-to-RS485 converters (FTDI FT232, CH340) drop the handle when the host suspends the device — this is a separate but identical symptom.
6. Run a third-party Modbus master (e.g. mbpoll on Linux, or Modbus Poll from WinTech) against the same COM port from another laptop. If it reads successfully while WinCC does not, the failure is isolated to the WinCC runtime, not the cable or PLC.
7. Export the WinCC runtime diagnostic log (DiagLog.txt, located in the project runtime directory, typically C:\ProgramData\Siemens\Automation\WinCC RT Advanced\[ProjectName]\) and search for Modbus, COM, and disconnected. You will see entries like Channel 'Modicon_Modbus_1': connection error 0x80072EE7 or COM port handle invalid.

Solution 1 — Programmatic Offline / Online Cycle (Recommended for V16)

The most reliable workaround in TIA V16 is to drive the connection lifecycle from a C or VBScript in the runtime. Tie the cycle to a periodic check of any tag whose value is known to change (e.g. the PLC real-time clock or a heartbeat counter).

VBScript Example (Global Script, scheduled task @ 5 s)

' --- WinCC Advanced V16 — Modicon Modbus RTU auto-reconnect ---
' Scheduled task: cyclic, 5000 ms
' Trigger: runtime start
Dim conn, heartbeat, q

Set conn = HMIRuntime.Screens("System").ScreenItems("ConnectionStatus")
heartbeat = SmartTags("PLC_Heartbeat")
q = SmartTags("PLC_Heartbeat").QualityCode

' If the heartbeat tag is BAD for more than 15 s, recycle the connection
If (q And 0xFF) = 0x01 Then
    HMIRuntime.Trace "Modbus RTU channel degraded, q=" & Hex(q) & ", cycling..." & vbCrLf
    HMIRuntime.ConnectionState = 0        ' 0 = offline
    HMIRuntime.Sleep 1500
    HMIRuntime.ConnectionState = 1        ' 1 = online
End If

C Script Variant (ANSI C, scheduled task)

/* TIA Portal V16 — WinCC Advanced — Cyclic Modbus RTU reconnect
   Compile target: WinCC Advanced C-Script (ANSI C, no MFC) */
#include "apdefap.h"

void CyclicReconnect(void)
{
    DWORD q = GetTagQuality("PLC_Heartbeat");
    static DWORD badSince = 0;
    DWORD now = GetTickCount();

    if ((q & 0xFF) == 0x01) { /* BAD */
        if (badSince == 0) badSince = now;
        if ((now - badSince) > 15000) {
            SetConnectionState(g_hConnection, 0);  /* offline */
            Sleep(1500);
            SetConnectionState(g_hConnection, 1);  /* online  */
            badSince = 0;
        }
    } else {
        badSince = 0;
    }
}

Solution 2 — Operator-Triggered Reconnect Button

Wire a physical or screen button to the same offline / online transition. This is the minimum viable fix and is the pattern Siemens support recommends when the customer cannot accept an automatic script.

1. Add a button to the diagnostics screen. Event: Press.
2. Configure the Press event with a system function: SetConnectionState("Modicon_Modbus_1", 0).
3. Configure the Release event with: SetConnectionState("Modicon_Modbus_1", 1).
4. Add a delay of 1–2 s between the two events using a visibility animation or a small VBScript wrapper. A pure release-without-delay will not close and reopen the handle.

This pattern is documented for WinCC Advanced in the TIA V20 WinCC communication readme as the manual recovery path when channel supervision does not recover automatically.

Solution 3 — System-Level Hardening

Solution 4 — Migration to TIA V18 / V19 / V20

Starting with TIA Portal V18, the Modicon Modbus RTU driver received an internal channel supervisor that reopens the COM port on a configurable number of consecutive failed transactions. In V20, this is exposed as the Connection supervision property group on the connection object.

For projects that can be upgraded, the long-term remedy is to move to TIA V20 (or the highest version your customer licenses) and enable Connection supervision → Enable automatic reconnection on the Modicon Modbus RTU connection. The behaviour and the new properties are documented in the WinCC V20 Communication readme.

Verification

1. With the project running, open the runtime diagnostics screen and note the initial @ConnectionState value: 1 (connected).
2. Pull the RS-485 cable from the panel-side DB-9. Within one polling cycle the value drops to 0.
3. Reconnect the cable. The chosen solution must return the value to 1 within the configured back-off (1–15 s) without operator action (Solution 1 and 4) or via a single button press (Solution 2).
4. Confirm all polled tags transition from Quality = BAD back to Quality = GOOD. Tag value updates should resume on the next poll.
5. Repeat the test 10 times back-to-back. A reliable fix does not produce any tag-quality oscillation or duplicate transaction errors in the diagnostic log.
6. Capture the runtime diagnostic log (DiagLog.txt) before and after the test. A healthy reconnect shows entries like Channel 'Modicon_Modbus_1': connection established, attempt 2/3. Persistent entries of COM handle invalid indicate the system-level fix in Solution 3 is incomplete.

Troubleshooting Matrix

Best Practices for Stable Modbus RTU Channels

• Use a dedicated native COM port (COM1 / COM2) on the panel rather than a USB dongle when the installation is permanent. USB-RS485 converters are a long-term reliability risk in industrial cabinets.
• Always set the Modbus connection's Polling cycle to a fixed value (e.g. 500 ms). Do not use 0 (on-demand) for serial channels — it interacts badly with the driver's error state machine.
• Implement a heartbeat tag (a counter incremented every 100 ms in the PLC) and supervise it from a scheduled C / VBScript. A healthy heartbeat proves end-to-end Modbus health, not just link state.
• Place the channel supervision script in the Global Script area, not in a screen script, so it survives screen changes and alarms.
• Keep the TIA Portal installation on the latest service pack for the version you are licensed for. Each SP contains COM-stack stability fixes.
• For sites with frequent cable maintenance, plan a TIA V18 / V19 / V20 upgrade so the auto-reconnect feature can be used instead of a user-written workaround.