Siemens DP/DP Coupler Not Resetting After PROFIBUS Fault

Problem Description

The Siemens DP/DP Coupler (catalog number 6ES7158-0AD02-1XA0 and successor 6ES7158-0AD03-1XA0) is a gateway used to interconnect two independent PROFIBUS DP networks. Field reports describe a recurring fault scenario in which the coupler enters a latched bus-fault state after a downstream DP slave is powered off and back on. Expected behavior is automatic recovery once the slave reappears on segment 2. Actual behavior is that both BF1 and BF2 LEDs remain solid red, with two additional status LEDs flashing red, and the coupler does not return to data exchange until the PROFIBUS connector is physically removed and reinserted on at least one side.

This latched behavior is abnormal. The DP/DP Coupler firmware is designed to clear bus faults as soon as the underlying cause disappears and at least one valid telegram has been received on the affected segment. Persistent latching typically indicates one of three underlying problems: marginal PROFIBUS physical layer conditions that the coupler is misinterpreting as a permanent failure, a corrupted internal diagnostic state that requires a power-cycle, or a hardware defect in the coupler's PROFIBUS ASIC.

DP/DP Coupler Operating Principles

The DP/DP Coupler operates as a transparent PROFIBUS DP slave on each side while internally maintaining a configured I/O image. Each DP master on either segment sees the coupler as a normal DP slave with a fixed I/O footprint that is set in the coupler's DIL switches or GSD-based configuration. The device passes cyclic I/O data between the two masters, decoupled electrically, and supports up to 244 bytes of input and 244 bytes of output per direction (configuration dependent).

Key internal components:

• PROFIBUS ASIC (SPC3 or SPC56 / VPC3+): Handles the physical layer and PROFIBUS protocol on each segment.
• Microcontroller (C165 / VPC3+ controller): Manages configuration, diagnostics, and the dual-port RAM between the two segments.
• Isolated DC/DC converter: Provides galvanic isolation between segment 1 and segment 2 supplies.
• Watchdog / Firmware supervisor: Monitors firmware execution and triggers a soft reset on severe protocol errors.

On a normal PROFIBUS segment drop, the SPC3 / VPC3+ loses the start delimiter, exits data exchange, and signals BF (Bus Fault) to the host microcontroller. Once the segment returns and the coupler receives a valid token/telegram, the ASIC exits the BF state autonomously. The microcontroller's supervisor should then clear BF1 and BF2. If firmware on the microcontroller is hung or the ASIC is latched in a watchdog state, the LEDs remain red and only a power-cycle recovers operation.

LED Status Reference

Correct interpretation of the LED pattern is the first step in diagnosis. The DP/DP Coupler faceplate exposes the following indicators:

The reported symptom of "two red LEDs solid on (both sides) and two red LEDs flashing" matches the BF1 solid + BF2 solid + SD1 flashing + SD2 flashing pattern. This combination is firmware- or ASIC-level, not a simple bus drop. It indicates the supervisor is in a continuous reset loop or that the diagnostic state machine is not advancing.

Root Cause Analysis

Four root causes account for the majority of "coupler will not auto-reset" cases observed in production environments:

1. Marginal PROFIBUS Physical Layer

A repeater or slave near the end of segment 2 that generates reflections, has a damaged connector, or carries shield currents will produce intermittent telegrams. The coupler's SPC3/VPC3+ averages telegram validity; a marginally valid stream can keep the BF latch asserted. Removing and reinserting the connector momentarily re-establishes a clean contact and clears the latch until the underlying problem returns.

2. Termination / Grounding Defect

Missing or duplicate termination on segment 2 produces signal reflections that the ASIC may interpret as a permanent collision. Improper shield bonding (daisy-chained shield wire, missing ground bus bar at the cabinet entry) introduces common-mode noise. The SIMATIC NET PROFIBUS Network Manual Appendix C.6 specifies shield bonding, equipotential bonding, and termination resistor placement; deviation from these rules is the single most common cause of stuck-bus behavior in field installations.

3. Watchdog / Firmware Hang

The DP/DP Coupler firmware (versions 2.x for 6ES7158-0AD02, 3.x for 6ES7158-0AD03) contains a state machine that should clear BF on the next valid telegram after the fault condition is removed. If the supervisor task is starved (interrupts disabled by noise on the RS-485 lines corrupting the microcontroller stack) the BF latch can remain asserted indefinitely. A clean power-cycle reloads the firmware image and clears the latched state.

4. Hardware Defect in the PROFIBUS ASIC or DC/DC

Failed isolation, leaky electrolytic capacitors in the 24 V input, or a degraded RS-485 transceiver on the segment-2 side will latch the BF LED. Replacement is the only repair; the coupler is not field-serviceable. Reference the Helmholz DP/DP Coupler Manual (158-0AD02) for an alternative replacement unit with the same electrical and functional interface.

Diagnostic Procedure

Follow this sequence before swapping hardware. Each step isolates one root cause category.

1. Capture the LED pattern with a photo, including both BF and SD indicators. Note the segment in which the slave was powered off.
2. Record 24 V supply voltage at the coupler's power terminals. Acceptable range is 20.4 V to 28.8 V. Below 19 V the DC/DC converter drops out and the firmware reinitializes; this can be mistaken for a software fault.
3. Inspect the segment-2 connector at the last DP slave. Check that the PROFIBUS connector's integrated termination resistor is ON only on the first and last device on the segment. All intermediate devices must have termination OFF.
4. Measure the shield continuity from the cabinet entry ground bar to the slave's PROFIBUS connector shield clamp. Resistance should be less than 1 ohm and there should be no series breaks in the shield path.
5. Capture a PROFIBUS trace using a PROFIBUS analyzer (e.g., Softing PROCENTEC, Siemens BT200) on segment 2. Look for repeating telegrams with CRC errors, telegram gaps greater than the configured slot time, or absence of token passing.
6. Check the master diagnostic buffer for entries containing Diagnostic interrupt, Slave not found, or Identifier byte mismatch. These confirm whether the master also sees the segment as down.
7. Verify PROFIBUS address uniqueness. Two slaves with the same address produce silent contention that the coupler reports as bus fault on one side.
8. Power-cycle the coupler (24 V off, wait 10 seconds, 24 V on). If the BF clears and returns when the affected slave is repowered, the root cause is on segment 2, not the coupler.

Diagnostic Decision Matrix

Reset Methods

Three reset levels are available. Apply them in order from least to most disruptive.

Level 1: Soft Reset via PROFIBUS

Most SIMATIC masters expose a DPV1 slot that can issue a Set Slave Diagnostic or Reset command. In STEP 7 / TIA Portal:

1. Open the device configuration of the DP/DP Coupler.
2. Right-click the device in the network view and select Online & Diagnostics.
3. Navigate to Functions > Reset to factory settings or use Reset / Restart.
4. Confirm. The coupler will re-enter data exchange on both segments if the bus is healthy.

Level 2: Power-Cycle Reset

1. Turn off the 24 V supply to the coupler at the circuit breaker or terminal strip.
2. Wait a minimum of 10 seconds to allow the DC/DC converter to fully discharge.
3. Restore 24 V. Observe the LED sequence: ON green -> BF1/BF2 solid red during startup -> both BF clear within 5-15 seconds as masters establish data exchange.

Level 3: Bus Segment Reset

If a power-cycle does not clear the BF latch, the only field-level recovery is to interrupt the bus segment long enough for the master to re-tokenize. Two implementations exist:

• Connector pull-and-replug: Remove the 9-pin D-sub PROFIBUS connector from the coupler for at least 5 seconds. The master detects the slave's absence, logs a diagnostic event, and re-integrates the slave on reinsertion.
• Repeater segment power-down: If a repeater is between the master and coupler, removing repeater power for 10 seconds produces a clean bus reset on the downstream segment without affecting the master segment.

Hardware Replacement Procedure

When the diagnostic matrix points to a coupler hardware defect, replacement follows a defined sequence. The DP/DP Coupler manual documents the standard swap procedure.

1. Note the DIL switch settings on the bottom of the failed unit. The switches select PROFIBUS address (typically 1-99), operating mode, and diagnostic behavior. Photograph the switch bank.
2. Record the configured I/O length in the STEP 7 / TIA Portal hardware catalog for the DP/DP Coupler. The slot count and module identifiers must match between both masters.
3. Power down the 24 V supply to the failed unit.
4. Remove the 9-pin D-sub connectors from both segment interfaces. Label each connector to avoid segment swap.
5. Remove the failed unit from the DIN rail.
6. Snap the replacement unit onto the DIN rail.
7. Set the DIL switches to match the original configuration.
8. Reconnect segment 1, then segment 2.
9. Apply 24 V power. Both BF LEDs should clear within 15 seconds as both masters re-establish data exchange.
10. Verify cyclic I/O is being updated in both master diagnostic views.

Termination and Grounding Best Practice

PROFIBUS DP operates at 9.6 kbit/s to 12 Mbit/s on RS-485. At 1.5 Mbit/s and above, segment length is limited to 200 m, and termination quality directly controls signal integrity.

Termination resistors (220 ohm in series with 390 ohm to +5 V on both data lines) are physically present in every PROFIBUS connector but enabled by a switch. Confirm that exactly two devices on the segment have termination ON, and that these are the devices at the two physical ends of the trunk.

Shield bonding must follow the SIMATIC NET PROFIBUS Network Manual Appendix C.6 guidance:

• Use PROFIBUS cable with foil plus braid shield (e.g., Siemens 6XV1830-0EH10 or equivalent).
• Bond the shield at both ends to a low-impedance ground bus using the connector's integrated shield clamp.
• Maintain equipotential bonding between cabinets; a potential difference above a few volts across the shield path will induce common-mode currents and cause intermittent faults.
• Keep PROFIBUS cables separated from VFD motor cables by at least 200 mm in parallel runs, or use a partitioned cable tray.

Firmware and Configuration

Verify the firmware version currently installed on the coupler. From STEP 7 (TIA Portal) or STEP 7 V5.x, open the coupler's Online & Diagnostics view and read the firmware version from the device information area. Cross-reference against the latest firmware release published on the Siemens support portal. Known issues related to bus-fault latching were addressed in firmware 2.1.4 and later for the 6ES7158-0AD02 part number.

To update firmware:

1. Download the firmware file (a .upd package) from the Siemens support portal.
2. Connect the engineering station to the segment containing the coupler via a PROFIBUS CP or USB-to-PROFIBUS adapter.
3. Open the device, navigate to Online & Diagnostics > Firmware Update.
4. Select the .upd file and trigger the update.
5. Wait for the update to complete. The coupler will perform an internal reset and return to data exchange if the bus is healthy.

Verification

After applying any of the reset methods or hardware replacement, confirm correct operation with the following checks:

1. LED state: ON green solid, BF1 and BF2 off. SD1 and SD2 off or briefly flashing during normal operation.
2. Master diagnostics: Open the master's online view (STEP 7, TIA Portal, or third-party master). The DP/DP Coupler appears as a slave with status OK on both master projects.
3. Cyclic I/O: Force a test value at one master and read it back at the other master. Confirm the propagation delay is within the expected cycle time (typically 1-10 ms per direction at 1.5 Mbit/s).
4. Diagnostic buffer: Clear the master diagnostic buffer and run the system through one full production cycle. Confirm no PROFIBUS-related entries appear.
5. Fault injection test: Power off the last segment-2 slave. The BF2 LED should illuminate on the coupler. Restore power to the slave. The BF2 LED must clear within 15 seconds without operator intervention. This validates auto-recovery.

Preventive Measures

Stuck-bus conditions on the DP/DP Coupler are almost always a symptom of segment-level physical layer issues. A preventive program reduces incident frequency and shortens recovery time.

• Schedule annual PROFIBUS physical layer inspections including shield continuity, termination resistance, and connector wear.
• Install a PROFIBUS diagnostic repeater (e.g., Siemens 6GK1500-0AA10 with diagnostics) to continuously monitor segment health.
• Maintain a spare DP/DP Coupler with pre-configured DIL switch settings and pre-loaded GSD on the engineering station for fast swap-out.
• Track all coupler-related master diagnostic events. Two or more BF events per quarter on the same coupler indicates an emerging physical layer or hardware problem.

FAQ