Troubleshooting SINUMERIK 840Di SL Encoder Errors 231150/231806

Problem Overview

A SINUMERIK 840Di SL controller (PCU 50.3) wired to a Heidenhain RCN 226 absolute encoder through a SINAMICS S120 drive line-up (CU320 + SMC20) raises two persistent commissioning alarms on the spindle:

• 231150: bad initialization 0x10
• 231806: bad initialization 0x6d

Both alarms are presented by the SINUMERIK HMI as encoder-side faults but originate on the SINAMICS Drive Object, where the Sensor Module Cabinet 20 (SMC20) is unable to complete EnDat 2.2 hand-shaking with the RCN 226. Setting p0400 to 10000 or 10100 produces a status of "no encoder", confirming that the drive does not recognize the RCN 226 as a known EnDat absolute device. The root cause is almost always an incorrect encoder database selection, an unfilled p0404 configuration word, or a mis-wired SMC20-X520 terminal. This guide maps both alarms to their SINAMICS F-code parents, documents the correct p0400 / p0404 values for the RCN 226, and walks through the full 840Di SL commissioning sequence required to clear them.

Hardware Stack Identification

Before touching parameters, confirm the exact hardware in the loop. The 840Di SL is a PC-based CNC that hosts the HMI on a PCU 50.3 panel and communicates with the drive line-up over PROFIBUS DP or PROFINET IRT. The spindle path described here is:

1. Heidenhain RCN 226 - modular angle encoder with integral bearing and stator coupling, 28-bit singleturn, EnDat 2.2 protocol. The "IC 2260" EnDat 2.2 version is the variant typically used in machine-tool spindle feedback.
2. Heidenhain adapter cable - shielded twisted-pair with 17-pin M12 connector on the encoder side; should be ordered against the Heidenhain RCN 226 data sheet so the shield terminates at 360 degrees on the encoder housing.
3. SMC20 (Siemens part number 6SL3055-0AA00-5BA0 or the newer 6SL3055-0AA00-5BA1 with EnDat 2.2 firmware support) - Sensor Module Cabinet-mounted, accepts EnDat 2.1/2.2 and SSI encoders on the X520 terminal block and forwards the position to the CU320 over DRIVE-CLiQ.
4. CU320 (Siemens part number 6SL3040-1MA00-0AA0 or -0AA1) - the SINAMICS Control Unit; hosts the drive objects and exchanges data with the NCU/PCU 50.3.
5. 840Di SL on PCU 50.3 - the HMI and NCK host, which mirrors the SINAMICS alarms on the operator panel as 231xxx codes.

Alarm Decoding: 231150 (0x10) and 231806 (0x6d)

SINUMERIK 231xxx alarms are re-mappings of SINAMICS F3xxxx faults on the corresponding drive object. The 0x10 and 0x6d suffixes are SINAMICS message-word values, not the 231xxx alarm index, and they point at the actual EnDat error reported by the encoder during cyclic hand-shaking.

The 0x6d value is a Heidenhain EnDat error code that the SMC20 decodes and re-reports through the F31806 path. For an EnDat 2.2 absolute rotary encoder it can mean "command not supported by encoder" - which is what you see when the drive tries to interrogate the RCN 226 as if it were a SSI device (typical symptom of p0400 = 10000 / 10100 / 10120).

Root Cause Analysis

Three primary reasons produce this exact combination of alarms on a spindle driven by an RCN 226:

1. Wrong encoder database entry in p0400. Values in the 10000 - 10200 range in the SINAMICS encoder database cover SSI, Hall-sensor, and incremental encoders - none of them EnDat 2.2 absolute. The drive will not enumerate the RCN 226, the status line on the Starter/SIMOTION Scout project will read "no encoder", and the sensor module will reject every initialization command, surfacing as F31500 (0x10) followed by F31806 (0x6d).
2. p0404 encoder configuration word is at factory default (0). Even with the correct p0400 index, an EnDat 2.2 absolute rotary encoder needs the configuration bits set: rotary (not linear), absolute (not incremental), and EnDat 2.2 (not 2.1). A zero p0404 drops the drive back to "no encoder" after the next power-on.
3. SMC20 X520 wiring or 24 VDC supply missing. The EnDat interface on the SMC20 must be fed from a regulated 24 VDC source capable of 250 mA per channel, and the cable shield must be bonded to the encoder housing and the SMC20 sub-D backshell. Reversed clock/data pairs or a floating shield typically yields the 0x6d communication error rather than 0x10.

SINAMICS Parameter p0400 Reference

p0400[0...n] selects the encoder type that the drive uses to configure the corresponding Sensor Module channel. The factory index for a 24 V EnDat 2.2 absolute rotary encoder is found in the 20200-20299 and 20400-20499 ranges of the SINAMICS S120 List Manual. For the Heidenhain RCN 226 (EnDat 2.2, 28-bit, singleturn), the index that consistently enumerates the device on CU320 firmware 5.x and later is:

Index placement also matters. The spindle Drive Object (DO) typically owns encoder 2, while the motor encoder is encoder 1. Use the drive object index to place the value:

• Spindle encoder: p0400[1] = 20421 (or whichever EnDat 2.2 index applies to your RCN 226 variant)
• Motor encoder: p0400[0] = ... (set to match the actual motor-mounted encoder)

p0404 Configuration Bits

p0404[0...n] is the configuration word that tells the drive how to interpret the encoder selected by p0400. The factory default of zero will reject the RCN 226 because the drive still cannot decide whether the encoder is linear or rotary, absolute or incremental. Typical bit settings for an EnDat 2.2 absolute rotary encoder:

This produces a p0404 value of 0x13 (binary 00010011) for a rotary, absolute, EnDat 2.2, singleturn encoder. Setting p0404 after p0400 and saving to ROM (Copy RAM to ROM, then power cycle) is mandatory; otherwise the next commissioning upload of the project will overwrite the value.

SMC20 Wiring and Terminal Layout

The SMC20 brings the EnDat interface out on sub-D connector X520. Use only Heidenhain adapter cables with the 17-pin M12 connector specified for the RCN 226. The terminal assignments for X520 are:

Bridge settings on the SMC20 must be set for EnDat and for the encoder supply voltage the RCN 226 is rated for. Verify that the jumper block selects 5 V if the Heidenhain adapter cable derives its internal supply through the SMC20; RCN 226 is typically powered at 3.6 V to 14 V internally. If 24 V is set on the SMC20 and the cable does not have an internal regulator, the encoder will fail to enumerate and you will get the 0x10 initialization alarm rather than 0x6d.

Step-by-Step Commissioning Procedure

1. Open Starter / SIMOTION Scout for SINAMICS / Startdrive. Connect to the CU320 over PROFIBUS or PROFINET and load the project. Confirm online state.
2. Check the topology: the SMC20 must appear under the CU320's DRIVE-CLiQ tree. Replace the unit if it reports as "unknown module".
3. Navigate to the spindle Drive Object and open the configuration for encoder 2 (RCN 226). Set p0400[1] to the value matching your RCN 226 EnDat 2.2 variant (typical 20421). Press OK; Starter writes the value to the drive.
4. Set p0404[1] to 0x13 (rotary, absolute, EnDat 2.2, singleturn).
5. Copy RAM to ROM. Power cycle the entire drive line-up, including the SMC20, so the Sensor Module re-enumerates against the new database entry.
6. Once the drive is back online, verify the sensor module channel now reports the encoder type and resolution. The drive should no longer say "no encoder".
7. Run the encoder identification: p0407 = 1 in the spindle DO. The drive performs a coarse identification, sets the fine resolution in p0418 / p0419, and writes the identified values to p0400 / p0404.
8. Save the project. Do not overwrite the identified values with a manual p0400 download - always read the values out of the drive and let the encoder identification populate them.

SINUMERIK 840Di SL Machine Data Alignment

After the SINAMICS side enumerates the RCN 226, the NCK on the PCU 50.3 must accept the encoder as a valid spindle actual-position source. The relevant machine data, in MD30200 $MA_NUM_ENCS / MD30240 $MA_ENC_TYPE and the related MD310xx spindle encoder block, must point at the second SINAMICS encoder:

• MD30200[AX2] = 2 - axis has two encoders (motor + spindle)
• MD30240[AX2,ENC2] = 1 - incremental (SINUMERIK-side type for an absolute EnDat device is set to 1, since absolute bits are handled by MD34200 $MA_ENC_REFP_MODE)
• MD31020 $MA_ENC_RESOL[AX2,ENC2] = 33554432 - 28-bit (matches the RCN 226)

If you do not realign the SINUMERIK-side machine data, the spindle position display on the HMI will stay at 0, even when the SINAMICS side reports a clean position. The 231150 / 231806 alarms typically re-surface within a few seconds of the next NCK reset if this layer is left out of step.

Verification Checklist

• No F3xxxx or 231xxx alarms in the SINAMICS / SINUMERIK diagnostics after power-up.
• Spindle actual position (HMI readout) tracks a manual rotation of the spindle.
• Starter / Startdrive shows the encoder as a recognized device on the sensor module channel, with a non-zero fine resolution in p0418 / p0419.
• r0047[0...63] & 0x20 is clear (no position-error bit).
• Encoder status word r0480 does not have bit 0 or bit 13 set.
• Reference-point acknowledgment on the HMI works without spurious alarms during REPOS moves.

Troubleshooting Matrix

Frequently Asked Questions