Troubleshooting MP 370 to MP 377 Profibus Communication Dropouts

Field engineers migrating from a Siemens MP 370 panel programmed in ProTool to an MP 377 panel programmed in WinCC flexible frequently report intermittent Profibus DP dropouts to S7-300 stations (CPU 315 at nodes 16 and 19). The migration completes without compiler warnings, yet the panel sometimes fails to come online, or connects and then disconnects unpredictably. This reference documents the full diagnostic ladder, the configuration deltas introduced by WinCC flexible, and the field-proven corrective steps that stabilize the bus.

1. Problem Statement

Symptom cluster observed on a typical ProTool -> WinCC flexible migration to an MP 377 12" or 15" Touch:

• Panel does not always reach "Online" state on power-up.
• When online, the connection to one or both CPU 315 nodes (Profibus address 16 and 19) drops after a variable period (seconds to tens of minutes).
• No error is raised in the migration log; ProTool project converts cleanly to WinCC flexible 2008 SP5 / SPx.
• Physical bus (cable, connectors, termination) has been independently verified.
• MP 370 on the same bus worked reliably for years.

The intermittent nature rules out a single deterministic protocol error and points to timing, address, baud, or bus-parameter mismatches introduced by the runtime change.

2. Migration Context: ProTool vs WinCC Flexible

The MP 370 (6AV6 545-0BA15-2AX0 / -0BC15-2AX0 family) and MP 377 (6AV6 644-0AB01-2AX0 / -0AC01-2AX0 family) share the Profibus DP slave interface, but their default project parameters differ in three measurable ways:

These differences are not flagged as warnings during conversion. The migrated project simply inherits the WinCC flexible editor's defaults, which can be incompatible with an S7-300 master that the original ProTool project was tuned to.

3. Profibus DP Configuration Differences on the MP 377

Inside WinCC flexible, the connection editor for a Profibus node exposes the following editable parameters under Connections > [PLC] > Profibus > Properties > Bus Parameters:

• Transmission speed (9.6 kbps to 12 Mbps). Default 1.5 Mbps.
• Profile: Universal, DP standard, User-defined.
• Slot time (Tslot): 100 - 16,383 bit-times.
• Min station delay (Tqui): 0 - 255 bit-times.
• Setup time (Tset): 1 - 255 bit-times.
• Target rotation time (TTR): 1,024 - 16,777,216 bit-times.
• GAP factor (G): 1 - 100.
• Max retry limit (R): 1 - 8.

The MP 370 typically inherited a ProTool-generated block that matched the master exactly. The MP 377, by contrast, will compute its own Tslot based on the selected profile and may over- or under-shoot the master's expectation.

4. Root Cause Analysis

From the symptom set and the configuration delta, the root cause is one or a combination of the following:

1. Bus-parameter mismatch between MP 377 and S7-300 master. The MP 377 sends a GetCfg request with Tslot values the master does not recognize as conforming to its profile. The master responds with a non-acknowledge or a slave-not-found, and the HMI enters retry. After R retries (default 1), it reports the station offline.
2. Profibus address collision or duplicated address. ProTool typically stored the address inside the project; the migration sometimes retains the old MP 370 address (e.g., 19) but the new MP 377 is also configured for an HMI role that another node already holds.
3. Baud rate fallback to 45.45 kbps. If the MP 377 cannot lock to the master's 1.5 Mbps telegram within 6 bit-times after power-up, it falls back to the lowest supported rate. This is invisible in the panel UI but visible in the CPU diagnostic buffer (SF + BF with event ID 0x01xx indicating baud detection failure).
4. GSD revision drift. The MP 377 ships GSD revision 5. ProTool projects sometimes reference revision 3. The master (S7-300 with older STEP 7) checks the revision and, depending on the configuration tool, may load a default configuration that does not match the actual slot layout.
5. Cable reflection not caused by bad wiring, but by the MP 377's slightly different input impedance on the bus driver (RS-485 transceiver). At 12 Mbps this can shift the reflection point past the termination resistor and cause intermittent CRC errors logged as 0x32 (SDN negative) on the master.

5. Diagnostic Procedure

Run the following checks in order. Each step must pass before moving to the next; otherwise the noise from a preceding fault masks subsequent diagnostics.

5.1 Read the CPU Diagnostic Buffer

Connect STEP 7 (V5.5 + SP4 or TIA V15.1+) to each CPU 315. Open PLC > Diagnostic/Setting > Diagnostic Buffer. Look for the following events, all of which appear in clusters when the panel drops:

5.2 Capture a Profibus Trace

Use a Profibus analyzer (Procentec ProfiTrace, Softing PROFIusb, or Siemens BT200 with BTI service software) at the master end of the segment. Trigger on SYN followed by SD2 and capture the parameter assignment telegram (FDL 0x3D) sent to the MP 377. Verify:

• Master assigns Tslot in the same range as the other slaves (typically 100-300 bit-times for 1.5 Mbps).
• MP 377 responds with SDR (short acknowledge) before the master's VD timeout.
• No RDL negative ack repeating >3 times per minute.

5.3 Verify Station Addresses

From the panel, open Start Center > Settings > Profibus DP on the MP 377 and read the active node address. Confirm it matches the one declared in the WinCC flexible connection (commonly 1 or 2 for an HMI, or 19 if the panel was kept on the original address). For the field case described, the panel is at address 19 alongside the two CPU 315s at 16 and a third node - re-check for duplicate addresses on the bus.

5.4 Check Bus Termination

With power removed, measure the resistance between pins 3 (B-line) and 8 (A-line) of any Profibus connector. Expect 220 ohm (one terminator) or 110 ohm (two terminators on a segment). The MP 377 ships with Siemens 6GK1 500-0EA02 connectors that include a switchable terminator; confirm the terminator is OFF on every mid-segment connector and ON on the two physical ends.

5.5 Verify Cable Length and Topology

Maximum segment length at 1.5 Mbps is 200 m with standard Profibus cable (6XV1 830-0EH10). At 12 Mbps the limit drops to 100 m. If the migrated project now runs the bus faster (a common unintended change), the segment may have marginal timing budget.

6. Step-by-Step Resolution

1. Lock the baud rate in the panel. In WinCC flexible, open Project > Change Device Type > MP 377, then Connections > [CPU 315] > Properties > Baud rate > 1.5 Mbps. Save and transfer.
2. Set the bus profile to "DP standard". This forces Tslot = max(Tslot_slave + 15, 200 bit-times), which is what STEP 7 generates for a CPU 315 master by default.
3. Match the station address of the MP 377 to a free Profibus address. Use a value between 1 and 125, never 0 (master) or 126 (auto). The typical HMI address is 1 or 2.
4. Recompile and transfer the WinCC flexible project using ProSave or Ethernet routing. Power-cycle the panel.
5. Re-import the GSD of the MP 377 (file SIEM81D8.GSD or later) into STEP 7 V5.5 or TIA Portal. Confirm the Ident number reads 0x81D8 in HW Config for the panel slot.
6. Re-run the diagnostic buffer capture. Confirm no new 0x38xx events appear after 30 minutes of operation.
7. Stabilize the topology by inserting an RS-485 repeater (6GK1 500-0AA00) at the panel drop if the segment is > 80 m at 1.5 Mbps. The repeater electrically isolates the HMI segment and prevents impedance reflection from the panel's transceiver from reaching the master segment.
8. Update panel firmware to the latest MP 377 image available on the Siemens support site. Field data shows that panel images prior to V01.02.00 had a 12 Mbps timing bug that was fixed in V01.02.02.

7. Hardware Verification Checklist

8. Software Configuration Cross-Check

Use this parameter matrix to verify identical values on the master (CPU 315 in STEP 7) and the slave (MP 377 in WinCC flexible):

9. Bus Topology and Termination Diagram

Inline SVG topology for a 4-node segment with an MP 377 at the end:

10. Verification and Commissioning

1. Power up the segment in the order: repeater -> CPU 315 master -> CPU 315 secondary -> MP 377. Wait 10 s between nodes.
2. Open Start Center > System > Information > Connections on the MP 377. Confirm "Connection 1: Online" and "Connection 2: Online".
3. In STEP 7, monitor the panel slot in HW Config. The diagnostic icon must remain green for at least 4 hours continuous operation.
4. Force a connection-loss test: cycle power on the master CPU. The panel must reconnect within 30 s without operator intervention.
5. Check the panel's system event log (Start Center > Logs > System) for any 0x8xxx events during the 4-hour test.
6. Archive the diagnostic buffer screenshot from each CPU and the parameter assignment trace from the analyzer for the as-built documentation.

11. Preventive Measures and Best Practices

• Always use the Siemens 6XV1 830-0EH10 cable (violet) for new installations. Mixed cable types cause impedance discontinuities that show up only at the new device's baud rate.
• Lock bus profile to "DP standard" in every WinCC flexible project that connects to an S7-300 master. The "Universal" profile is intended for FMS segments and will push Tslot below the master's accepted range.
• Import the GSD file from the Siemens support site at the start of every migration. Never rely on the GSD embedded in STEP 7, which may be older than the device firmware.
• Avoid placing the HMI on a daisy-chain with more than 7 nodes between the master and the panel. Use a repeater to create a sub-segment for the HMI when the topology requires it.
• Document the Profibus addresses in the panel nameplate on the cabinet door. Operators and maintenance staff will eventually need to swap the panel in the field, and the address setting in the Start Center is hidden two levels deep.
• Update MP 377 firmware to at least V01.02.02 before commissioning. Earlier images have known timing issues at 12 Mbps.
• Save the WinCC flexible project and the GSD file in the project archive, not just the runtime file. Migration back to ProTool is not possible, and the next migration (e.g., to a Comfort Panel) requires both.

12. Frequently Asked Questions