Siemens SINAMICS p1821[0...n] Direction Parameter and Drive Data

Overview

The Siemens SINAMICS parameter p1821[0...n] reverses the direction of rotation of the motor and the encoder actual value without changing the setpoint. The [0...n] indexing notation means the parameter exists once for each Drive Data Set (DDS) configured in the drive. On a default SINAMICS S120, G120, G120C, G120D, or SINUMERIK ONE simDrive, four Drive Data Sets are typically present, so p1821 appears as p1821[0] through p1821[3]. The active index follows whichever DDS is currently selected at runtime.

The reason p1821 is split across multiple indexes is that any parameter flagged with the "D" attribute in the SINAMICS parameter list is DDS-dependent. Each DDS is a self-contained copy of the drive configuration, and changing p1821[0] only affects DDS1. If a subcontractor reversed direction in DDS1 and the application always runs on DDS1, only that index needs to be changed. If the application switches DDSs at runtime, every index must reflect the intended rotation direction.

Function of p1821[0...n] — Direction of Rotation

Per the SINUMERIK ONE simDrive parameter documentation, p1821 performs a sign inversion on two quantities simultaneously:

1. Motor direction of rotation — The internal speed setpoint polarity is flipped before it is applied to the closed-loop controller.
2. Encoder actual value — The feedback direction is inverted so the controller still "sees" positive speed for the same mechanical direction after the reversal.

Because both setpoint and actual value are inverted together, the closed loop remains stable and the regulator does not need to be re-tuned. This is the recommended way to correct an installation where the motor or encoder was wired with reversed polarity, or where the mechanical definition of "positive" is opposite to the electrical default.

Parameter values:

What Are Drive Data Sets (DDS)?

A Drive Data Set is a complete copy of the drive's runtime configuration. Every DDS contains its own values for all DDS-dependent parameters. Switching the active DDS at runtime swaps the entire configuration in one cycle, allowing a single drive to behave as several different drives without re-parameterizing.

Standard SINAMICS drive firmware supports up to 4 DDSs (p0180 = 4 by default on most platforms). Each DDS references exactly one Motor Data Set (MDS) and one Encoder Data Set (EDS) per used encoder. The Command Data Set (CDS) is layered on top of the DDS for setpoint source selection.

For SINAMICS S120, the SINAMICS S120 List Manual identifies DDS-dependent parameters with a "D" suffix or the "DDS" column in the parameter table. Any parameter whose value depends on which DDS is active will appear as pXXXX[0] through pXXXX[3] in STARTER, Startdrive, or the drive's web server.

Index Assignment — p1821[0] to p1821[3]

The mapping between index and Drive Data Set is direct:

The active DDS is selected via:

• Manual selection: p0181 (default 0 = DDS1) — sets the active DDS at power-up.
• Control word bits: On CU320-2 and SINUMERIK NCU, the DDS selection bits are part of the drive-internal control word and can be interconnected from a higher-level PLC or from the NCK.
• PROFIdrive telegram: Some telegrams (e.g., Siemens telegram 111, 112, 116) contain DDS selection bits in the supplementary telegram.

After a DDS switch, the drive applies the new configuration on the next controller cycle (typically 1 to 4 ms on SINAMICS S120 with the standard current-controller clock of 125 µs).

Why Multiple Drive Data Sets Exist

Drive Data Sets solve real engineering problems where one physical drive must exhibit multiple personalities:

1. Multi-motor operation: Two different motors (e.g., a high-speed spindle and a low-speed torque motor) on the same power section, switched mechanically by a contactor. Each DDS points to its own MDS.
2. Dual encoder setups: Different encoders for different speed ranges (resolver at low speed, incremental at high speed). Each DDS uses a different EDS.
3. Tuning profiles: Soft start-up tuning for commissioning versus production-tuned parameters for normal operation. Operator selects via HMI.
4. Setpoint source switching: DDS1 receives setpoint from PROFIBUS/PROFINET; DDS2 receives from a local analog input — though this is more commonly handled with CDS, DDS-level interconnections are sometimes layered.
5. Backup/secondary configuration: A spare commissioning profile stored in DDS2 that can be activated if DDS1 becomes suspect.

In the scenario from the original question, the subcontractor likely defaulted the drive to DDS1 (the standard first data set), set p1821[0] = 1, and left DDS2 through DDS4 untouched. If the application only uses DDS1, this works. If it later switches to DDS2/3/4, the direction will silently revert to default.

Related Direction and Polarity Parameters

Several parameters influence direction in SINAMICS. Understanding the relationship prevents redundant or conflicting settings.

Accessing and Editing p1821

The parameter can be edited from any of the standard SINAMICS engineering tools. The procedure is identical across tools except for navigation:

• STARTER (classic): Project tree → Drive → Parameters → Search "p1821" → Inspect/change each index.
• Startdrive (TIA Portal): Drive device → Parameters → filter by number or name.
• SINAMICS Web server: Browse to drive IP → Parameters → search.
• BOP20 / IOP-2: Navigate to P1821, then use the index key (handshake icon on BOP20) to step through indexes.
• PLC direct write (PROFINET/PROFIBUS): Use PKW or non-cyclic parameter access (e.g., SFB52/SFB53 in STEP 7, RDREC/WRREC in TIA Portal). Write to parameter number 1821 + index.

Example: A WRREC to read/write p1821[2] from a SIMATIC S7-1500 uses parameter number 1821 (decimal) and index 2 in the data record.

Step-by-Step Commissioning Procedure for Direction Reversal

1. Identify how many DDSs the drive uses. Read r0050 (active DDS bit pattern) and p0180 (DDS count). If only one DDS is in use, only p1821[0] matters.
2. Lock the drive against unintended start. Issue OFF1 via control word, wait for r0899.0 = 0, and ensure pulses are inhibited (r0899.11 = 0 or no enable signal).
3. Backup current parameters. Use STARTER/Startdrive → Online → Upload (RAM to ROM) and save a project backup. This preserves the original direction setting if reversal is incorrect.
4. Modify p1821 for each active DDS. For each index that the application may activate, set the value (0 = default, 1 = inverted). Example: To invert on all four DDSs, write 1 to indexes 0, 1, 2, and 3.
5. Save to non-volatile memory. Issue "Copy RAM to ROM" (p0977 = 1) or use the tool's Save button. Without this step the change is lost on power cycle.
6. Power cycle the drive to ensure the change is loaded from the CF card / persistent storage on next boot.
7. Verify in commissioning mode. Run the motor at low speed with no load, observe r0061 and r0063. Positive speed setpoint must produce positive actual speed in the desired mechanical direction.

Verification and Diagnostic Checks

After modifying p1821, validate the change with the following checks before returning the drive to production:

Common Field Issues and Troubleshooting

SINUMERIK-Specific Notes

On SINUMERIK ONE (and 840D sl) with simDrive, the drive parameter p1821[0...n] interacts with the NCK's axis direction-of-rotation logic. The NCK expects consistent positive direction across:

• Axis-specific MD32100 $MA_AX_MOT_DIR (motor direction of rotation)
• Drive-side p1821 per active DDS
• Encoder-side adjustment MD34200 $MA_ENC_REFP_MODE and the absolute encoder offset

If the NCK issues a positive travel command and the drive reports r0061 < 0, the SINUMERIK will trip with axis-specific alarms ("Measuring system wrong polarity" or similar). Always validate direction in the NCK trace after a drive-side reversal. SINUMERIK ONE commissioning manuals also document the procedure for reversing an axis using the NCK command "Reference axis" with reversed direction.

Commissioning Recommendations

1. Document every DDS with a description in STARTER/Startdrive ("DDS1: Motor A, encoder 1", etc.).
2. Standardize on one DDS for production unless switching is genuinely required — fewer DDSs means fewer places where direction, ramps, and limits can drift.
3. Use p1821 for installation mismatch only. For routine forward/reverse at runtime, use the PROFIdrive control word direction bits or p1113.
4. Back up the project before any DDS-level change. A wrong reversal during a production run can cause mechanical damage.
5. Verify on every DDS if the drive switches DDS at runtime. A common commissioning oversight is testing only on DDS1 and missing the fact that DDS2–DDS4 retain the original direction.
6. Coordinate with PLC logic. If the PLC selects DDS bits based on machine state, document the DDS-to-state mapping in the PLC code comments.

FAQ