Resolving Siemens TP900 Comfort HMI Online Connection Failures in Virtual Machine Environments

Engineers running Siemens TIA Portal inside a virtual machine frequently encounter a situation where the S7-1500 CPU goes online without issue, but the TP900 Comfort panel refuses the online connection, returning the diagnostic: "Going online: the HW configuration of the HMI_RT_1 was changed but it was not compiled and loaded yet. The shown diagnostics is possibly inconsistent or wrong." The panel still accepts a project download, yet the online view remains unreachable. This article documents the layered root cause, the VM networking fix, and the engineering workflow that prevents the error from recurring on SIMATIC Comfort Panels operating behind hypervisor NAT.

1. Problem Statement and Symptom Matrix

Two distinct symptoms are typically reported simultaneously, but they have different root causes and different fixes. Treat them as two parallel failure modes that must be cleared independently.

2. Decoding the HW Configuration Warning

The diagnostic string has a fixed meaning in the TIA Portal project model:

1. The device HMI_RT_1 (the HMI runtime target inside the panel) was added or had its hardware configuration changed since the last full hardware compile.
2. The new configuration was not compiled and downloaded to the physical panel before the user requested "Go online."
3. Therefore, the diagnostic buffer and the online view that TIA Portal will display may be inconsistent with the device's actual runtime state, because the panel image that the engineering system thinks is loaded is not the image actually running.

The message is informational, not strictly an error. It is a prompt to recompile and reload so the engineering system and the device return to a known-consistent state. On its own, however, it does not cause the online connection to fail.

3. Layered Root Cause Analysis

The online connection failure has two independent contributors that must be cleared sequentially.

3.1 Compilation State Mismatch

TIA Portal maintains a project-side model of the panel hardware (the HMI device configuration under Devices & Networks). When you change a connection, add a tag, alter a screen reference, or even add the HMI to a subnet, the project tree is marked "changed." The compiler must be re-run to produce a fresh HMI configuration image, and that image must be transferred to the panel before the online view can synchronize. If you skip this step, TIA Portal refuses to display online diagnostics from a runtime whose image it cannot recognize.

3.2 Virtual Machine Network Mode

This is the dominant cause. When TIA Portal is installed inside a Windows VM (VMware Workstation, VMware ESXi, VirtualBox, Hyper-V, Parallels) the PG/PC interface typically reaches the S7-1500 over the host's physical NIC through one of three network modes:

Why does the PLC online work under NAT but the HMI online does not? The S7-1500 CPU has a richer set of routing/return-path capabilities: it responds on the IP that received the request, and the TIA Portal transfer path often succeeds via a one-way route. The HMI Comfort runtime, however, uses Siemens' S7Online protocol for the online view, which depends on broadcast-based device discovery (DCP / ARP) and on a clean symmetric return path. NAT breaks the broadcast and breaks the return path simultaneously.

4. Prerequisites

Before you begin the corrective procedure, confirm the following.

• TIA Portal installed inside the guest OS, with the same project used for the S7-1500 CPU program and the TP900 Comfort device.
• The TP900 Comfort panel is powered, has a valid IP address in the same subnet as the S7-1500 CPU, and is reachable from the physical LAN (verify with ping from a host on the physical network).
• Hypervisor access to change the VM's network adapter mode (bridged / NAT / host-only).
• Administrative rights inside the guest OS to change the Set PG/PC Interface assignment in TIA Portal.
• Access to the S7-1500 CPU's IP, the TP900 Comfort panel's IP, and the project file (.ap15 / .ap16 / .ap17 / .ap18 depending on TIA version).

5. Step-by-Step Resolution

Step 1 — Recompile and reload the HMI configuration

1. Open the project in TIA Portal.
2. Right-click the HMI_RT_1 device (TP900 Comfort) in the project tree.
3. Select Compile > Hardware (rebuild complete).
   This forces TIA Portal to regenerate the HMI device description from the current project state.
4. Right-click the TP900 Comfort and choose Download to device > Hardware and software (only if different) to push the recompiled configuration to the panel.
5. Wait for the transfer to complete and for the Comfort panel to restart the runtime.
6. Retry "Go online" and confirm the HW configuration warning is cleared.

Step 2 — Change the VM network adapter to bridged mode

1. Shut down the guest OS (do not pause or save state; perform a full power-off so the virtual NIC disconnects cleanly).
2. Open the hypervisor's VM settings.
3. Select the network adapter that is bound to the physical NIC connected to the plant LAN.
4. Change the mode from NAT to Bridged (in VMware: Bridged: Connected directly to the physical network; in VirtualBox: Bridged Adapter; in Hyper-V: configure an External virtual switch bound to the physical NIC).
5. Power on the guest.
6. Inside the guest, open Set PG/PC Interface (accessible from the TIA Portal menu under Options) and assign the S7ONLINE access point to the bridged virtual NIC.
7. Verify the guest now has a routable IP on the same subnet as the S7-1500 and the TP900 Comfort (use ipconfig /all on Windows; confirm the IP is on the plant network, not in the 192.168.x.x NAT range).

Step 3 — Re-test the online connection

1. In TIA Portal, expand the TP900 Comfort device in the project tree.
2. Right-click and choose Go online.
3. TIA Portal should now establish the online view, populate the diagnostic buffer, and enable the Online & Diagnostics panel.
4. If a partner connection to the S7-1500 is configured on the HMI, TIA Portal will additionally report the connection status of HMI_Connection_1 (or whatever name you assigned).

6. VM Network Mode Configuration Reference

Each hypervisor exposes a slightly different vocabulary for the same concept. The table below maps the term you will see in the GUI to the underlying mode.

7. Why the PLC Online Works but the HMI Online Fails Under NAT

To understand why the bridged fix is the correct answer, it helps to see what each protocol stack expects from the network layer.

The online view needs bidirectional broadcast reachability, which is precisely what NAT strips out. The CPU's online view sometimes works under NAT only because the S7-1500 can be addressed by IP and tolerates a one-way discovery; the Comfort panel's online stack is stricter.

8. Verification Procedure

After applying both fixes, perform this end-to-end check before declaring the issue resolved.

1. From the guest OS command prompt, run ping <TP900_IP>. Expect replies with sub-millisecond times on the local subnet.
2. Run ping <S7-1500_IP> and confirm reachability.
3. In TIA Portal, click Online > Accessible devices. Both the S7-1500 and the TP900 should appear with their actual MAC addresses (not the VM's bridged MAC).
4. Right-click the TP900 and choose Go online. The status bar at the bottom of TIA Portal should display "Online" with a green indicator.
5. Open Online & Diagnostics > Diagnostics buffer on the HMI and confirm fresh entries are being captured.
6. Open the HMI's connection list (Online & Diagnostics > Connections) and confirm the partner connection to the S7-1500 shows Established.
7. Optionally, transfer a small project change to confirm that the round-trip transfer + online view cycle is healthy.

9. Troubleshooting Matrix

Use this matrix when the standard fix is in place but online still fails.

10. Hardening the Setup to Prevent Recurrence

Once the immediate problem is fixed, these engineering practices prevent it from coming back.

• Reserve a static IP for the VM host on the bridged NIC. Avoid relying on DHCP; some plant DHCP servers do not return DCHP offers across all VLANs, and the TIA Portal online path is sensitive to IP changes during the session.
• Document the bridged NIC binding in the project file so a colleague sitting at the same VM weeks later does not "helpfully" switch it back to NAT for security reasons.
• Adopt a habit of recompile-and-reload on every HMI change. This eliminates the HW configuration warning from the user experience and reduces confusion when a network fault occurs.
• Disable the VM's secondary NAT NIC if it is not used. A VM with two NICs (one bridged, one NAT) will sometimes bind S7ONLINE to the wrong one based on route metrics.
• Lock the panel's IP at the panel side (not just in TIA Portal) via the Windows CE Control Panel or the Comfort Panel's Settings > Network and Dial-up Connections. The Comfort runtime ships with a DHCP client active by default in some firmware lines, and an unexpected IP change will silently break the online view.

11. Commissioning Checklist for New VM Hosts

1. Confirm hypervisor version, bridged NIC driver, and Windows version of the guest.
2. Confirm that the plant switchport permits the VM's bridged MAC (port security allow-list).
3. Configure a static IP on the bridged NIC inside the guest.
4. Install TIA Portal and import the project.
5. Set the S7ONLINE access point to the bridged NIC.
6. Run Online > Accessible devices and confirm CPU and panel are both listed with correct MACs.
7. Compile and download the HMI configuration.
8. Verify online view opens and diagnostic buffer populates.
9. Document the configuration in the project's README or commissioning report.

12. Cross-Platform and Hardware Considerations

Engineers sometimes ask whether a native Windows installation behaves differently from a VM guest. The answer is yes, in two specific ways.

• Native install: The OS owns the NIC directly, DCP broadcasts are received without mediation, and the S7ONLINE access point has a single, unambiguous binding. Online view opens reliably.
• VM guest with bridged NIC: Effectively the same as native from the network's point of view, with one caveat — the hypervisor's virtual switch may add filtering (especially in Hyper-V with protected network). Promiscuous mode settings and the virtual switch's port isolation features can interfere; if you observe intermittent drops, inspect the vSwitch configuration.
• VM guest with NAT NIC: Always problematic for the online view, regardless of hypervisor. The transfer path may "work" by accident if the panel and CPU happen to be on the same subnet as the host's NAT adapter; this is a fragile state and should be considered broken.

For engineers using a USB-to-Ethernet adapter plugged into the laptop, the same rules apply — the USB NIC must be visible inside the guest (USB passthrough or USB arbitration by the hypervisor), and S7ONLINE must be assigned to it. A common failure mode is a USB NIC that is visible to the host OS but not presented to the guest; from inside the VM, the NIC is "missing," and TIA Portal silently falls back to another access point that does not reach the panel.

13. When the Panel is Behind a Router (eWon / Industrial Gateway)

Many plant networks have a remote-access router such as the eWon Flexy in front of the HMI. In that case the panel's online view must traverse the router's NAT layer, and bridged mode in the VM is necessary but not sufficient. Additional considerations apply.

• The eWon must be configured to forward TCP/102 to the panel's IP.
• For TIA Portal to discover the panel, the eWon must pass through DCP/ARP frames. Some eWon firmware versions suppress broadcast traffic by default; check the eWon's filtering rules.
• If only the CPU is reachable through the eWon (the panel is on a separate subnet behind the eWon's LAN side), the HMI online view requires a static route in the eWon's routing table pointing to the panel's subnet.

These layered issues are out of scope for a single-vm troubleshooting exercise, but the principle is the same: when an online view fails, trace the path broadcast-by-broadcast and ensure every NAT hop is explicitly bridged or routed.

14. Key Parameter Reference

Parameters most often needed when working this issue.

15. Frequently Asked Questions