Troubleshooting WinCC PROFIBUS EXTF BUS1F: 0 Tank Level Recovery

Problem Overview: WinCC Tank Levels Stuck at Zero with EXTF and BUS1F LEDs

A maritime automation retrofit using a Siemens S7-300/S7-400 PLC on a vessel was migrated from a working HMI PC to a replacement PC that was supplied as "plug and play." After connecting the purple PROFIBUS cable, the WinCC Runtime starts, but every tank level reads 0 and every sensor appears unresponsive. WinCC diagnostics return "No IM/PROFIBUS network detected." On the Siemens rack the operator observed two steady red LEDs labeled EXTF and BUS1F.

The same PROFIBUS cable was then moved back to the original PC and worked correctly. After a monitor swap on the new PC (with all cables disconnected and reconnected), the fault returned. Re-attaching the cable to the original PC subsequently reproduced the fault as well, confirming that the disturbance was induced into the PROFIBUS physical layer rather than into the HMI PC application layer.

This pattern is characteristic of a PROFIBUS DP physical-layer fault: the network cable was energized, de-energized, and reconnected while the bus was live, which produced reflections and bus-fault conditions on the slaves. When the cable was finally terminated incorrectly, the CPU detected the failure and latched EXTF and BUS1F. The HMI loses process values because the WinCC tag database is read through the same DP segment.

Affected Hardware and PROFIBUS Topology

The reported architecture is a single-segment PROFIBUS DP master/slave network. Components in scope:

• SIMATIC S7-300 or S7-400 CPU with an integrated MPI/DP interface (PROFIBUS interface 1).
• IM (Interface Module) and DP slaves distributed in the engine-control room, e.g., ET 200M, ET 200S, or third-party DP slaves used for tank level sensing.
• One operator HMI PC running WinCC Runtime, connected through a CP 5611 / CP 5612 / CP 5621 communications processor or the native PROFIBUS port of an industrial motherboard.
• Siemens PROFIBUS bus connector with integrated termination switch and PG socket (typically 6ES7972-0BA12-0XA0 or 6GK1500-0FC10).
• PROFIBUS DP cable (violet/purple, type A, two-wire shielded twisted pair, characteristic impedance 150 Ω).

LED Meanings: EXTF and BUS1F on Siemens S7 CPUs

The Siemens CPU front panel LEDs map directly to fault conditions on the integrated PROFIBUS interface. Refer to the SIMATIC S7-300 CPU 31x Technical Data Manual for the canonical LED definitions.

A simultaneous steady EXTF + BUS1F pattern strongly indicates that the CPU cannot exchange I/O data with the DP slaves on interface 1 and has received diagnostic interrupts reporting slave loss. The most common underlying causes, in priority order, are:

1. Termination missing, double-terminated, or set on a mid-segment device.
2. Cable shield not bonded or broken at a connector.
3. Signal wires A (green) and B (red) swapped or shorted to shield.
4. Reflected wave on a live-cable disconnect (TDR-like reflection when the segment was unplugged under power).
5. Slave address conflict or a slave whose 24 V supply collapsed during the disturbance.

Root Cause: Live PROFIBUS Disruption and Incorrect Termination

PROFIBUS DP runs at RS-485 levels (typically 9.6 kbit/s to 12 Mbit/s) with a 1.5 V differential signal between conductors A and B. The bus is a single linear segment terminated at both ends with 220 Ω terminating resistors in parallel with 390 Ω pull-ups and pull-downs, giving the specified 150 Ω line impedance. When the segment is opened or shorted while the bus is live, the termination impedance becomes a reflection point. The result is a standing wave that can:

• Drive receivers into bus errors for several hundred milliseconds.
• Cause DP slaves to drop off the bus and assert diagnostic interrupts.
• Cause the CPU to mark interface 1 as failed (BUS1F) and latch EXTF.

In the reported case, the PROFIBUS connector at the HMI was unplugged, swapped between PCs, and reconnected while the PLC and slaves remained powered. Combined with the cable being handled (twisted, kinked) and a probable change to the connector's termination switch, the segment was left either un-terminated, double-terminated, or terminated at a middle node. Once that condition is met, the CPU will not recover automatically until a power cycle or a bus reset is performed and the segment is repaired.

PROFIBUS DP Physical Layer Requirements (IEC 61158 / IEC 61784)

The physical layer of PROFIBUS DP follows IEC 61158-3 and the cable installation follows the rules summarized in the SIMATIC NET PROFIBUS Network Manual. Key engineering values:

Inline PROFIBUS Topology Diagram (SVG)

Diagnostic Procedure in WinCC and STEP 7

Before opening any cable, reproduce the fault on the engineering station to confirm the failure is physical-layer and not HMI-side.

1. Open WinCC Explorer → Tools → Status of Driver Connections. A status of "No IM/PROFIBUS network detected" indicates that the WinCC channel driver (typically SIMATIC S7 PROTOCOL SUITE on the PROFIBUS softnet) cannot see any active partner.
2. In STEP 7, open HW Config → Online → Accessible Nodes on PG/PC interface CP5611/CP5621 (PROFIBUS, 1.5 Mbit/s, profile DP). Expected result when BUS1F is steady: only the CPU itself appears; no slaves listed.
3. Open PLC → Module Information → Diagnostic Buffer on the CPU. Look for events of type "Station failure," "DP slave diagnostic," and "Bus fault on PROFIBUS interface 1." The diagnostic buffer timestamps confirm whether the fault coincides with the cable swap.
4. Use SIMATIC Manager → Options → Set PG/PC Interface → Diagnostics to read bus statistics: repeated receive errors, token retries, and "slave lost" counters all point at the physical layer.

Reference: WinCC V7.5 Communication Manual and S7-300 CPU 31x Manual.

Step-by-Step Recovery Procedure

1. Stop the WinCC Runtime on the HMI PC and exit the WinCC Explorer. Disconnect the CP 5611/5621 PROFIBUS cable from the PC end first.
2. Take the S7 CPU to STOP mode (mode selector to STOP). This protects the DP slaves from another live disturbance and clears BUS1F only after a bus reset.
3. Inspect every PROFIBUS connector in the segment: rack-mount, field-mount, and the cable transition at the HMI. For each, confirm the termination switch is OFF for any device in the middle of the segment and ON at exactly the two end devices.
4. Verify the connector part number. Recommended Siemens types: 6ES7972-0BA12-0XA0 (35° cable outlet, with PG socket), 6ES7972-0BB12-0XA0 (90°), or 6GK1500-0FC10 (for 6XV1 830 cable).
5. Confirm wiring of the violet cable:
     - Pin 3: conductor B (red insulation stripe)
     - Pin 8: conductor A (green insulation stripe)
     - Pin 5: data ground (drain wire and bare shield braid, only one continuous connection to ground)
     - Pin 6: VP (5 V terminated power, only used by terminating resistors; do NOT connect to anything else)
     - Shield: clamped under the connector strain relief, bonded to the metallic D-sub housing.
6. Use a multimeter to measure between pin 3 and pin 8 with the segment powered and master in STOP. A healthy terminated end reads ~110 Ω (220 Ω in parallel with 220 Ω across the segment) or ~220 Ω at an unterminated end. A reading of 0 Ω indicates a short between A and B; an open reading (>10 kΩ) at both ends indicates no termination is enabled.
7. Inspect the cable for kinking, crushed sections, or excessive twist. PROFIBUS cable has a minimum bend radius of 10× outer diameter (≈75 mm for the standard violet cable). Replace any section that has been deformed.
8. Re-seat every connector, including the one at the HMI. Confirm the locking screws are tightened; a loose D-sub will intermittently break the bus and trigger BUS1F.
9. Switch the CPU back to RUN. The diagnostic buffer should now show the slaves returning ("Station return"). BUS1F and EXTF should clear within one DP cycle (typically 5–50 ms at 1.5 Mbit/s).
10. On the HMI PC, restart WinCC Runtime. Verify that the tank level tags now show live values and that the WinCC driver status reads "OK" or "Connected" for the SIMATIC S7 PROFIBUS channel.

Verification Tests

After the recovery steps, perform three checks before declaring the bus healthy:

1. Oscilloscope eye-pattern check. Probe across pins 3 and 8 at the master end with the segment powered and DP cycling. The differential eye must be at least 1.0 V peak-to-peak and free of ringing. Anything below 0.8 V indicates a termination problem.
2. Long-run diagnostic soak. Leave the system in RUN for 24 hours with WinCC trending active. Open the CPU diagnostic buffer and confirm zero new "Station failure" or "Bus fault" entries.
3. Cable redundancy test (if applicable). If the vessel has a redundant PROFIBUS ring, force a switchover to the standby fiber/copper pair and confirm that WinCC tags remain live and EXTF/BUS1F stay off.

Cable Handling and Connector Re-Termination Rules

• Always strip the violet cable jacket in a single cut and fold the braided shield back over the jacket before inserting into the D-sub housing. Do not twist the shield into a pigtail.
• Maintain at least 200 mm of separation between PROFIBUS and 230/400 V power cables. Cross at 90° if they must intersect. See SIMATIC NET PROFIBUS Network Manual, Edition 2008, Section 4.3 for installation guidelines.
• Termination must be ON at exactly two physical ends. If you add a diagnostic repeater or a fiber-optic converter at the end of the segment, the terminator at the far end of the segment is now inside that device, not at the last slave.
• Never leave a stub or drop cable connected to the bus without the device at its end being powered. An unpowered ET 200 is a stub antenna and will reflect the signal.
• When you replace the HMI PC, the PROFIBUS connector's termination switch is often the part that gets toggled by mistake. Mark the ON position with paint or a label before installation.

PC and HMI Replacement Precautions

Replacing an HMI PC in a running plant carries specific risks for PROFIBUS:

1. Power down the WinCC PC and unplug its PROFIBUS connector before moving the PC, even if you only plan to swap the monitor.
2. If the replacement PC uses the same PROFIBUS CP (e.g., CP 5621 on PCIe), verify the driver is installed and the PG/PC interface in Set PG/PC Interface is set to S7ONLINE (STEP7) -> CP5621 (PROFIBUS) before reconnecting the cable.
3. Confirm that WinCC's channel unit "SIMATIC S7 PROTOCOL SUITE → PROFIBUS" has the correct MPI/DP address (typically 1 for the HMI) and that this address does not conflict with the CPU's PROFIBUS address.
4. Take the S7 CPU to STOP before the cable swap, then back to RUN only after WinCC has been started and the connection status is verified. This avoids the BUS1F latching that triggers EXTF.

Field-Commissioning Checklist

• [ ] CPU in STOP, DP slaves powered, segment termination verified at both ends (110 Ω between pin 3 and pin 8).
• [ ] All shields bonded at the connector backshells; ground loops avoided.

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