Resolving S7-1200 'Connection Could Not Be Established' Errors in VMware Workstation

When commissioning an S7-1200 CPU (such as the 1214C DC/DC/Rly) from a TIA Portal instance running inside a VMware Workstation virtual machine, engineers frequently encounter a state where the PLC is visible to the engineering station, the device LEDs can be flashed, the PLC responds to ping, yet an online connection fails with the error "Connection to address 192.168.0.1 failed. Online: Connection could not be established. The connection partner is not responding."

This article dissects the root causes of this specific failure mode, presents a ranked set of corrective actions, and documents the verification steps that confirm a stable TIA Portal ↔ S7-1200 link over Ethernet when the engineering tool is hosted inside a VMware Workstation 8 / Windows XP (or Windows 7) guest. It is written as a field-ready troubleshooting reference for control engineers and commissioning technicians.

1. Problem Description

1.1 Symptom Cluster

The failure presents the following observable signals from the engineering station:

• TIA Portal Online → Accessible Nodes enumerates the S7-1200 CPU at IP 192.168.0.1.
• The Flash LED function operates correctly — the CPU's STOP/RUN and ERROR LEDs blink in response to the command.
• ping 192.168.0.1 from a Windows command prompt inside the VM returns replies with sub-millisecond latency.
• Selecting Go Online immediately returns the error: "Online: Connection could not be established. The connection partner is not responding."

1.2 Reference Network Parameters

The combination of discoverable, pingable, and flashable with non-connectable is the diagnostic fingerprint that points to a layer-4 or layer-7 block between the guest OS and the S7-1200. The S7-1200 itself is responding; something further up the stack is not.

2. Root Cause Analysis

Five interlocking conditions produce this exact failure mode. They are listed in the order in which they should be ruled out during troubleshooting.

2.1 VMware Virtual Network Adapter Mode

VMware Workstation offers three primary virtual switch modes for the guest's network adapter:

NAT mode is the single most common cause of the reported error, because the S7-1200 initiates no inbound connection to the guest — the guest must reach it. Symmetric NAT traversal works for short bursts but fails on the S7 connection establishment handshake when the PLC's reply is interpreted by the NAT engine as a new, untracked session.

2.2 Auto-Assigned Host IP Outside the PLC Subnet

With the guest's network card set to Obtain an IP address automatically, the VMware DHCP server typically issues an address in the 192.168.x.x range that may collide with or fall outside 192.168.0.0/24. A guest at 192.168.182.130 with mask 255.255.255.0 sees 192.168.0.1 as off-link and routes through the VMware NAT gateway, which then fails as described in §2.1.

2.3 Windows Firewall or Third-Party Security Suite Blocking TCP/102

S7 communication uses ISO-on-TCP (RFC 1006) encapsulated in TCP port 102. The Windows Firewall that ships with XP SP3 and Windows 7, plus commercial endpoint protection suites, frequently block inbound or outbound TCP/102 once they detect that the binary is not on the trusted list. The firewall logs the drop silently; the application simply times out.

2.4 Multiple Active Network Adapters on the Guest

A laptop running VMware typically has at least three active network paths: physical Ethernet, Wi-Fi, and the VMware virtual adapters (VMnet1 host-only, VMnet8 NAT). TIA Portal binds its S7 sockets to the first adapter that matches its PG/PC interface selection. If the wrong adapter is selected, packets destined for the PLC are emitted onto a network where no route to 192.168.0.1 exists.

2.5 Incorrect PG/PC Interface Selection in TIA Portal

The Set PG/PC Interface dialog in TIA Portal must be configured to use the S7ONLINE access point bound to the physical or bridged NIC. Selecting TCP/IP(Auto) in the presence of multiple TCP adapters (e.g., virtual NAT adapter + bridged NIC) can cause TIA to attempt the handshake on the wrong interface, yielding the same connection failure.

3. Prerequisites

Before applying the corrective procedures, verify the following:

1. CPU 1214C DC/DC/Rly is powered and in STOP or RUN — both states accept online connections.
2. The PROFINET port (X1) is connected to the same physical LAN segment as the host PC's Ethernet NIC.
3. The CPU has a valid IP (192.168.0.1 by default) and subnet mask (255.255.255.0). Use Online → Accessible Nodes in TIA Portal or the SIMATIC Automation Tool to confirm.
4. You have administrator rights on the VMware guest OS to change network adapter properties and firewall rules.
5. VMware Tools is installed inside the guest — it provides the virtual NIC drivers required for bridged mode.

4. Solution 1 — Configure the VM Network Adapter as Bridged

This is the primary fix and resolves the majority of reported incidents.

Step-by-step

1. Power off the virtual machine.
2. In the VMware Workstation library, right-click the VM and choose Settings.
3. Select Network Adapter in the Hardware tab.
4. Change Network connection from NAT to Bridged.
5. Under Bridged to, select the physical NIC that is physically cabled to the S7-1200's PROFINET port. If uncertain, leave it on Automatic.
6. Check Replicate physical network connection state if you want the guest's link to follow the host NIC's link state (useful for laptops that dock/undock).
7. Click OK and power the VM back on.

Verification

From a Windows command prompt inside the guest:

Confirm that the bridged adapter has received an IP from the same subnet as the PLC (for example, 192.168.0.50 / 255.255.255.0). If the address is still in the 192.168.182.x range, bridged mode is not actually applied — repeat the steps or reinstall VMware Tools.

5. Solution 2 — Assign a Static IP in the Correct Subnet

If the bridged network has no DHCP server, or if VMware's DHCP service is disabled, assign a static address inside the guest.

1. Open Control Panel → Network and Sharing Center → Change adapter settings (Windows 7/10) or Control Panel → Network Connections (Windows XP).
2. Right-click the bridged Ethernet adapter → Properties → Internet Protocol Version 4 (TCP/IPv4) → Properties.
3. Select Use the following IP address and enter:

   Field	Value
   IP address	192.168.0.50
   Subnet mask	255.255.255.0
   Default gateway	(leave blank for direct link)

4. Click OK twice and verify with ipconfig.

6. Solution 3 — Allow TCP/102 Through the Firewall

The S7 transport uses ISO-on-TCP on TCP port 102 (0x66). This must be permitted on both inbound and outbound rules.

6.1 Windows Firewall (Windows 7 / 10 guest)

1. Open Windows Firewall with Advanced Security (wf.msc).
2. Select Inbound Rules → New Rule.
3. Rule type: Port. Protocol: TCP. Specific local port: 102. Action: Allow the connection. Profile: Domain, Private, Public (or restrict to Private for lab use).
4. Name: Siemens S7-1200 Online (TCP/102).
5. Repeat for Outbound Rules.

6.2 Windows XP SP3 Firewall

1. Control Panel → Windows Firewall → Exceptions tab.
2. Click Add Port: Name = S7ONLINE, Port = 102, Protocol = TCP.
3. Confirm the rule is checked and click OK.

6.3 Third-Party Suites

Symantec Endpoint, McAfee, Kaspersky, and Bitdefender all perform application-aware inspection on TCP/102. Add the TIA Portal executable (Siemens.Automation.Portal.exe) to the trusted list, and add an explicit allow rule for port 102 in both directions. If the suite supports deep packet inspection of RFC 1006, disable that specific inspection for the engineering subnet only.

7. Solution 4 — Configure the PG/PC Interface in TIA Portal

The PG/PC Interface assignment is the binding between the S7ONLINE access point and a physical or virtual Windows network adapter. It must match the bridged NIC.

1. In TIA Portal, open the project and select Online → Online & Diagnostics for the target device, or press Ctrl+Shift+O from the project tree.
2. Click Online access in the project navigation.
3. From the PG/PC Interface drop-down, select the bridged adapter with the appropriate protocol:

   Adapter Type	Protocol	When to Use
   Intel / VMware Accelerated AMD PCNet Adapter	TCP/IP	Recommended for S7-1200 over standard Ethernet
   Intel / VMware Accelerated AMD PCNet Adapter	ISO Industrial Ethernet	Use only if a SIMATIC NET SOFTNET-IE software package is installed
   Any NIC	TCP/IP (Auto)	Acceptable for single-NIC guests; avoid with multiple TCP adapters

Persist the assignment via Start → Control Panel → Set PG/PC Interface (in older TIA Portal versions) so that the choice survives project restarts.

8. Solution 5 — Disable or Subnet-Separate Unused Adapters

If a Wi-Fi adapter, host-only VMnet, or a second physical NIC is active and shares a subnet with the PLC link, Windows can route S7 packets to the wrong interface. Two clean ways to disambiguate:

8.1 Disable Unused Adapters

1. Open Network Connections.
2. Right-click each unused adapter (Wi-Fi, VMware host-only VMnet1, Bluetooth PAN) and choose Disable.
3. Re-test the online connection.

8.2 Subnet Separation

Keep all adapters active but configure them on disjoint subnets. For example:

Because no two adapters advertise overlapping prefixes, Windows always selects the bridged Ethernet as the route to 192.168.0.1.

9. Verification Procedure

Run the following sequence inside the guest VM after applying the fixes:

1. ipconfig /all — confirm the bridged adapter has the correct IP and mask.
2. ping 192.168.0.1 — confirm round-trip time below 5 ms and 0% loss.
3. telnet 192.168.0.1 102 — a blank screen (or a connection-succeeded indication) confirms TCP/102 is reachable; a "Could not open connection" message indicates a firewall block.
4. In TIA Portal, Project tree → Online → Go online. The status should transition to Online with a green check on the CPU icon.
5. Open Online & Diagnostics → Diagnostics and verify that the device name, firmware version, and serial number are read back correctly.

10. Extended Diagnostics

10.1 Wireshark Trace Inside the Guest

Install Wireshark on the guest and capture on the bridged adapter while attempting Go Online. Filter with tcp.port == 102. The expected exchange is:

1. Guest → PLC: SYN to 192.168.0.1:102
2. PLC → Guest: SYN, ACK
3. Guest → PLC: ACK (handshake complete)
4. Guest → PLC: TPKT/COTP CR (Connection Request, TPDU 0xE0)
5. PLC → Guest: TPKT/COTP CC (Connection Confirm, TPDU 0xD0)

If the trace shows SYN with no SYN, ACK from the PLC, the firewall on the PLC is rejecting the connection (rare, but documented when the S7-1200 has access control lists enabled in V4.x firmware). If the trace shows RST, ACK from the PLC, the PLC is rejecting the connection for security reasons — review the Connection mechanisms settings in the CPU properties under Protection & Security.

10.2 Siemens Support Article Cross-Reference

Siemens has published a Knowledge Base entry that documents online connection problems with S7-1200 in virtualized environments. Reference: Siemens Entry ID 38721248 — Online connection to S7-1200 fails. The article confirms that the combination of bridged networking, correct subnet placement, and TCP/102 firewall allowance is the supported configuration.

10.3 CPU Protection & Security Settings

From TIA Portal V13 onward, the S7-1200 supports Connection mechanisms configured in Device properties → Protection & Security. The following options can independently block an online session even when the IP and transport are correct:

If the CPU has been configured with Read access only and the password is unknown, the connection will be permitted for read-only diagnostic operations but a download will fail with error 33:1443 ("Protection violation"). This is not the same error as the connection-not-responding message, but it surfaces in the same troubleshooting path and should be ruled out by selecting Full access with a valid password.

11. Common Pitfalls and Edge Cases

12. Summary of the Correct Working Configuration

FAQ