Resolving WinCC Flexible MP 177 Transfer Error via PC Adapter

Problem Overview

Engineers commissioning a SIMATIC MP 177 multi-panel frequently encounter the runtime error "The connection could not be established" when attempting to download or transfer a WinCC flexible project via MPI/PROFIBUS using a Siemens PC Adapter. The error is generated by the WinCC flexible ES transfer dialog when the engineering station cannot complete an S7 connection handshake with the target panel over the selected bus.

The MP 177 (order number 6AV6642-0BC01-1AX1 for the PROFIBUS variant, or 6AV6642-0BA01-1AX1 for the PN variant) is a Windows CE 5.0-based multi-panel with a 6-inch or 10-inch resistive touch display. It supports three transfer channels: MPI/PROFIBUS, Industrial Ethernet, and USB host-to-host. When a PROFIBUS-based transfer is the only option - typically because the panel sits in a machine that lacks Ethernet drops or because the panel is already wired to an S7-300/S7-400 PLC over PROFIBUS - the engineer must verify that the PC Adapter, the panel, and the bus segment are configured as a single coherent PROFIBUS network.

Per the SIMATIC HMI MP 177 Operating Instructions (article 6AV6691-1DP02-0AB0, edition 08/2008), a project transfer via MPI/PROFIBUS requires the panel to be physically on the same PROFIBUS segment as the PC Adapter, or to be reachable through a transparent repeater or DP/DP coupler. The PC must be the master on that segment during the transfer window. The same constraints apply to any S7-300/S7-400 PLC on the bus; the panel's MPI/PROFIBUS port is the only one the adapter must see.

Hardware Identification: Siemens PC Adapter USB A2

The adapter referenced in the source thread is the PC Adapter USB A2, MLFB 6ES7 972-0CB20-0XA0. This is the second-generation USB-to-MPI/PROFIBUS adapter, superseding the legacy 6ES7 972-0CA23-0XA0. Identifying the exact version is critical because driver packages and supported baud rates differ. The "A2" suffix indicates the USB 2.0 full-speed variant with an integrated MPI/DP transceiver; the even newer PC Adapter USB A2 6ES7 972-0CB35-0XA0 (revision 4) adds isolated USB and supports up to 19.2 kbps on the MPI side with improved ESD protection.

Note that the 24 V supply on the adapter is mandatory: the adapter cannot power the PROFIBUS transceiver from the USB port alone. If the 24 V jack is left unconnected, the adapter enumerates on USB but the MPI/DP channel reports "bus fault" and the WinCC flexible transfer dialog stalls at "Connecting..." before returning The connection could not be established. This is the single most common field failure when using the PC Adapter for the first time.

Root Cause Analysis: Why the Transfer Fails

The error class on the engineering side can be split into four fault domains. Identifying which domain the failure lives in is the first troubleshooting step.

1. Physical layer - No PROFIBUS segment exists between the PC Adapter and the MP 177. The user has configured the adapter and the panel identically but has not cabled them together. PROFIBUS requires a linear bus topology with terminating resistors enabled at the two end nodes. The MP 177 has an internal termination that is enabled by a slide switch or by a DIP on the PROFIBUS connector. The 6ES7 972-0BA12-0XA0 and 6ES7 972-0BB12-0XA0 connectors both include a termination switch and a PG socket for tapping off a service node.
2. Bus parameter mismatch - The transmission rate, highest station address (HSA), or PROFIBUS profile differ between the PG/PC interface, the panel, and (if present) the PLC. The MP 177 defaults to "MPI" profile at 187.5 kbps with HSA 126. The PC Adapter must match. When the panel was previously on a different DP master, its stored parameters may not match the new commissioning laptop. Also note that PROFIBUS DP and MPI use the same physical layer but different bus profiles; the panel's "MPI/PROFIBUS" mode can be toggled, and the wrong profile (DP when MPI is expected, or vice versa) is a silent mismatch that yields the same connection error.
3. Access point routing - Windows "Set PG/PC Interface" assigns a default access point that is not the PC Adapter. The S7DOS / S7EHM driver in WinCC flexible must be bound to the access point "S7ONLINE" pointing to PC Adapter (PROFIBUS). If the active access point is "TCP/IP" or a virtual Ethernet driver, the project transfer opens the wrong socket. WinCC flexible's Accessible Nodes function will then either return nothing or list an Ethernet node that is not the panel.
4. Panel transfer mode - The MP 177 is not in transfer mode. The panel's "Transfer" channel in the HMI Control Panel must be enabled, and "Wait for transfer" (or "Automatic transfer" on power-up) must be selected. Without that, the panel never opens the S7 server on MPI/PROFIBUS and rejects the connect attempt. Note that a previously loaded project can lock the Control Panel: a long-touch into the empty area of the screen opens the service login only if the project does not include a full-screen window. If a project is running, use the panel's Service button (or the rear Service key on the 10-inch variant) to enter the Control Panel.

Of these, the most common field failure - and the one implied in the source thread - is domain 1: the PC Adapter is connected to the laptop, the panel is on a different segment, and the engineer expects a peer-to-peer handshake over a bus that does not physically exist. The second most common is domain 4, because the panel's "Transfer" setting is buried in the Windows CE Control Panel of the MP 177 and is sometimes disabled by a previously loaded project. The third is domain 2, because PROFIBUS bus profiles are not always obvious and a "DP" panel will not accept an "MPI" transfer (or vice versa) without a parameter rewrite.

MPI/PROFIBUS Network Prerequisites

Before opening WinCC flexible, the engineer must physically and logically assemble the bus.

1. Confirm that the MP 177's IF1B (MPI/PROFIBUS) port is connected to a PROFIBUS cable. Siemens specifies 6XV1 830-0EH10 (violet, PVC jacket) for fixed installation and 6XV1 830-3EH10 (halogen-free, grey) for confined spaces. Maximum segment length is 1,000 m at 187.5 kbps, 200 m at 1.5 Mbps, and 100 m at 12 Mbps. The cable has a defined input and output end - pin 1 to pin 1, pin 2 to pin 2, and the shield must be bonded at every connector with 360° termination.
2. Plug the PC Adapter into the bus using a PROFIBUS connector with PG socket (6ES7 972-0BA12-0XA0 for 90° outlet, or 6ES7 972-0BB12-0XA0 for 35° outlet). Engage the termination switch on the connector only if the PC Adapter is at a physical bus end. The two switch positions are OFF (default for inline tap) and ON (for end-of-segment). The MP 177 itself does not have a terminator; the terminator lives in the connector at the panel.
3. Connect the 24 V supply to the PC Adapter's power jack. The MPI/DP LED remains dark without 24 V even when USB enumeration succeeds. The power input accepts 19.2-28.8 V DC and draws 150 mA typical.
4. Verify that the highest PROFIBUS address (HSA) and the baud rate on the panel match the PC Adapter. The panel's settings are in Start > Settings > Control Panel > Transfer > MPI/PROFIBUS. Default is 187.5 kbps, address 1, HSA 126, profile "MPI". The PC Adapter's matching settings live in the "Set PG/PC Interface" tool under Properties > PROFIBUS.
5. Disable any firewall on the engineering laptop that may block the S7DOS service. The S7DOS service uses local inter-process communication and outbound UDP for node discovery, so the Windows Defender Firewall must allow S7DOS64.exe or the legacy S7DOS.exe. On Windows 11, the rule may need to be created manually under Windows Defender Firewall with Advanced Security > Inbound Rules.
6. If the panel sits behind a DP/DP coupler, both segments must be electrically isolated and the coupler must be in operation. The PC Adapter must be on the segment that physically connects to the panel; the coupler will not bridge S7 connection requests from one segment to the other without a configured slot.

Configuring PG/PC Interface (Step-by-Step)

The PG/PC interface is the single most error-prone configuration in the chain. The following sequence applies to Windows 7, 10, and 11 with the SIMATIC PC Adapter USB driver installed.

1. Open Start > Siemens Automation > Set PG/PC Interface (or run SIMATIC_S7PCInterface.exe from the WinCC flexible installation folder). The shortcut is also reachable from the Windows Control Panel under "PG-PC Interface" if the option is enabled in the WinCC flexible installer.
2. In the Access Point of the Application dropdown, select S7ONLINE. STEP 7 and WinCC flexible both use this access point. Do not confuse it with S7ONLINE_STEP7, which some tools bind independently.
3. In the Interface Parameter Assignment Used list, choose PC Adapter (PROFIBUS). If only "PC Adapter (MPI)" is visible, the driver was not installed for PROFIBUS - re-run the PC Adapter USB setup from the WinCC flexible DVD or download the latest SIMATIC PC Adapter USB driver from Siemens Support. The PROFIBUS variant of the driver is the one that registers the bus as "PC Adapter (PROFIBUS)" and supports the 12 Mbps DP rate.
4. Click Properties. On the PROFIBUS tab, set:
   • Transmission rate: 187.5 kbps (or the rate of the active bus)
   • Highest station address: 126
   • Profile: DP or MPI depending on whether the panel is in DP mode
   • Bus time-out: leave at 10 s for first commissioning
5. On the MPI tab, set the PC Adapter's own MPI address to 0 (the default master) and the local PROFIBUS address to a value not used by any master on the bus (commonly 0 or 1). The "PC Adapter" address is the address that the bus master sees; the "Own MPI address" is the address of the PC itself.
6. Click OK and then OK again. Windows registers the access point change. A reboot is not normally required, but if the transfer still fails, a reboot of the engineering PC reinitialises the S7DOS driver cleanly.

Verify the configuration by running Accessible Nodes from WinCC flexible (menu Project > Transfer > Accessible Nodes, or in TIA Portal Online > Accessible Nodes). The MP 177 should appear as MP 177 with its PROFIBUS address. If it does not, the bus parameter set is still wrong or the bus is not terminated properly.

Configuring the MP 177 Panel-Side Connection

On the panel, navigate the Windows CE-based operating system to align the transfer channel.

1. Press the Service button (or, on units without a service button, touch the empty area of the screen for 5 seconds, then confirm the service login) to open the HMI Control Panel. On the 10-inch MP 177 (6AV6642-0DC01-1AX1) the service key is on the rear and is recessed to prevent accidental activation.
2. Open Transfer.
3. Set Channel 1 to Enable and select MPI/PROFIBUS from the protocol dropdown. Channel 2 is reserved for Ethernet and is independent.
4. On the Properties page, set:
   • Transmission rate: 187.5 kbps (or your network rate)
   • Address: 1 (must not collide with the PLC if one is on the bus)
   • Highest station address: 126
   • Profile: DP or MPI matching the PC Adapter
5. Enable Remote Control / Wait for transfer so the panel opens the S7 server on power-up. Without this flag, the panel boots into the project and never listens for a transfer on the bus.
6. If the project currently on the panel is locked, set Transfer startup to Yes to force the panel into transfer mode after the configured delay. The delay is 5 s by default and is configurable to 10 s, 30 s, or 60 s.

Configuring WinCC Flexible Transfer Settings

In WinCC flexible ES, the transfer dialog is the last gate before the panel download.

1. Open the project in WinCC flexible 2008 SP5 (or TIA Portal with a WinCC flexible-converted project; native TIA Portal targets the MP 177 only as a migration candidate and not all features are supported).
2. From the menu bar, choose Project > Transfer > Transfer Settings.
3. In the Mode group, select Serial / MPI / PROFIBUS / Ethernet as appropriate. For this scenario choose MPI/PROFIBUS. Selecting the wrong mode opens the wrong S7DOS channel and the dialog will report "no nodes found" rather than a clean connection refusal.
4. Set the panel address to 1 (matching the panel's IF1B address). The address field accepts decimal values from 1 to 126.
5. Optionally tick Overwrite password-protected recipe data and Reset recipe data on panel if you need a clean transfer. These flags are only honoured by panels that have recipe support compiled in; the MP 177 base variant does not have recipes and the flags are silently ignored.
6. Click Transfer. The dialog opens a connection attempt and reports the bus state in the status bar. The first 5-10 seconds are handshake; the actual transfer starts when the panel acknowledges the S7 read/write request.

If the status bar shows Connecting... for more than 5 seconds and then returns The connection could not be established, the S7DOS handshake failed. The most likely causes in priority order are: (a) bus not physically connected, (b) baud rate mismatch, (c) PG/PC access point not bound to PC Adapter, (d) panel not in transfer mode.

Initiating the Transfer

1. Power-cycle the MP 177 if you have changed any bus parameter. The new address and baud rate only take effect after a restart; the panel keeps the old parameters in RAM until the next cold boot.
2. On the panel, confirm the transfer prompt. If Automatic transfer is enabled, the panel will offer the transfer dialog within 5-10 seconds of boot. The prompt reads Transfer: Wait / Cancel with a countdown bar.
3. Click Transfer in WinCC flexible. Progress is shown in percent. A typical 4 MB project takes 1-3 minutes at 187.5 kbps and under 30 seconds at 1.5 Mbps. The PC Adapter's MPI/DP LED blinks rapidly during the transfer; a steady LED with no progress indicates the handshake succeeded but the project is stalled (likely a corrupted project file or a recipe mismatch).
4. After the transfer completes, the panel restarts automatically and boots into the new project. The first boot of a newly transferred project can take up to 30 s while the panel initialises its file system and the project database.

Alternative Transfer Methods

When PROFIBUS is unavailable or the engineer is in a lab without a PLC, the MP 177 supports two faster paths.

Ethernet (Direct Cross-Over Cable)

For the PN variant (6AV6642-0BA01-1AX1) and the DP variant with a CP module, a direct cross-over Ethernet cable (Siemens 6XV1 850-2GH20 or any standard Cat 5e cross-over) connects the panel to the laptop. Configure the panel's PROFINET address (default 192.168.1.100) and the PC's PROFINET interface to the same subnet (e.g. 192.168.1.10 with mask 255.255.255.0). In WinCC flexible, set the transfer mode to Ethernet and use the panel IP as the target. This is the recommended path when both devices have free Ethernet ports. The Ethernet transfer uses TCP port 102 and is the same port the S7-300/S7-400 PLCs use, so any industrial firewall on the engineering laptop must allow it.

USB Host-to-Host

The MP 177 has a USB-A host port that accepts a USB-to-USB bridge cable (sometimes called a "USB link cable" or "USB data transfer cable", order code 6AV6 671-1CB00-0AX0 is the Siemens-branded variant). The cable contains a bridge chip (e.g. Prolific PL-25A1 or similar) and requires a Windows driver on the engineering side. The Windows CE side uses the panel's native USB function driver. Once the cable is installed, the panel appears in Accessible Nodes as a TCP/IP device, and WinCC flexible can transfer over the USB-NCM interface at USB 1.1 full-speed rates (12 Mbps theoretical, around 800 kB/s effective).

Diagnostics and Verification

After configuring all three sides, the following checks confirm the bus is healthy:

1. LEDs on the PC Adapter - The Power LED must be solid green when 24 V is applied. The MPI/DP LED must be solid green when the bus is idle and a master is active. A slow blink indicates bus error; a fast blink indicates active frame exchange. No light on MPI/DP indicates no 24 V, no driver loaded, or a broken PROFIBUS cable.
2. WinCC flexible Accessible Nodes - Project > Transfer > Accessible Nodes should list the MP 177 with its DP/MPI address. The bus is logically sound if the panel appears here but the transfer dialog still fails - the fault is in the project settings, not the bus. A common cause at this stage is a TSAP mismatch: WinCC flexible uses TSAP 01.01 by default for MPI and 02.01 for Ethernet; the panel accepts these on first commissioning but can be reconfigured in the project's "Connections" editor.
3. PROFIBUS diagnostics in STEP 7 - If a PLC is on the same bus, open PLC > Diagnostics/Set > PROFIBUS Diagnostics in STEP 7. The panel's station should show with its address and live status. Frame errors point to termination or shielding problems; absence of the station points to address or baud rate mismatch. STEP 7's "Bus Statistics" counter shows CRC errors, retries, and the bus cycle time - the cycle time at 187.5 kbps is 5-10 ms per node, well below the 50 ms PROFIBUS limit.
4. Port check with the S7DOS tool - Run S7DOSDiag.exe from the WinCC flexible install directory (typically C:\Program Files\Siemens\WinCC flexible 2008\WinCC flexible\). It reports the access point, the bound driver, and the live state of the bus. A "driver not found" status means the PG/PC access point is misconfigured. The same tool can be used to test the connection without launching WinCC flexible, which is useful for scripting a connection check in a CI pipeline.
5. MP 177 system information - On the panel, Start > Settings > Control Panel > System > About shows firmware version and DP address. Verify the DP address matches the transfer dialog. The "Version" tab reports the image version (typically V12.00.00 for the last production firmware of the MP 177) and the build date; a panel with old firmware may have known transfer bugs that are fixed by an OS update from the WinCC flexible DVD.

Common Errors and Resolutions

Field-Proven Tips

• Always use the violet PROFIBUS cable (6XV1 830-0EH10). The grey PROFINET cable looks identical at a glance and has the wrong pinout - PROFINET uses pins 1 and 2, PROFIBUS uses pin 3 (B-line) and pin 8 (A-line). Mixing them up is one of the most common causes of "no nodes found" on a freshly assembled segment.
• Before opening WinCC flexible, prove the bus with Accessible Nodes. If the panel is not visible, do not start the project transfer - the result is the same error and you will spend the time arguing with the dialog instead of fixing the bus. Accessible Nodes is the canary; if it chirps, the bus is alive.
• When the panel sits behind a DP/DP coupler, the PC Adapter must be on the same segment as the master that has the coupler configured. The coupler does not transparently bridge the broadcast storm for the S7 connection lookup. Both sides of the coupler must have their own HSA and baud rate settings; they are independent buses.
• For first commissioning, prefer a 187.5 kbps bus. Most MP 177 stock firmware defaults to this rate, and many PLCs default to it as well. Faster rates can be applied after the first successful transfer, once the bus is known to be clean. At 12 Mbps the cable must be impedance-controlled and the connectors must be torqued; a single loose connector will generate CRC errors and stall the transfer.
• Keep a spare 6ES7 972-0BA12-0XA0 PROFIBUS connector with PG socket in the toolbox. The 35° and 90° outlet versions are not interchangeable with the panel cutout in some enclosures. The PG socket is required to tap the PC Adapter into an existing segment without breaking the bus.
• For TIA Portal projects, migrate the WinCC flexible project using TIA Portal > Project > Migrate WinCC flexible project. The migration is lossy - some ProTool-derived screens and certain script constructs are not carried over - but the runtime behaviour on the MP 177 is identical once migrated.
• If the panel's image has been corrupted (e.g. by an interrupted transfer), the MP 177 supports a "Restore" mode from the Windows CE startup. Hold the Service key while powering on; the panel enters restore mode and accepts a fresh image over the same channels. This is the recommended recovery path; do not attempt to reflash the image over the boot loader without a Siemens Service request.

For complete panel documentation, including firmware update procedures and connector pin-outs, consult the official MP 177 Operating Instructions on the Siemens Industry Online Support portal (article 6AV6691-1DP02-0AB0).

Frequently Asked Questions