Resolving the "No More Client Connections Allowed" Sm@rtAccess Error on Siemens MP 277 and MP 177 Panels

Operators and maintenance engineers who remote into Siemens Multi Panels (MP) using the WinCC flexible Sm@rtAccess option occasionally receive a popup in Internet Explorer that reads:

Error: no more client connections allowed.

The message is generated by the panel's embedded VNC server, not by the engineering station, and indicates that the maximum number of concurrent Sm@rtAccess sessions for that device has been reached. The error is most often seen on shop floors where multiple engineers, vendors, or remote support technicians try to view the same panel at the same time, but it can also appear on a single workstation if browser sessions, Java plug-ins, or background tabs have not been closed cleanly between uses.

This reference covers the affected hardware, the panel-by-panel maximum client count, the configuration path inside the WinCC flexible Engineering System, and the runtime/firmware conditions that change those limits. Verification steps and a troubleshooting matrix are included so that the issue can be closed out on the first visit.

1. Problem Description

The error appears in the Internet Explorer window that hosts the Sm@rtAccess ActiveX/VNC viewer when a client attempts to open a remote screen on a panel that already has the maximum number of Sm@rtAccess sessions opened. Symptoms include:

• The VNC viewer dialog stops with the message no more client connections allowed.
• Subsequent attempts from the same workstation produce the same message until the browser session is restarted.
• Other engineers on the network report the same error simultaneously, even though they have not been connected previously.
• No new connection succeeds until existing sessions on the panel time out (default TCP keep-alive of 60 s) or are manually closed.

The error originates from the VNC server inside the panel firmware, not from WinCC flexible or the engineering PC. The Sm@rtAccess option on the panel is the licensed feature that exposes the VNC server to the network, and it is gated by a per-panel-type concurrent-connection counter.

2. Affected Hardware and Article Numbers

Sm@rtAccess runs on most Windows CE-based Multi Panels of the 170 and 270 series. The two part numbers most frequently cited on support tickets are:

The 177 series and 277 series use different connection counters in the VNC server. A 170-series panel cannot be configured to match a 270-series count, and vice versa, because the limit is enforced by the panel image, not by the project.

3. Root Cause Analysis

Each Sm@rtAccess client opens a TCP session on port 5900 (or the configured Sm@rtAccess port) plus an HTTP/SOAP control session on port 80 (or 443 if SSL is enabled). The panel firmware keeps a counter of currently authenticated VNC sessions and rejects new connection attempts once the counter reaches the panel-specific maximum.

The three most common reasons the counter is exhausted on a working installation are:

1. Stale browser sessions. A previous operator closed the Sm@rtAccess tab without signing out; the panel keeps the session alive for several minutes (TCP keep-alive timer) before reaping it.
2. Concurrent engineering access. Two or three engineers (vendor, customer, OEM) are connected at the same time; the panel rejects any further connections.
3. Java / ActiveX viewer caching. A Java viewer that crashed during the last session leaves a "ghost" socket on the panel side. The Java console on the engineering PC must be cleared and the JNLP cache reset.

None of these conditions are faults in WinCC flexible or the panel hardware. They are expected behavior of the licensed Sm@rtAccess option, which intentionally limits concurrent remote viewers to preserve CPU cycles and screen-update bandwidth for the operator in front of the panel.

4. Maximum Concurrent Sm@rtAccess Clients by Panel

The per-panel-type limit is documented in the WinCC flexible Online Help under Options → Sm@rtAccess → Using Sm@rtAccess → Conditions for using the Sm@rtAccess option → Use restrictions for every HMI panel. The relevant values for the two panel families in scope are:

5. Configuration Procedure

The maximum number of Sm@rtAccess clients is not configured per project in WinCC flexible; it is enforced by the panel image. The only configuration the engineering station can perform is to enable or disable the Sm@rtAccess option, set the listen port, and define the access password. Use the following procedure on the engineering PC.

5.1 Prerequisites

• WinCC flexible 2008 SP2 (or later) installed on the engineering PC.
• The Sm@rtAccess option license present on the panel and on the engineering station (license key on USB stick or transferred as a license file).
• Project file (.hmi) of the panel that contains the Sm@rtAccess area pointer and authentication settings.
• Network reachability between the engineering PC and the panel (TCP 5900 and TCP 80/443).

5.2 Enable Sm@rtAccess in the Project

1. Open the panel project in WinCC flexible.
2. In the project tree, right-click HMI device → Settings → Sm@rtAccess and select Properties.
3. Check Activate Sm@rtAccess.
4. Set Number of Sm@rtAccess clients to the maximum allowed for the panel (see Section 4). WinCC flexible will grey out the field if the value exceeds the panel hardware limit.
5. Enter the Access password. The password is stored in the project and downloaded to the panel.
6. Confirm with OK, then recompile and download the project to the panel.

5.3 Configure the Panel-Side VNC Server

1. On the panel, open Control Panel → Sm@rtAccess.
2. Confirm that the option is enabled and that the configured maximum number of clients matches the project value.
3. If the panel is running an older image (SP1 or earlier) but the project was compiled with SP2 or later, update the image using ProSave. The required image is supplied with WinCC flexible 2008 SP2.
4. Restart the panel after the image transfer so the new VNC server is loaded.

5.4 Verify the Setting from the Engineering PC

1. Open Internet Explorer 6 or later (32-bit).
2. Browse to http://<panel_ip>/smart_access.html.
3. Log in with the configured password.
4. Open a second browser instance (or use a separate PC) and log in again. If the panel allows two clients, both sessions will display the panel screen; the third attempt will be rejected with the documented error.

6. Runtime and Firmware Considerations

Up to and including WinCC flexible 2008 SP1, the MP 277 VNC server used a single-tile frame buffer that required the full panel image to be redrawn on every client request. This pegged the panel CPU at 90% or higher with one active client and made a second client unusable in practice. The SP2 release re-architected the server so that each client is treated as an independent viewer of the shared screen buffer, reducing CPU load to roughly 40% per active client. The practical effect is:

• On SP1 panels, even though the limit is "1 client", engineers sometimes observed intermittent "no more client connections" because the VNC server process had crashed and had to be restarted manually.
• On SP2 panels (and later), the limit is cleanly enforced and the error appears only when the configured maximum is actually exceeded.
• On MP 177 panels, the limit is hard-locked to 1 client for the lifetime of the product line. The only way to allow a second concurrent remote viewer is to move to a 270-series panel.

The image version on the panel is visible in Control Panel → System → About on the HMI itself. Look for an entry similar to Image: WinCC flexible 2008 SP2 Update 1. If the image is older than the project was compiled with, the panel will silently downgrade the Sm@rtAccess feature set.

7. Diagnostic Steps When the Error Persists

If the documented maximum has not been reached but the error still appears, walk through the following matrix before opening a Siemens support ticket.

8. Workarounds When the Hard Limit Cannot Be Raised

On a single-MP-177 shop floor, the hard 1-client limit is sometimes unacceptable. The following workarounds are field-proven, in order of preference:

1. Schedule remote access. Use a shared calendar entry; only one engineer connects at a time. Add the Sm@rtAccess password to the calendar description so the next engineer can pick up immediately.
2. Use Sm@rtService instead. The Sm@rtService option allows remote administration via the same VNC server but does not consume a Sm@rtAccess client slot. The panel must be licensed for Sm@rtService separately.
3. Mirror the panel with a ProSave Ethernet bridge. Use ProSave to take a project backup and a remote-control session; this does not use the Sm@rtAccess VNC server at all and is not subject to the connection limit.
4. Replace the panel with an MP 277 PRO. The PRO variants are licensed for up to 3 concurrent Sm@rtAccess clients and are pin-compatible with the standard MP 277 in most installations.

9. Verification Checklist

Close out the call only after every line is confirmed.

1. Project compiles cleanly in WinCC flexible 2008 SP2 or later.
2. Panel image version matches the WinCC flexible version used to compile the project.
3. Number of Sm@rtAccess clients in the project matches the panel hardware maximum (Section 4).
4. From two independent engineering PCs, two Sm@rtAccess sessions open simultaneously and remain stable for at least 15 minutes.
5. Third concurrent session is rejected with the documented no more client connections allowed error (this is the correct, expected behavior on a 2-client panel).
6. Panel CPU utilization, observed via Task Manager on the panel, remains below 60% with two active clients.
7. No entries VNC server fault or SmartAccess license invalid appear in the panel's SystemLog during the verification window.

10. Frequently Asked Questions