Troubleshooting Simotion D435 Fault 7841 Infeed OFF1/OFF2/OFF3

1. Problem Statement: Simotion D435 Fault 7841 During Roller Operation

A SIMOTION D435 controller (MLFB 6AU1 435-0AA00-0AA1, hardware release G) running SIMOTION Scout reports fault 7841 and brings the entire machine to a controlled standstill. The fault appears only during the rolling station cycle, where two hydraulic rollers shape a hot component gripped by a manipulator. Roller rotation and the manipulator's reverse motion are released simultaneously. Cycles that bypass the roller station never trigger the fault.

The diagnostic pattern is consistent: fault 7841 is a controller-level follower fault that mirrors a SINAMICS S120 infeed shutdown. Clearing the infeed alarm and acknowledging the technology objects restores operation, but the fault returns on the next roller cycle, sometimes minutes later, sometimes hours.

1.1 Observable symptoms

• Fault 7841 is logged in the D435 diagnostic buffer; the time stamp coincides with the roller-engage transition.
• All axes (manipulator + roller drives) drop out at the same instant, indicating a global stop rather than a single-axis trip.
• The Smart Line Module (6SL3130-6TE23-6AA3) reports the line contactor opening, DC-link undervoltage, or "infeed not ready" depending on the active alarm mask.
• The fault is intermittent. Identical recipes can run cleanly for hours before tripping, which points to a stochastic trigger - typically a transient on a control word bit.
• Operation is restored only after the operator clears the D435 fault and re-acknowledges the infeed via S120 commissioning or the HMI button.

1.2 Why this fault is misleading

Fault 7841 surfaces on the controller, but the cause lives in the SINAMICS drive line-up. The D435 simply propagates the infeed "not ready" state to all configured axes, which forces an emergency stop on every Technology Object. Engineers chasing the fault on the Simotion side waste time on axis configuration, follow-up error masks, and TO programming when the actual root cause is a one-bit drop in the PROFIBUS control word.

2. Affected Hardware Stack

The Smart Line Module feeds a 600 V DC link that supplies the four motor modules. If the SLM drops out, the DC link collapses within tens of milliseconds and every connected motor module reports "DC link undervoltage" - which the D435 then translates into a global technology stop and the visible 7841.

3. Reading Fault 7841 - The Correct Diagnostic Path

Begin at the controller, but resolve at the drive. Follow this sequence:

1. Open the D435 online diagnostics in SIMOTION Scout (Project > Accessible nodes > D435 > Diagnostics). Note the time stamp of fault 7841 and the listed follow-up faults.
2. Switch to the SINAMICS S120 commissioning view (Start > SINAMICS > Commissioning). Open the alarm history of the active CU320-2 / Control Unit hosting the SLM.
3. Correlate the S120 alarm time stamp with the D435 7841 entry. The S120 alarm that appears first is the root cause; anything on the D435 is downstream.
4. Inspect the control word (STW1) of the infeed at the moment of the trip. In the S120 expert list, navigate to r2090.0 and the bit-field r2090.0 : B0:1F1 (PROFIBUS PZD1 receive, bit serial). Compare the bits to the standard telegram 1 assignment.

4. Root Cause - OFF1 / OFF2 / OFF3 Drop via Telegram 1

The default Scout configuration for a Smart Line Module wires the enable signals to the bits of telegram 1 (STW1) received over PROFIBUS. Telegram 1 contains a 16-bit control word where the following bits control the line contactor and pulse enable:

The standard Siemens advice for an SLM is:

• OFF1 (bit 0) should be a controlled 1→0 transition only on a real shutdown, not during normal cycle interlocks.
• OFF2 (bit 1) should remain continuously at 1 during normal operation. It is intended for an immediate pulse disable (category 0 stop).
• OFF3 (bit 2) is the quick-stop input. It should be tied to the safety circuit / E-stop hardware and not toggled by the application program.

On the affected machine, the controller logic was driving these three bits directly. During the roller-engage sequence, a transient or program race condition allowed one of the three bits to drop to 0. The SLM interpreted this as a stop command, opened the line contactor, and the DC link collapsed. The D435 then reported 7841 because the infeed was no longer ready.

5. Why the Roller-Only Correlation

The fault pattern correlates with the roller station for two reasons:

1. The roller station is the only part of the cycle that demands simultaneous torque from multiple motor modules and the SLM. Any weakness in the DC link (recoverable from the line) is exposed by a high inrush event.
2. The manipulator's reverse-motion trigger is timed with the roller-engage command in the program. The bit toggles for both events occur in the same SCOUT code block, increasing the chance that an OFF bit is unintentionally cleared when a different event is being signaled.

Cycles that do not run the rollers do not exercise this combined transition, and the fault does not appear.

6. The Enable Pulses (EP) Hardwired Signal - The Foundation

Every SINAMICS S120 book-size line and motor module has a hardware terminal on the Control Unit (typically EP / Enable Pulses, X122 on the CU320-2, terminals 3 and 4, or the equivalent on the SLM itself). This terminal must be at +24 V for the drive to release pulses, regardless of what the fieldbus says. The PROFIBUS bits are a software enable layered on top of a hardware enable.

If EP is wired to a digital output of the D435 that can glitch, the line module will also glitch. Check the wiring of EP before changing any parameter.

7. Solution Strategy - Keep the Line Module Powered Throughout the Cycle

The fundamental fix is to decouple the SLM enable from the application control word. The S120 has its own internal safety path; OFF1/2/3 on the line module are meant to be used for shutdown, not for per-cycle logic. The motor modules carry the per-axis safety; the line module should stay up as long as the machine is energized and EP is high.

7.1 Why this is permissible

The S120 motor modules and integrated safety functions (STO, SS1, SLS, etc., where licensed) are designed to remove torque from the motors independently of the line module state. The motor modules can execute OFF1/2/3 ramps and disable pulses while the SLM remains closed and the DC link stays charged. As long as the safety category permits, you do not need to open the line contactor for routine stops or zone interlocks.

7.2 The target behavior

• SLM stays enabled for the full operational envelope of the machine, from main power on to main power off.
• OFF2 on the SLM is held at 1 by a constant 1-signal, not by program logic.
• OFF3 on the SLM is wired to the hardware E-stop chain (via EP or a hardwired safety relay), not to a software bit.
• OFF1 on the SLM can be a 1-signal sourced from the D435 only if the application truly needs to open the line contactor; otherwise it is a constant 1.
• Motor modules continue to receive and act on their own OFF1/2/3 bits and TO commands; this is where the per-axis safety is enforced.

8. Procedure: Forcing the SLM Enable to a Constant "1" via the Expert List

When the SLM is initially configured, Scout asks whether the OFF1/2/3 signals should come from the PROFIBUS telegram or from a fixed "1". The simplest path is to redo the supply configuration with the "constant 1" option. When that is not possible on a running machine, change the parameters manually in the S120 expert list.

1. Open SIMOTION Scout online and connect to the D435.
2. Switch to the S120 commissioning view of the CU320-2 that hosts the SLM.
3. Open the Expert list (Commissioning > Expert list).
4. Locate the source selector for the SLM control word. The relevant parameters are on the line-side Control Unit; in modern S120 firmware they are p0840 (BI: ON/OFF1 source), p0844 (BI: 1. OFF2 source), p0845 (BI: 1. OFF3 source), and p0852 (BI: Enable operation / pulse enable source). Older firmware revisions may use p840.x and the p800-series block.
5. For each of p0840, p0844, p0845, and p0852, disconnect the PROFIBUS bit source and rewire to a constant 1. In SINAMICS parameter terms, a constant high is typically BICO-sourced from the fixed signal "1" (source index r2090.15 of the receiving DO or the equivalent p1 fixed signal in the BICO source list).
6. Confirm the change is applied to the drive object of the line module, not to a motor module. The BICO source is per DO.
7. Save the parameters to the CF card of the CU320-2 (Copy RAM to ROM).
8. Cycle power to the line section and verify the SLM powers up and stays up without any PROFIBUS bit activity.

9. Alternative Path: Hardwired Digital Input for OFF1/2/3

If the application logic truly needs to command the SLM (for example, a controlled energy-saving mode during long idle periods), use a hardwired digital input on the SLM terminal block or on the CU320-2, not a PROFIBUS bit. This isolates the SLM enable from any fieldbus transient.

1. Allocate one 24 V output on the D435 (or a relay) dedicated to "SLM enable".
2. Wire that output to the S120 terminal reserved for the OFF1 / OFF2 / OFF3 input (X122 or X132, depending on the CU variant).
3. Set p0840 / p0844 / p0845 to the digital-input BICO source (e.g. r722.x for CUD digital inputs).
4. Apply the following sequencing convention: OFF2 first (constant 1), then OFF3, then OFF1 after a short delay (typically > 50 ms). This sequencing mirrors the drive's internal state machine and avoids edge-triggered faults.
5. Pull the wire down (0 V) only for a genuine, hardwired shutdown of the entire machine, not for cycle events.

9.1 Sequencing convention (OFF2 → OFF3 → OFF1)

The Siemens commissioning manual recommends the following order for a controlled release of the SLM:

1. OFF2 = 1 must be present first. Without OFF2 = 1, the SLM stays in pulse-disable.
2. OFF3 = 1 is applied next. Some S120 firmware versions require OFF3 to be present before OFF1 is accepted.
3. OFF1 = 1 is applied last, with a delay (commonly 50-100 ms) to allow the SLM to lock the internal state machine to "run".

This sequence is also the order for a controlled shutdown, reversed: OFF1 → 0 first (ramp stop), wait for ramp to complete (n_act < n_min via p1226 / p1227), then OFF3 = 0, then OFF2 = 0.

10. PROFIBUS Noise / Telegram Stability Check

A second-order cause to investigate is PROFIBUS signal integrity. A telegrams-from-D435-rolling-station link that glitches during a high-inrush event (rollers engaging) suggests that:

• The PROFIBUS cable shielding is not properly bonded at one end, or the shield is broken at a connector.
• The bus terminator is missing on one or more segments, or two terminators are present (only the two physical ends of a segment should be terminated).
• Bus connectors on the motor modules are not the diagnostic repeater type, or the repeater is daisy-chained without respecting the segment length budget.
• EMC from the hydraulic valve block / VFD-driven pumps is coupling onto the bus cable, especially if the bus shares a tray with motor power cables.

10.1 Practical steps

1. Open the SCOUT online diagnostics for the PROFIBUS nodes and inspect the bus statistics. Look for "telegrams lost", "telegrams faulty", or "address check failed" counters. A non-zero counter during the fault window is direct evidence of a physical-layer problem.
2. Re-terminate the bus with the correct 220 Ω + 390 Ω termination network at both segment ends. Disconnect any inline connectors that are not at the physical end of a segment.
3. Verify shield bonding. The shield should be 360° bonded at every connector backshell and grounded at one end of the segment (typically the D435 end) with a low-impedance connection to the cabinet ground bar.
4. Physically separate the PROFIBUS cable from any VFD output cables by at least 200 mm, and avoid parallel runs longer than 1 m. Cross at 90° if the cables must intersect.
5. Add a PROFIBUS repeater or a fiber-optic link between the D435 and the S120 line-up if the run exceeds 50 m or runs through a high-EMC area (welding transformers, large VFDs, induction heaters).

11. Parameter Cross-Reference for the SLM

12. Verification After the Fix

Run the following checks in order, logging every result in the commissioning report:

1. Single-shot SLM enable: with the D435 in "stop" and the PROFIBUS disconnected from the SLM, the SLM should still power up. If it does, the BICO fix is correct.
2. PROFIBUS disconnect test: physically unplug the PROFIBUS connector at the SLM during a cycle. Fault 7841 should not appear. If it does, the OFF bits are still sourced from the telegram on at least one parameter.
3. E-stop test: trigger the hardwired E-stop (which drops EP, the dedicated safety input, or a hardwired OFF3). The SLM must drop, the motor modules must drop, the machine must reach a safe state. This is the safety regression test.
4. Roller cycle endurance: run 200 consecutive roller-engage cycles and confirm zero 7841 events. Capture the PROFIBUS bus statistics for the run window.
5. Diagnostic buffer review: the D435 diagnostic buffer should show no 7841 entries for the test window. Any S120 alarms (F07841, F30003, F07900 family) on the infeed DO must be investigated separately.

13. Troubleshooting Matrix

14. Field-Proven Caveats

• Fault 7841 on the D435 is a follower fault. Clearing it without fixing the S120 side will only buy one cycle. Always resolve the infeed alarm first.
• Some S120 firmware versions (4.x and earlier) require the OFF bits to be applied in a strict sequence on the line module; check the firmware release notes for the exact CU320-2 firmware loaded in your machine before relying on a constant-1 source.
• When the SLM enable is moved to a constant 1, the line contactor will close immediately on precharge complete. There is no per-cycle "soft" shutdown of the SLM anymore. If your process depends on the DC link collapsing between parts, redesign the cycle to use the motor modules' own disable path.
• Never disable the safety function of OFF3 on the SLM if the risk assessment assigns a stop category to the SLM. OFF3 is a hardwired stop in the functional sense, even if the wire is a PROFIBUS bit.
• After a BICO change, run an acceptance test with the safety officer present. Document the new wiring and BICO mapping in the machine file.

15. Related Standards and Documentation

Refer to the official Siemens Industry Online Support for the device manuals applicable to your firmware version:

• SIMOTION D4x5 Commissioning and Configuration Manual (covers D435, D445, D455) - Siemens Industry Online Support.
• SINAMICS S120 / S150 List Manual (parameter list for p0840, p0844, p0845, p0852, p8641 and the entire S120 parameter set) - Siemens Industry Online Support.
• SINAMICS S120 Function Manual "Drive Functions" (OFF1/OFF2/OFF3 state machine and BICO interconnection rules) - Siemens Industry Online Support.
• SIMOTION SCOUT Configuration Manual (telegram 1 mapping and TO-axis-to-DO wiring) - Siemens Industry Online Support.
• PROFIBUS Installation Guideline (Pi, bus termination, shielding, segment length) - PROFIBUS & PROFINET International (PI).

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