Resolving Siemens 802D sl Error 26002: Encoder Parameterization

Siemens SINUMERIK 802D sl Error 26002 is a drive-side alarm raised when the controller detects that the configured number of encoder marks does not match the connected feedback device. On the 802D sl platform, the alarm commonly surfaces on the Z axis when a motor-mounted incremental encoder is present but the system has been parameterized for an external encoder on the SMC30 module, and on the spindle when MD31020 (ENC_RESOL) disagrees with the physical resolution of the spindle encoder. The official alarm description for 26002 is "Axis %1 encoder %2 parameterization error" and is published in the SINUMERIK 802D sl Diagnostics Manual (06/2009 edition). The remainder of this reference covers the alarm structure, the encoder identification workflow, the SMC30 topology, and a field-proven procedure for clearing the fault without disturbing a running machine.

1. Error 26002 Alarm Format and Channel Behavior

The 802D sl controller raises 26002 as a channel alarm with a Clear criterion: Channel not ready. The alarm text expands to identify the affected axis and the encoder number being parameterized. The two typical field strings are:

• 26002 Axis Z encoder 1 parameterization error — most often raised when the Z axis drive has been configured to look at the second encoder (encoder 2, the SMC30) but the SMC30 is either unpopulated, mis-wired, or its MD is inconsistent with the physical device.
• 26002 Axis SP encoder 1 parameterization error — the spindle's first encoder (typically the motor-integrated encoder) has a resolution value stored in MD31020 that does not agree with the drive parameterization in the SINAMICS commissioning tool.

When 26002 is active, the affected axis is not released to the position controller. The Channel not ready clear criterion means that the controller suppresses program start (NC-Start is blocked) until the alarm is acknowledged and the underlying mismatch is corrected. Per the Siemens support note on 26002, if the encoder is an absolute type, the controller also expects that any pending drive alarms that could indicate encoder problems (e.g. lost commutation, low battery, EnDat protocol warnings) be evaluated before the parameterization is changed.

2. Root Cause Analysis

Error 26002 is fundamentally a configuration-consistency fault, not a hardware failure. The NCK side (SINUMERIK) and the drive side (SINAMICS S120 / CU320-2 DP on the 802D sl) each maintain an image of the connected feedback device. If the two images disagree, the drive raises the alarm and the NC surfaces it as 26002. The four most common root causes observed in the field are:

1. MD31020 (ENC_RESOL) mismatch. The number of encoder marks per revolution stored in MD31020 does not match the value configured in the SINAMICS drive parameter p0408 or p0421. In the case described in the source, MD31020 = 2048 for the Z axis and 1024 for the spindle, but the motor nameplate or the SMC30 module reports a different value.
2. Wrong encoder selected in SINAMICS commissioning (IBN). During drive commissioning, the engineer selected a default encoder type (e.g. resolver, EnDat 2.1, SSI) without cross-checking the actual motor encoder fitted to the machine. The electrical drawing typically documents the SMC30 presence but not the resolution of the motor-side encoder.
3. Misidentification of motor vs. external encoder. On the 802D sl Z axis, the motor-integrated encoder is normally encoder 1 on the drive. The SMC30 is intended for a second, external measuring system. If the controller has been parameterized as if an external encoder is present on the SMC30 (encoder 2) but no such device is wired, both encoder images become invalid.
4. Pending absolute-encoder faults. For absolute encoders (EnDat 2.2, DRIVE-CLiQ, SSI absolute), a low backup battery, a corrupted absolute position, or a broken commutation track will also cause 26002. The Siemens support article explicitly notes that drive alarms indicating encoder problems should be evaluated for absolute encoders before changing parameterization.

3. Machine Data MD31020 (ENC_RESOL) Reference

MD31020 is the NCK-side display machine data ENC_RESOL — Encoder marks per revolution. The valid index and the value entered here must match the resolution of the physical encoder that the drive reports back. Common values encountered on 802D sl machines are listed below.

On the 802D sl, MD31020 is indexed per encoder. The axis-specific addressing follows the convention MD31020[AXn, ENCidx] where AXn is the logical axis (X, Y, Z, SP, A4, A5) and ENCidx is the encoder index (1 = motor encoder, 2 = external encoder on SMC30). The source thread shows the Z axis running MD31020[AX3,1] = 2048 and the spindle MD31020[AX4,1] = 1024, which is a normal configuration for an 802D sl with a 2048-line Heidenhain ERN-type motor encoder on the Z feed motor and a 1024-line HIPERFACE or SSI encoder on the spindle.

4. Identifying the Encoder Type and Resolution

When the electrical drawing is silent on the motor encoder, use the following diagnostic order. Each step produces a deterministic answer before any parameter is changed.

1. Read the motor nameplate. The 1FK / 1FT / 2SP1 / PH / SH motor family fitted to 802D sl machines always has a part number block that decodes the encoder. For Siemens motors, the 11th–14th digit of the MLFB encodes the encoder type (e.g. 1FK7060-2AF71-1AB1 = EnDat 2048 1Vpp). For non-Siemens motors, look for a Heidenhain / Sick / Hengstler sticker on the encoder housing.
2. Read p0407 / p0408 / p0421 from the drive. In the SINAMICS commissioning tool (STARTER or the integrated Sinamics IBN on the HMI), connect to the drive and read p0407 (Encoder type), p0408 (Encoder resolution in lines/rev), and p0421 (Absolute encoder resolution). These are the ground-truth values that the drive has auto-detected from the encoder.
3. Cross-check the SMC30 module. If encoder 2 is in use, navigate to the topology view in the Sinamics IBN and confirm the SMC30 is online (green) and that the encoder at its X521/X522 terminal is the one the drawing specifies. SMC30 supports incremental 1Vpp, HTL, and SSI; an HTL device on an X521 input configured for 1Vpp will produce 26002.
4. Check drive-side alarm history. 26002 is often a secondary symptom. Look for F3xxx encoder faults (e.g. 30001, 30002, 30003, 30101, 30200) in the SINAMICS diagnostic buffer. If any of these are present, resolve them first — 26002 will clear once the underlying drive alarm is acknowledged and the encoder is healthy.

5. SMC30 Module Configuration on the 802D sl

The SMC30 (Sensor Module Cabinet-Mounted, 6SL3055-0AA00-5CA2) is the second-encoder interface used when a machine has an external linear or rotary measuring system in addition to the motor encoder. On the 802D sl it is wired into the DRIVE-CLiQ ring of the SINAMICS S120 line-up and is commissioned as a topology node in the Sinamics IBN tool.

If the machine does not actually have an external measuring system on the SMC30, the correct configuration is to leave the SMC30 topology in the project but mark encoder 2 of the Z axis as not configured. This is done by setting MD30200[AX3,2] = 0 (NUM_ENCS) or by setting MD31020[AX3,2] = 0 and MD32700[AX3,2] = 0. After that, the SINAMICS commissioning must mirror the change by setting the drive parameter p0142 of the encoder 2 module to 0 or removing the SMC30 from the topology.

6. Step-by-Step Resolution Procedure

The following procedure applies to a 802D sl with a feed axis (Z) reporting 26002 for encoder 1 and/or encoder 2, and a spindle reporting 26002 for encoder 1. Run the steps in order.

1. Bring the machine to a safe state. Reset (NC-Stop), cancel all active programs, isolate the drives from the mains via the main contactor if an encoder is to be physically disconnected.
2. Open the SINUMERIK HMI and acknowledge the alarm. Press the Alarm softkey, select 26002, and use CANCEL. If the alarm reappears immediately, the inconsistency is hard-coded and must be fixed at the parameter level.
3. Connect Sinamics IBN to the drive. Use the HMI menu path Commissioning → Drive commissioning (Sinamics IBN) or STARTER via Ethernet on X127. Online with the drive and read the encoder parameters p0400, p0407, p0408, p0421 for both encoder 1 and encoder 2 of the affected axis.
4. Compare drive values to NCK MD values. For the Z axis in the source, p0408 = 2048 must equal MD31020[AX3,1] = 2048. For the spindle, p0408 = 1024 must equal MD31020[AX4,1] = 1024. If p0408 reports 0, the drive has not auto-detected the encoder; this points to a wiring, DRIVE-CLiQ, or hardware fault rather than a parameter error.
5. Set MD31020 to the verified value. In the SINUMERIK HMI menu Commissioning → Machine data, navigate to the axis-specific MD31020. If the value is correct, do not change it. If the drive reports a different number of lines, update MD31020[AXn,ENCidx] to match. The recommended procedure in the source thread is to also reduce the resolution (e.g. 512) for a suspect absolute encoder, but this is only a fallback for the case where the absolute encoder absolute-value format cannot be read and a per-revolution count of 512 is being used as a placeholder.
6. Correct the encoder type if necessary. If the motor nameplate shows EnDat but the drive was commissioned as incremental (p0407 = 9999 with manual p0408 entry), reset the drive to factory defaults and re-run automatic encoder identification. On the 802D sl this is available from the IBN tool's Encoder → Identify function. Power-cycle the drive after the identification completes.
7. Reconcile the SMC30 / encoder 2 image. If the machine does not have an external encoder, set MD30200[AX,2] = 0 in the NCK and remove the SMC30 topology from the SINAMICS project. If an external encoder exists, ensure its wiring (X521 for incremental 1Vpp, X531 for SSI) matches the SMC30 DIP-switch / firmware setting and that MD31020[AX,2] equals the value reported in p0408 of the SMC30.
8. Save and back up the data. Issue Commissioning → Series commissioning → Save on the HMI to write the MD to the CompactFlash card. On the drive side, save with Copy RAM to ROM in STARTER or the equivalent HMI softkey.
9. Power cycle the NCK and drive. A full NCK power-on (not just an alarm reset) is required for MD31020 changes to take effect, because the encoder configuration is read during NCK startup.

7. Verification

After the power cycle, perform the following checks before returning the machine to production:

1. Confirm that the alarm buffer is empty and that the Alarm history does not show 26002 recurring on power-up.
2. Open the Axis/Spindle service screen on the HMI. The actual position value must be stable and follow the axis within ±1 increment during a manual jog. If the position counts in the wrong direction, swap the sign in MD32100 (AX_MOT_DIR) or in p0410 (direction reversal), not in MD31020.
3. Run an axis-specific reference-point approach (G74) on each feed axis. Successful referencing indicates that the encoder is electrically healthy and that the absolute or incremental reference is consistent with the controller's expectation.
4. For the spindle, run a speed-controlled mode at 50, 500, and the maximum programmed RPM and verify that the actual-speed value tracks the setpoint. A discrepancy of more than 1 % indicates a scaling problem in MD31020 vs. p0408.
5. Run a short NC program that includes a G01 move on the affected axis and an M03/M05 spindle start. If 26002 does not return, the parameterization is consistent.

8. Common Pitfalls and Field-Proven Cautions

• Don't conflate 26002 with 26126 / 26127. Those are position-control-loop alarms (following error / contour monitoring). 26002 is a configuration alarm that fires before the position loop is closed, often at NCK power-on or after a topology change.
• Don't set MD31020 = 0 on encoder 1 to silence the alarm. On encoder 1 this disables the position feedback entirely and will produce a different alarm (25000, 25010). If the alarm is on encoder 2, leaving the value at 0 is correct only if no external encoder is present.
• Absolute encoder reset is not the same as 26002 fix. An absolute encoder that has lost its position (e.g. after a battery change on the SMC30) requires a homing reference point approach (MD34200) or an absolute-encoder adjustment (p0441, p0442) on the drive. Setting MD31020 will not recover the absolute reference.
• Check the firmware version. 802D sl controllers shipped before SW 2.x had tighter restrictions on p0407 values. If the SINAMICS firmware has been updated independently of the NCK software, an inconsistency between the two can present as 26002. Always match the NCK SW release to the SINAMICS firmware release as documented in the 802D sl commissioning manual.
• The spindle's 1024 in the source is a common Siemens default. When the spindle encoder is a Sick / Stegmann HIPERFACE single-turn with 1024 lines, MD31020 = 1024 is correct. Do not change it to 2048 unless the encoder nameplate confirms 2048 lines.

9. Related Alarms and Diagnostic Path

When 26002 is the leading alarm, the following secondary alarms are likely to be present in the alarm history and should be examined:

Each of these alarms is documented in the SINUMERIK 802D sl Diagnostics Manual (06/2009). The official 26002 reference in the Siemens Industry Online Support database is entry 109777872.

10. Quick Reference Summary