MP277 to CPU 315T-2 DP: Resolving Duplicate Access Point Error

1. Problem Description

When configuring a SIMATIC MP277 (WinCC flexible 2008 SPx project) against a SIMATIC S7-300 CPU 315T-2 DP (6ES7 315-6TH13-0AB0 or compatible variant), engineers frequently add the first connection to the CPU and then attempt to add a second connection over the remaining physical interface. The first connection compiles cleanly, but as soon as the second connection is created and the project is rebuilt, both connections turn orange in the WinCC flexible "Connections" editor and in STEP 7 NetPro.

The orange state is WinCC flexible's signal that the connection definition is plausible (right PLC family, right rack/slot convention) but is not fully resolvable in the engineering database. It does not mean runtime communication is impossible, but it does mean the project will not download or will only partially download, and online diagnostics cannot be opened from the panel until the conflict is cleared.

2. Root Cause Analysis

The conflict is generated by the Access Point of the Application assignment on the connection object. Every WinCC flexible / STEP 7 connection carries a hidden parameter that names the Windows-side logical access point the engineering tool will use to reach the target when compiling, downloading, or performing online diagnostics. The default name is:

Access Point = S7ONLINE

This name is the same string used by:

• STEP 7 V5.x "Set PG/PC Interface" dialog
• WinCC flexible Transfer channel
• SIMATIC Manager "Accessible Nodes" lookup
• Any PC-side application that follows the S7ONLINE convention (ProSave, S7-PCT, etc.)

When two WinCC flexible connections to the same target CPU both reference S7ONLINE, the engineering database cannot decide which physical interface (TCP/IP or PROFIBUS) the S7ONLINE access point should bind to for that specific connection. WinCC flexible therefore reports both connections as unresolved and turns them orange.

The relationship between the access point string and the physical hardware is defined in the Windows control panel Set PG/PC Interface application (entry point: Control Panel → Set PG/PC Interface on a classic WinCC flexible engineering station). The mapping is one-to-many: a single access point can be assigned to one physical interface at a time, but multiple distinct access point names can each point to a different interface.

3. CPU 315T-2 DP Communication Architecture and Connection Resources

Per the official SIMATIC S7-300 CPU 31xT manual (entry ID 21362915), the CPU 315T-2 DP provides:

The 14 connection resources are shared across:

• PG (programming device) online
• OP/TP (HMI panels, including the MP277)
• S7 connections defined in NetPro (PUT/GET, USEND, BSEND, etc.)
• PROFINET IO controller / device relationships
• Time synchronization (where configured)

The CPU 315T-2 DP reserves one resource for PG and one for OP by default. Adding the MP277 twice (once over PROFINET, once over PROFIBUS) consumes two OP resources, leaving the engineer with the full 14 minus 2 minus any NetPro-defined S7 connections. For a typical 2-connection HMI configuration, the 14-resource ceiling is not the proximate problem; the access point naming is.

4. MP277 Interface and Connection Model

The MP277 8" and 10" panels (e.g., 6AV6 643-0CB01-1AX1 Touch, 6AV6 643-0BA01-1AX1 Key) provide two physical interfaces relevant to this configuration:

A single MP277 can run two S7 connections to the same CPU simultaneously, one per interface, and WinCC flexible supports this with the connection type S7 300/400. Each connection is an independent PLC integration with its own tag namespace, its own area pointers, and its own error-status tag (or "connection-status" bit) used by the runtime to flag a loss of the partner.

The two connections must each declare which physical interface of the MP277 they ride on, and the engineering transfer channel of WinCC flexible must know which interface to use during download. This is exactly where the access point parameter takes over.

5. Prerequisites

Before applying the fix, confirm the following:

1. STEP 7 V5.5 (or V5.4 + SP5) is installed with the CPU 315T-2 DP support package (HSP ready).
2. WinCC flexible 2008 SP5 (or 2008 SP3 with HF13 or higher) is installed.
3. Either the WinCC flexible project is integrated into STEP 7 (recommended) or it is a standalone project with the connection partners correctly typed.
4. The MP277 is reachable on at least one interface (green Transfer LED behavior, or Control Panel > Transfer > accessible).
5. Windows "Set PG/PC Interface" shows at least one valid access point assignment for the physical interface currently in use.

6. Step-by-Step Resolution

1. Open the WinCC flexible project (or the STEP 7 project with the integrated WinCC flexible station).
2. In the project tree, open Connections under the MP277 station.
3. Confirm you have two connection objects, both targeting the CPU 315T-2 DP rack/slot, e.g., Connection_1 (PROFINET) and Connection_2 (PROFIBUS). Both will show the orange icon.
4. Select Connection_1. In the Properties inspector, switch to the Parameter tab.
5. Locate the Access Point field. It will show the default value S7ONLINE.
6. Leave Connection_1 set to S7ONLINE.
7. Select Connection_2. Open its Parameter tab.
8. Change the Access Point field from S7ONLINE to a new unique name, for example S7ONLINE1.
9. Open Windows Control Panel → Set PG/PC Interface. In the Access Point of the Application dropdown, select S7ONLINE1 and assign it to the interface that is NOT used by S7ONLINE (typically PROFIBUS MPI if S7ONLINE is already TCP/IP). Click OK.
10. Return to the WinCC flexible project. From the menu choose Project → Rebuild All (or right-click the station → Rebuild All). The orange indicators should clear on both connections.
11. Save the project (Ctrl+S).
12. Transfer the project to the MP277 using whichever interface the user intends to be the primary transfer channel. The panel's Transfer dialog (under Control Panel) must be set to allow transfers on the matching interface.

6.1 STEP 7 NetPro Alternative

If the WinCC flexible station is integrated into STEP 7, the same parameter is exposed in NetPro:

1. Open SIMATIC Manager → Project → Open NetPro.
2. Right-click the connection line between the MP277 and CPU 315T-2 DP. Choose Object Properties.
3. Open the connection's Special / Parameter tab. Find the Access Point property.
4. Rename the second connection's access point to S7ONLINE1.
5. Save and close NetPro. Reopen WinCC flexible and run Rebuild All.

7. Matching the Transfer Mode and Baud Rate

A second, independent cause of "both connections orange" or "transfer fails after the first download" is mismatched transfer settings between the panel and the CPU. After fixing the access point, verify these on both ends:

If the user changed S7ONLINE to point at PROFIBUS for the second connection, the panel's PROFIBUS transfer channel and the CPU's MPI/DP interface must both operate at the same baud rate and both must agree on the panel's DP/MPI address. Mismatched baud rate is the most common cause of "transfer starts, then fails partway through" errors after the access point fix is applied.

8. Rebuild and Transfer Procedure

After the access point is renamed and the PG/PC interface is re-assigned:

1. In WinCC flexible, choose Project → Compiler → Rebuild All. Confirm no warnings or errors are reported in the output window.
2. Power-cycle the MP277 (or soft-restart from the Control Panel).
3. On the panel, navigate to Control Panel → Transfer → Settings. Enable Transfer on the interface that matches the access point S7ONLINE (typically Ethernet/PROFINET) and, if needed, on the interface that matches S7ONLINE1 (typically PROFIBUS).
4. From the engineering PC, run Transfer → Download → Target → Ethernet or PROFIBUS as appropriate. WinCC flexible will resolve the S7ONLINE / S7ONLINE1 binding through the Set PG/PC Interface configuration.
5. After the first transfer completes successfully, the panel restarts into runtime and both connection status indicators in the project (under Connections → Status) should show green.

9. Verification

To confirm both connections are alive at runtime:

• On the MP277, open the configured connection-status area pointer (default tag name: <Connection>_Status or the user-defined name). The bit should report connection established for both connections within 5–15 seconds after panel start.
• In STEP 7, select the CPU and choose PLC → Accessible Nodes with the panel's PROFINET IP. The panel should appear as an OP partner on the active connection list.
• Open the CPU's diagnostic buffer (PLC → Diagnostic Buffer). Look for entries referencing the MP277's MPI/DP and PROFINET addresses. Each connection establishment logs an "S7 connection established" event with the partner's TSAP and address.
• Watch the MP277's mode LEDs during startup: the bottom-row LED cluster should show Transfer blinking during download, then Run steady once runtime is active and both connections are up.

10. Troubleshooting Matrix

11. Field-Proven Notes

• Always rename the access point on the second connection, not the first. The S7ONLINE name is referenced by ProSave and by STEP 7 — changing it on the primary connection breaks the engineering transfer channel.
• If the panel is later re-flashed with a different WinCC flexible project, the access point assignment in Set PG/PC Interface persists, but the connection objects are re-evaluated. Re-validate both connections after a clean install.
• Some older service packs of WinCC flexible 2008 (SP1 / SP2) do not surface the access point parameter in the connection editor at all. In that case, the only workaround is to define the second connection on a separate WinCC flexible station (a different MP277 device in NetPro) targeting the same CPU, then merge the area pointers manually. Upgrading to SP5 removes the limitation.
• Do not rename S7ONLINE itself in the registry. The name is hard-coded in many SIMATIC tools. Use only the Set PG/PC Interface dialog to add S7ONLINE1 as a new logical access point.

12. Related Documentation

• SIMATIC S7-300 CPU 31xT Manual (Entry ID 21362915) — connection resource model, interface description for CPU 315T-2 DP.
• WinCC flexible 2008 Communication manual (Entry ID 18796062) — connection types, access point, area pointers, status bits.
• STEP 7 V5.5 NetPro documentation (Entry ID 18652604) — integrated WinCC flexible station, S7 connection definition.
• SIMATIC PROFIBUS DP interface description (Entry ID 19289347) — baud rate, HSA, DP address configuration for the CPU MPI/DP port.