Resolving TIA Portal V16 Too Many Online Connections Error

Resolving the TIA Portal V16 "Too Many Online Connections" Error on S7-1500 and SINAMICS G120

The "Too many online connections. Please terminate some connections and try again" message appears in the bottom-right status bar of TIA Portal V16 (Update 3 and similar builds) when the engineering station attempts to open an additional online session against an S7-1500 CPU or a SINAMICS G120 drive over PROFINET, but the controller's PG/OP connection resource table is already saturated with stale or pending sessions. The symptom is highly misleading: the project tree typically shows only one or two devices as "online", yet the CPU refuses every new connect attempt. This article documents the resource limits, the connection lifecycle inside TIA Portal, and the field-proven procedures to recover the engineering session without rebooting the controller.

1. Problem Description

1.1 Observed Symptom

When the engineer selects Go online on an S7-1500 CPU or a SINAMICS G120 (G120C, G120P, G120X, or G120 with CU250S-2 PN / CU240E-2 PN control unit) inside a TIA Portal V16 project, the bottom-right status panel of the TIA Portal main window displays:

Too many online connections. Please terminate some connections and try again.

The online operation times out after approximately 10–30 seconds without establishing a session. The diagnostic flash in the project tree may still display the device as "online with a green check", although the actual session is not usable.

1.2 Environment Where the Fault Appears

1.3 Initial Misdiagnosis

The error string contains the word "many", which leads many engineers to assume they have exceeded the maximum number of simultaneously accessible nodes. In practice, the maximum number of online connections of an S7-1500 CPU is bounded by the configuration, not by the project tree. The fault almost always reproduces with one (1) online node because the TIA Portal client retains orphaned TCP sessions on its own PG socket side.

2. Root Cause Analysis

2.1 S7-1500 Online Connection Resources

An S7-1500 CPU supports up to 32 PG/OP connections in the basic configuration (configurable via the CPU properties → Connection resources → Maximum number of PG connections). Each TIA Portal instance consumes one connection resource per online node. The same TIA Portal application opens separate sessions for the device view, the inspector window, the online & diagnostics view, the trace viewer, and the watch tables. Under load, a single human-driven online session can occupy 2 – 4 resources on the CPU side.

2.2 TIA Portal Client-Side Session Lifecycle

TIA Portal maintains a logical connection set per online window. When you close a project online session or click Go offline, the application is supposed to issue a OPCOMM_CLOSE / T_disconnect to the CPU. Three conditions prevent the close from completing:

1. Forced process termination: If TIA Portal is closed via Task Manager / End Task, or the workstation enters sleep/hibernate, the OPCOMM close is never sent. The CPU holds the resource in ESTABLISHED state until its keep-alive timer (default 30 s) eventually expires.
2. Network interface reset: Disabling the Ethernet adapter, switching WLAN profiles, or unplugging the cable while the session is open tears down the socket on the PG side without notifying the CPU.
3. Background tasks: The firmware downloader, the "Accessible nodes" browser, and the Topology Editor each open their own short-lived sessions. If the user clicks Go online repeatedly, these stack up before the keep-alive timer fires.

Once the CPU-side resource count reaches the configured maximum, every further connect request is rejected and TIA Portal reports the symptom. Even if only one device shows a green check, the CPU is still holding the orphaned sessions from earlier attempts.

2.3 SINAMICS G120-Specific Behaviour

A SINAMICS G120 with PROFINET control unit supports a single STARTER / TIA online connection at any one time. If a previous session was held open by STARTER, Startdrive, or a PLC routing path through the S7-1500, the next online attempt from TIA Portal produces the same error string. The G120 does not differentiate between the two source applications; it only sees one outstanding TCP session on PROFINET slot 0.

3. Solution Procedure

Follow the steps in order. Each step is more invasive than the previous; stop when the next Go online succeeds.

3.1 Step 1 — Use the TIA Portal "Connections" Taskbar

The most direct path is the Connections taskbar, which lists every online session that TIA Portal currently considers open on the engineering station.

1. In the TIA Portal main window, locate the taskbar strip at the bottom of the window.
2. Right-click the taskbar strip and ensure Connections is ticked, or use View → Taskbar → Connections.
3. The Connections panel appears with columns: Status, Online State, Device, Type, Address.
4. Identify every entry showing Connecting, Online, or Inconsistent state against the target PLC or drive.
5. Right-click each entry and choose Go offline / Disconnect.
6. Wait 5 seconds, then retry Go online on the target device.

3.2 Step 2 — Terminate Sessions From the CPU's Online View

1. In the project tree, right-click the S7-1500 CPU and choose Online & Diagnostics.
2. Navigate to Diagnostics → Connection overview (or Connections depending on firmware). This opens the CPU-side view of every PG/OP/PG routing connection currently held.
3. Identify connections whose Local end point is the engineering PC's IP and whose Connection state is ESTABLISHED or CONNECTING.
4. If the firmware exposes Disconnect, terminate the stale entries. Otherwise, proceed to Step 3.

3.3 Step 3 — Close the Online & Diagnostics Windows and Trace Viewers

Each open Online & Diagnostics editor, watch table, and trace viewer retains its own implicit session. Closing them releases resources without rebooting TIA Portal.

1. Close every open tab related to online monitoring (Watch tables, Force tables, Trace, Online & Diagnostics, Inspector window online mode).
2. Switch the Inspector window from Online to Offline mode via the small selector in the upper-right of the Inspector pane.
3. Wait 10 seconds for the CPU keep-alive to age out the closed sessions.
4. Retry Go online.

3.4 Step 4 — Disable and Re-Enable the Ethernet Adapter

If the error persists after Steps 1 – 3, the PG socket table is contaminated. Cycling the network adapter forces Windows to release every TCP socket bound to that interface.

1. Open Control Panel → Network and Sharing Center → Change adapter settings.
2. Right-click the PROFINET / engineering Ethernet adapter and choose Disable.
3. Wait 5 seconds.
4. Right-click the same adapter and choose Enable.
5. Wait for the link to re-establish (green icon), then retry Go online in TIA Portal.

This step is sufficient in the majority of field reports and avoids a TIA Portal restart.

3.5 Step 5 — Close Background TIA Portal Components

TIA Portal V16 ships with helper services such as TIA Portal File Handler, TIA Administrator, and the Siemens Automation License Manager. If two TIA Portal instances were launched (for example, one project per monitor) they share connection resources. Close all but one instance via Task Manager.

3.6 Step 6 — Restart the Engineering PC

As a last resort before restarting the controller, reboot the PC. This clears all PG-side sockets. Do not restart the S7-1500 or the G120 unless the CPU's diagnostic buffer confirms it has run out of PG connection resources.

4. Verification

4.1 Confirm a Healthy Online Session

1. In TIA Portal, right-click the S7-1500 CPU → Go online. The bottom-right status bar should display Online with a green tick.
2. Right-click the SINAMICS G120 → Go online. Confirm the green tick.
3. Open Online & Diagnostics on the CPU, navigate to Diagnostics → Connection overview, and verify that the engineering PC shows a single ESTABLISHED PG connection per online node.
4. Open a watch table, force it online, and read at least one tag to confirm bi-directional communication.

4.2 Confirm CPU Resource Headroom

Open the CPU's Properties → Communication → Connection resources in the project configuration and verify:

• Maximum number of PG connections ≥ 4
• Maximum number of OP connections ≥ 4
• Maximum number of S7 connections per the application requirement

Lower these values only if the application requires reserving resources for HMI panels. The S7-1500 default of 32 PG connections is usually more than adequate for a single engineering station.

4.3 Confirm G120 Single-Session Behaviour

After a successful online session with the G120, attempt to start STARTER / Startdrive on the same PC against the same drive. If STARTER reports Online connection already exists, the TIA Portal session is the holder; closing TIA Portal releases the drive. If the inverse occurs (TIA Portal fails after STARTER was used first), restart STARTER and disconnect its session before attempting TIA Portal.

5. Preventive Configuration

5.1 Reduce Implicit Session Count

Disable the following TIA Portal options unless they are actively required:

• Options → Settings → Online & Diagnostics → Automatically establish online connection when opening project
• Project tree → Show accessible nodes when not in active use (each refresh opens a broadcast session)
• Background topology / device configuration scans in Options → Settings → Hardware configuration

5.2 Reserve PG Connection Resources for Engineering

In the S7-1500 hardware configuration, set:

5.3 Keep Firmware Current

The connection lifecycle bug was reduced in later TIA Portal versions. As of TIA Portal V17 Update 4 and V18, the engine releases orphaned sessions more reliably when an Ethernet adapter is reset. If the project permits, upgrading to TIA V18 / V19 reduces the frequency of this symptom. Validate the project migration against the S7-1500 firmware compatibility list before upgrading.

5.4 Use a Stable Engineering Network

Wireless PROFINET adapters and USB-to-Ethernet dongles drop the TCP session faster than the CPU keep-alive, leading to orphaned sessions. Use a wired Intel / Realtek PCIe or on-board Ethernet port for engineering. Disable Windows power management for the adapter (Device Manager → Network adapter → Power Management → uncheck "Allow the computer to turn off this device to save power").

6. Field-Proven Diagnostic Flowchart

The following sequence summarises the diagnostic ladder for engineering use:

1. Is the status bar showing "Too many online connections"? → Go to Step 1 (Connections taskbar).
2. Are stale entries visible in the Connections panel? → Disconnect them, retry.
3. Are all entries clean? → Close all online windows, switch Inspector offline, retry.
4. Still failing? → Disable / re-enable Ethernet adapter, retry.
5. Still failing? → Close all TIA Portal instances, retry.
6. Still failing? → Restart the engineering PC.
7. Still failing after restart? → Read the CPU diagnostic buffer; investigate routing, firewall, or proxy interference.

7. Common Pitfalls

7.1 Assuming the Project Tree Reflects the Real Connection State

The green tick in the project tree is a logical state, not a probe of the CPU. An entry can show "online" while the underlying TCP session has been torn down by Windows or by the adapter reset.

7.2 Closing the Wrong TIA Portal Window

TIA Portal V16 keeps the connection state in memory. If multiple project windows are open, closing one does not free the CPU resource used by another. Close every project window or use File → Close all projects.

7.3 Misreading the Drive IP Address

If the SINAMICS G120 has been re-parameterised (for example via STARTER), its PROFINET name may have changed. A stale entry in the project tree pointing to an obsolete IP will produce the same error. Refresh the accessible nodes and re-assign the device name from the project.

7.4 Firewall Interference on Windows 10 / 11

Windows Defender Firewall blocks unsolicited TCP replies on port 102 (ISO-on-TCP / S7 communication) when the network profile is set to Public. Set the engineering network to Private in Settings → Network & Internet → Status → Properties before commissioning.

8. Related Siemens Documentation

For deeper protocol details and lifecycle management of online connections, refer to the official Siemens documentation:

• Establishing or changing an online connection – TIA Portal V20 documentation
• S7-1500 Communication function manual (entry ID 109751326 on the Siemens Industry Online Support portal)
• SINAMICS G120 operating instructions (entry ID 109751329 on the Siemens Industry Online Support portal)

The TIA Portal help system also contains a searchable entry on "PG connection resources exceeded" which links to the same root cause covered above.