Resolving SINAMICS S110 'Switch-On Disable Is Activated' Error

1. Problem Overview

A single-axis SINAMICS S110 drive controlled by a CU305 Control Unit with a PM340 Power Module and a DRIVE-CLiQ feedback-equipped servo motor will refuse to start from the BOP (Basic Operator Panel) or AOP30 (Advanced Operator Panel). Pressing the green ON button on the connected control panel produces the message "Switch-on disable is activated", while parameter r0002 (or the Status Word 2 mirror) reports Infeed de-activated/not operational and the ON/OFF1 LED remains gray rather than green or yellow.

This symptom block is one of the most frequently reported startup issues on S110 hardware revisions with firmware V4.x and V5.x. Although the message looks generic, it is a deterministic state inside the SINAMICS operating-state machine and is always the result of a downstream condition that must be cleared before STW1.0 (ON/OFF1) is accepted by the drive. In the case captured in field experience, the root cause was a physically defective DRIVE-CLiQ interface on the motor's encoder connector — a fault that is invisible until the drive runs a topology scan.

2. SINAMICS S110 System Architecture

Understanding the S110 requires recognizing that it is a single-axis derivative of the modular SINAMICS S120 family. The relevant hardware stack in the field-reported configuration is:

The CU305 does not include a separately addressable Line Module; the PM340's input bridge provides the DC bus directly from the 3-phase line supply. "Infeed ready" therefore refers to the PM340's internal pre-charge and DC-link health, not to an external Line Module (ALM, BLM, SLM). A green Ready LED on the PM340 and an unlit Fault LED are prerequisites for the drive's controller to leave the "Infeed de-activated" sub-state.

3. Switch-On Inhibit (Switch-On Disable) State Explained

SINAMICS drives implement the PROFIdrive state graph defined in PROFIdrive profile V4.2. The relevant substates and their relationship to the S110 are:

The control-panel display "Switch-on disable is activated" maps to state 7 with ZSW1.6 = 1. To leave state 7, all of the following must be true simultaneously:

• No active fault (Bit 3 = 0) — particularly no DRIVE-CLiQ topology fault.
• OFF2 inactive (Bit 4 = 0) — p0844 source must be high.
• OFF3 inactive (Bit 5 = 0) — p0845 source must be high.
• Infeed OK: DC-link pre-charge complete and r2 substate reporting the line as ready.
• Encoder/motor identification completed or valid parameter set loaded (DRIVE-CLiQ component fully online).

If any one of these conditions fails, the drive latches into state 7 regardless of the panel button input.

4. DRIVE-CLiQ Communication Fundamentals

DRIVE-CLiQ is Siemens' proprietary serial interface used to connect motors, encoders, Terminal Modules, and Sensor Modules to the Control Unit. The same cable carries 24 V power and 100 Mbit/s Ethernet communication over shielded twisted pairs with a dedicated, keyed RJ45-style connector. On the S110 with CU305, the topology is fixed and short:

• X100 / X101 / X102 — DRIVE-CLiQ ports 1, 2, 3 on the CU305. Port 1 is normally routed to the PM340's power-unit interface.
• DRIVE-CLiQ port on the motor — connects the integrated encoder (typically an EnDat 2.2 / 22-bit absolute encoder with absolute multiturn) to the CU305.

Each DRIVE-CLiQ node has a unique electronic type plate. When the CU305 powers up, it polls every port, identifies each component, and runs a CRC + version check against its expected topology. If the cable is open, shorted, or the connector electronics are damaged, the topology scan fails and the drive refuses to commission.

5. Common Causes of Switch-On Disable on S110

The following table enumerates the dominant root causes that have been observed in field service. Each row includes the diagnostic indicator and the fault/alarm code most commonly raised by the S110 firmware.

6. Diagnostic Workflow: Step-by-Step

Use this sequence before swapping any hardware. The list is ordered from cheapest, fastest checks to invasive replacements.

1. Inspect the LED stack. Check the CU305 RDY LED (steady green = OK, flashing at 2 Hz = commissioning mode, red = fault) and the PM340 READY LED. The gray ON/OFF1 LED on the panel confirms state 7.
2. Read the fault buffer. In STARTER, right-click the drive → Online → Diagnostics → Fault Buffer. Note the timestamp of the most recent F-code; F01303, F01611, F08501, or F30002 each redirect the troubleshooting path.
3. Verify the topology view. In STARTER, open Configuration → DRIVE-CLiQ Topology. A red device at the motor port is the visual confirmation of a DRIVE-CLiQ component fault.
4. Inspect cabling. Power down, unplug, and inspect each DRIVE-CLiQ cable. Check the locking tabs, the 24 V pins in the connector, and the cable jacket for damage along the drag-chain path.
5. Read r9944 — DRIVE-CLiQ error counter. A non-zero, rising value confirms a communication-quality issue. Cross-check r9945 for the offending port number.
6. Test the motor on a known-good axis. Swap the suspect motor with a verified-good unit. If the fault transfers to the swap axis, the motor's DRIVE-CLiQ electronics are defective.
7. Test the cable independently. Replace the DRIVE-CLiQ cable with a known-good 6FX2002-1DCxx lead. If the fault clears, the cable has an intermittent open or shield break.
8. Reload project to drive. In STARTER, perform Load to Target System after Copy RAM to ROM. A corrupted parameter image can hold the drive in state 7 even when hardware is healthy.

7. Reading the Fault Buffer and Status Indicators

Three CU305 LED states carry diagnostic meaning during a switch-on-inhibit condition:

The control-panel display field has its own indicators. The ON/OFF1 LED has four colors:

• Off — drive in state 7 (switch-on inhibit) — the symptom reported here.
• Green — drive in state 1 (ready to switch on).
• Yellow — drive in state 3 (operation enabled).
• Red — fault active.

To inspect the panel's r2 mirror (ZSW2), navigate Menu → Diagnostics → Status Word. A hex value of 0x60 (binary 0110 0000) indicates OFF3 active and switching-on inhibited. A hex value of 0x40 indicates pure switch-on inhibit with no OFF command active. This is the cleanest way to distinguish an OFF2/OFF3 source wiring fault from a DRIVE-CLiQ topology fault.

8. DRIVE-CLiQ Motor Interface Repair

If the diagnostic workflow isolates the fault to the motor's DRIVE-CLiQ connector or encoder, choose one of two repair strategies based on motor type:

8.1. Replaceable Encoder Option (1FK7, 1FT7)

1. Power down the drive and lock-out the line supply (5-minute capacitor discharge on PM340 ≥ 18 kW).
2. Remove the motor from the machine; support the shaft to avoid axial shock to the bearings.
3. Dismount the encoder cover and inspect the connector PCB for lifted pads, cold joints, or fractured traces around the 24 V pins (typical failure point when cable is yanked at angle).
4. Order a replacement encoder kit (e.g., 1FK7xxx-xAxx-x family with the matching order code suffix) and install per the motor's mechanical instructions.
5. Re-mount the motor, reconnect DRIVE-CLiQ, and torque the connector locking screws to spec.

8.2. Non-Replaceable Encoder Option (1PH8 induction motors)

On 1PH8 motors, the DRIVE-CLiQ interface is integrated into the encoder housing and cannot be field-repaired. The full motor assembly must be replaced. Before replacement, verify the encoder order code (p0400 / r0400) and confirm the spare motor matches the encoder type exactly — mixing EnDat 2.1 and 2.2 encoders on the same axis will re-introduce F01304.

9. Control Panel Commissioning via STARTER

The BOP/AOP control panel "Assume Control Priority" function only becomes fully un-ghosted once the drive has completed topology commissioning and a valid drive object exists. The procedure to reach that point:

1. Open STARTER (V4.x) or Startdrive (V15+) and create a new project. Select Online → Accessible Nodes to discover the CU305 on PROFINET or PROFIBUS.
2. If the project is empty, STARTER offers Upload from Target System. Use this if the drive was previously configured and you have lost the project file.
3. If the drive is in factory-fresh state, perform Restore Factory Settings (p0970 = 1) and confirm.
4. Run Configure DDS to select the line voltage, motor type, encoder type, and current limits. STARTER will filter valid combinations automatically.
5. Download the configuration: Load to Target System. The CU305 stores volatile parameters in RAM; select Copy RAM to ROM after the download to persist to the flash.
6. Open the Control Panel (icon top-right). Click Assume Control Priority; the line-module tick box should appear un-greyed for the PM340's integrated infeed. Tick it.
7. The LM box becomes active. Click it — this issues the pre-charge command and brings the DC link to rated voltage.
8. The ON/OFF1 and OFF2 buttons are now active. Press ON; the drive should transition through states 1 → 2 → 3 if no fault is latched.

If the LM box remains grey even after a successful topology scan and parameter download, the drive is internally latched on a DRIVE-CLiQ component fault that prevents it from declaring the infeed ready. This is the exact condition captured in field reports.

10. Verification Procedure

After the motor (or cable) is replaced, run this verification checklist before returning the axis to production:

1. Topology scan: In STARTER, right-click drive → Online → Topology. All DRIVE-CLiQ nodes must display green.
2. Fault buffer clean: Online → Diagnostics → Fault Buffer must show no active faults. Clear with the ACK button if any are present.
3. Control panel test: Use the AOP/BOP to issue ON. Drive should reach state 3 (yellow ON/OFF1 LED) within 200 ms of the command.
4. Direction check: Run a slow jog (p1130 limited, p1145 EN active) and confirm rotation matches p1820 direction inversion setting.
5. Encoder sign: Verify p0410[0] = 0 and the position actual value in r0480 increases in the direction of positive motor rotation.
6. Motor ID run: If the motor was replaced, run p1900 = 1 (or p1910 = 1 for standstill identification) and wait for completion.
7. Save and backup: Copy RAM to ROM, then Upload from Target System and archive the project file.

11. Preventive Maintenance Recommendations

• Cable routing: Route DRIVE-CLiQ cables in a separate trough at least 100 mm from VFD output power cables. Cross at 90° if a crossing is unavoidable.
• Connector care: Avoid pulling on the connector body; pull on the cable 25 mm behind the connector. Locking tabs must be audibly clicked into place.
• Drag-chain rules: Use only chain-rated DRIVE-CLiQ leads (6FX2002-1DCxx-xxxx with the AG or AD suffix). Observe minimum bend radius (typically 7× cable diameter).
• PM340 cooling: Verify fan operation at every scheduled service. A clogged heat sink trips F30004 on newer firmware and prevents the drive from leaving state 7.
• Firmware hygiene: Keep STARTER, CU305 firmware, and DRIVE-CLiQ component firmware on a coordinated version matrix. Mismatched firmware is a frequent cause of F01304 at next commissioning.
• Spare inventory: Carry one each of: DRIVE-CLiQ cable (3 m), encoder module (matching the line's installed motor family), and a PM340 of the matching current rating for fast swap-and-go troubleshooting.