Siemens 828D Alarms 231885/25201: Drive LED Diagnostics Guide

Problem Overview

On a SINUMERIK 828D-controlled vertical machining center (VMC), the HMI displayed a stack of three simultaneous axis alarms on machine power-up:

• 231885 Axis SP1 Servo_3.3:3(3): Encoder 1 Drive-CLiQ(CU): Cyclic Data Transfer error. Component Number: 20, fault cause 33
• 207016 Axis SP1 Servo_3.3:3(3): Drive Motor Temperature sensor fault. 0.
• 25201 Axis SP1 drive fault

The most diagnostic feature visible to the maintenance technician is the LED pattern on the SINAMICS S120 drive modules. Both the RDY (ready) and DC Link LEDs were OFF on every drive in the line-up, despite the DC link being measured at approximately 557 VDC and the 24 V auxiliary rail being correct at the top of the line-up. The alarm repeated on power-cycle, ruling out a transient handshake problem.

This pattern - DC link charged but drive electronics dark - is the signature of a failed Drive-CLiQ handshake between the SINUMERIK 828D's NCU and the SINAMICS S120 modules. A single defective encoder on the bus is sufficient to drag down the entire topology.

Affected Hardware and Drive Topology

The 828D is delivered in two main variants:

• PPU 24x (panel processing unit, e.g., 828D PPU 240.3 / 241.3 / 260.3 / 261.3) - vertical HMI form factor
• NCU 730.3 (numerical control unit, e.g., 828D NCU 730.3 PN / 731.3 PN / 732.3 PN) - separate NC box

Regardless of variant, the motion control side is identical: the NCU/PPU communicates to a SINAMICS S120 line-up over Drive-CLiQ, a real-time serial protocol running on standard RJ45 patch cables (CAT5e or higher) at 100 Mbit/s. The typical axis topology is:

• Active Line Module (ALM) - rectifier + active front-end, occupies DRIVE-CLiQ ports X100–X105
• Single Motor Modules (SMM) or Double Motor Modules (DMM) - one or two axes per module
• Sensor Modules (SMC20 / SMC30 / SME20 / SME25 / SME120) - one per non-Drive-CLiQ encoder
• Direct Drive-CLiQ encoders - integrated into Siemens 1FK7 / 1FT7 / 1PH8 motors with the EnDat 2.2 DRIVE-CLiQ interface

Each encoder is treated as an independent component in the topology. Component Number 20 reported in alarm 231885 corresponds to the position of that encoder in the configured DRIVE-CLiQ topology. Because Drive-CLiQ is a daisy-chained bus, a single short or noise source propagates to every downstream node and can disable every drive module's RDY state.

Alarm Code Reference

The three alarms are causally linked: the failed cyclic data transfer (231885) is reported by the drive electronics, which then pulls the drive-ready line. Because the motor temperature sensor is wired through or near the encoder connector on most Siemens servo motors, a damaged encoder cable can also be the physical cause of the 207016 alarm. The 25201 alarm is the NC-level consequence of the underlying drive hardware fault.

SINAMICS S120 LED State Reference

The RDY and DC Link LEDs on the front of every SINAMICS S120 Motor Module, Active Line Module, and Sensor Module provide the fastest possible diagnostic. Read them before any HMI diagnostics begin.

RDY LED States

DC Link LED States

The combination observed in the field - 557 VDC physically present but the DC Link LED dark on every module - is a strong indicator that the LED driver itself is unpowered because the module's 24 V logic supply is missing or because the internal electronics are held in reset by a DRIVE-CLiQ fault.

DC Link Voltage Analysis

A measured 557 VDC is consistent with the SINAMICS S120 active line module under partial load. The expected DC link voltage range for a 400 V class (380–480 V AC three-phase) supply is:

A reading of 557 VDC therefore confirms:

• The line filter, line reactor, and Active Line Module IGBTs are functional
• Pre-charge circuit completed successfully
• The DC link capacitors on every Motor Module are charged

The DC link side of the drive line-up is healthy. The fault lies entirely in the 24 V / DRIVE-CLiQ control side, which is consistent with the alarm stack observed.

Root Cause Analysis

Three independent observations converge on the same root cause:

1. Alarm 231885 fault cause 33 at component 20 points at a cyclic DRIVE-CLiQ integrity failure on the encoder reporting that component number.
2. Alarm 207016 on the same axis indicates the motor temperature sensor signal is also compromised - on Siemens servo motors, the KTY/PTC sensor is bundled into the same encoder cable harness.
3. All RDY and DC Link LEDs off across the line-up while the DC link is charged indicates the DRIVE-CLiQ bus is being held down by a faulted node. Because the bus is daisy-chained, every downstream module loses the ability to communicate with the CU and never advances to "ready".

The most likely physical root cause is one of:

• A mechanically damaged encoder cable (chafed insulation, broken shield, kinked at the drag-chain radius)
• Oxidized or contaminated pins on the DRIVE-CLiQ RJ45 connector at the encoder end
• A failed encoder subassembly on the motor itself
• A failed Sensor Module (SMC20 / SMC30 / SME) when the encoder is external to the motor

Field experience on 828D / S120 lines confirms that intermittent trips that clear on power-cycle but recur after a few operating days are typical of marginal cable damage: thermal cycling causes the conductors to make and break contact at the damaged point, eventually failing fully.

Step-by-Step Diagnostic Procedure

1. Lock out / tag out the main disconnect. Wait at least 5 minutes after opening the disconnect for the DC link to bleed down below 50 V (the SINAMICS S120 internal discharge resistors are sized to meet this 5-minute rule on 400 V class supplies).
2. Verify zero energy: confirm the DC link LED on every Motor Module is OFF and measure with a CAT III 600 V meter at the DC link test points before opening any cabinet door.
3. Document the alarm stack from the HMI's Diagnostics > Alarm list screen, including the component number and fault cause for each DRIVE-CLiQ alarm.
4. Read the topology: from the HMI, navigate to Commissioning > Drive system > Topology. Compare the actual topology against the configured topology. Any module marked "missing" or "wrong type" is a candidate.
5. Inspect every DRIVE-CLiQ cable in the line-up, working from the NCU/PPU outward. Look for crushed cable jackets, missing or damaged RJ45 latches, and evidence of oil or coolant ingress.
6. Re-seat each connector after a visual check. Power back up and observe whether the RDY LEDs come on steady green and whether the alarms clear.
7. If alarms persist on a single axis: identify which encoder reports component number 20 in the topology view. That is the encoder to suspect first.
8. Measure the encoder cable shield: with the cable disconnected at both ends, verify shield continuity from one end to the other and isolation from each signal conductor. Open shield = immediate replacement.
9. Substitute with a known-good cable if spares are available. Clearing of alarm 231885 with a substituted cable confirms cable damage.
10. If the encoder itself is the suspect: it can be temporarily deactivated in the parameter view to confirm it as the bus-holding component. See the next section.

Encoder Cable and Connector Inspection

For Siemens 1FK7 / 1FT7 / 1PH8 motors with integrated DRIVE-CLiQ encoders, the cable carries:

• 8 DRIVE-CLiQ data pairs + power pair on a single RJ45
• 8-conductor temperature sensor bundle in the same cable jacket
• A braided shield that must be 360-degree bonded at both connectors via the connector backshell

Common failure points in field installations:

Siemens pre-fabricated DRIVE-CLiQ cables (e.g., 6FX2002-1DC00-xxxx) are the recommended replacement. Field-built cables must use CAT5e or higher twisted pairs, a foil + braid shield bonded 360° at both ends, and the Siemens-compatible RJ45 plugs with metal backshells.

Encoder Deactivation via Parameters (Field Recovery)

To restore machine operation while the encoder hardware is being repaired, the encoder can be parked out of the configuration. This is the procedure that successfully recovered the machine in the original incident:

Procedure:

1. On the 828D HMI, navigate to Commissioning > Drives > Drive objects.
2. Select the affected axis (Servo_3 in the example).
3. Open the parameter list, search for p0142.
4. Identify the encoder index reporting component 20 (typically index 0 or 1).
5. Set that index to 0 and write to the drive.
6. Power-cycle the machine to re-build the topology.

Encoder Replacement Procedure

1. Identify the exact motor / encoder part number from the motor nameplate. Common 828D-compatible encoders include 1FK7 with AM24DQI, 1FT7 with AM22DQI, and 1PH8 spindle motors with built-in DRIVE-CLiQ.
2. Order the matching Siemens replacement encoder assembly. For motors with the encoder integrated into the rear housing, the entire rear cover / encoder block is replaced; for motors with a separate EnDat module, only the module is replaced.
3. Power down and lock out as per Step 1 of the diagnostic procedure.
4. Remove the motor terminal box cover. Disconnect the encoder connector and the temperature sensor connector.
5. Remove the encoder retaining ring or screws (depending on motor family).
6. Withdraw the encoder straight out along the motor axis. Do not pry.
7. Install the new encoder in the reverse order. Apply the locking compound to the retaining screws if specified.
8. Reconnect the encoder cable and the temperature sensor cable. Torque the connector backshell to the manufacturer value (typically 0.6–0.8 N·m for M12 connectors).
9. Restore p0142 to 1 for the replaced encoder index.
10. Run an automatic commissioning from the HMI: Commissioning > Drives > Automatic drive configuration. This re-runs the encoder identification and writes the commutation offset back into p0431.
11. Verify the encoder identification result against the motor nameplate data and accept the values.

Verification and Commissioning

1. Power on the machine. Observe the SINAMICS S120 modules: every RDY LED should reach steady green within ~5 s, and every DC Link LED should be on.
2. Confirm the HMI alarm list is clear of 231885, 207016, and 25201.
3. Navigate to Diagnostics > Drive state and verify that all axes report Ready / Follow-up.
4. From Diagnostics > Service Overview > Encoder, check the actual commutation offset in p0431 and the encoder signal quality counters in r0459. The cyclic counter r0459[0..n] should remain at zero for a healthy encoder.
5. Run each axis through its full travel in jog mode, monitoring the following-error r0061 and the position-tracking-error r0479. Both should stay below the configured limits (typically p2546 for position tolerance).
6. Run a circularity test (e.g., ball-bar or laser interferometer) on each axis. Circularity error below 0.020 mm confirms the encoder is reading correctly.
7. Run a representative part program for at least one full shift before releasing the machine back to production.

Preventive Maintenance Checklist

• Inspect DRIVE-CLiQ cables every 2,000 operating hours or 6 months, whichever comes first. Pay particular attention to drag-chain bend radii and any cable run that crosses coolant or chip return paths.
• Verify RJ45 connector backshells are torqued and latches intact. Replace any connector where the latch is broken.
• On spindle motors, check the encoder cable for evidence of oil wicking up the cable from the rear of the motor - early indication that the motor seal is failing.
• Read r0459 cyclic CRC counters at each quarterly PM. Any non-zero value trending upward indicates a marginal encoder cable that should be replaced preventively.
• Keep at least one spare DRIVE-CLiQ cable per axis type in the maintenance crib. Field recovery via cable swap is faster than waiting for an encoder delivery.
• Verify the cabinet cooling fans and filters are clean; heat-soaked drive electronics contribute to encoder failure on servo motors mounted near heat sources.
• After any machine crash or collision event, perform a full cable inspection - encoder cables are the most likely collateral damage from an axis impact.