Reversing the Direction of Rotation on a SINAMICS S120 with CU320-2 PN Using Parameter p1821

The SINAMICS S120 drive platform, paired with a CU320-2 PN control unit, is one of the most widely deployed modular drive systems in high-performance servo and vector applications. A recurring commissioning question is how to reverse the motor's direction of rotation without physically swapping two phases at the motor terminal box. Siemens provides parameter p1821 for exactly this purpose. The parameter changes the rotating field direction of the current controller, which is functionally equivalent to swapping two motor phases, but it does so purely in software.

This article consolidates the field-proven procedure for finding and setting p1821 in every supported engineering tool, the firmware prerequisites, the effect of direction reversal on speed, position, and encoder systems, the alternatives (negative setpoints, hardware phase swap), and the standard troubleshooting matrix used when the parameter is not visible in the expert list.

1. Overview of the Direction Reversal Function

Parameter p1821 — officially titled Direction of rotation reversal — inverts the rotating field inside the current controller. The effect is identical to swapping two of the three motor feeder cables (U, V, W):

• Speed setpoint (r0061 / r0063) remains positive when the drive accelerates in the new direction.
• Position actual value continues to rise monotonically while the motor physically moves backward relative to its prior direction.
• The setpoint/actual value signs stay consistent; only the mapping to the mechanical axis is inverted.

Siemens documents this behavior in the official knowledge base article SINAMICS S120: Direction of rotation reversal of a servomotor (ID 96838164). The KB article confirms that p1821 modifies the rotating field direction of the current controller and applies to Servo, Vector, and V/f axis types on the SINAMICS S120.

2. Prerequisites

Before modifying p1821, confirm the following:

1. CU320-2 PN firmware: minimum V4.4 for the PN variant. The parameter is present on every shipped CU320-2 PN from firmware 4.4 onward. Newer firmware releases (for example V4.5, V4.6, V4.7, V5.x) retain the parameter.
2. Engineering software: one of the following must be installed and online with the drive:
   • SIMATIC S7-STARTER (standalone commissioning tool, the traditional choice for SINAMICS S/G)
   • SCOUT (the SIMOTION-oriented variant of STARTER, same parameter tree)
   • SINAMICS Startdrive (integrated in TIA Portal, V14 SP1 and later)
3. Drive object context: the expert list must be opened against the correct drive object — typically the Drive_1 (Servo or Vector) object, not the Control Unit or an Infeed.
4. Access level: p1821 resides at access level 3 (Expert). The user account in STARTER/Startdrive must be set to Expert or higher; the default Standard view hides it.
5. Safety state: the drive must be in a safe state. Switch the drive to OFF1/OFF2 (disable pulses) before changing p1821. Direction reversal is not a runtime-tunable parameter on most drive objects; it requires a subsequent Calculate controller data and a power-on or restart of the drive object.
6. Mechanical clearance: the load must be permitted to move in the reversed direction. Confirm that limit switches, software position limits, and mechanical end stops are correctly wired for the new direction.

3. Parameter Specification — p1821 in Detail

On a Servo axis with an absolute encoder (EnDat, DRIVE-CLiQ encoder, SSI), changing p1821 automatically inverts the commutation angle by 180°, so the encoder's position counter continues to match the mechanical direction of rotation. No additional parameter is required to keep position and direction consistent.

4. Locating p1821 in SIMATIC S7-STARTER

STARTER is the canonical commissioning tool for the SINAMICS S120 with CU320-2 PN. The parameter is not visible in the Function view; it is only exposed in the Expert list of the drive object.

4.1 Step-by-step in STARTER

1. Open STARTER and load the project that contains the CU320-2 PN.
2. Go online with the target device (button: Online → Connect to target system).
3. In the project navigator on the left, expand the drive unit and click on the Drive_1 (Servo/Vector) object — not the CU320-2 PN or the Infeed.
4. Double-click Expert list (or right-click → Expert list) to open the parameter table.
5. In the toolbar, set the access level filter to Expert (icon: a key with the digit 3, or use the Tools → Expert list filter dialog).
6. Press Ctrl+F and search for p1821.
7. Confirm the row shows the drive object you intend to modify.
8. Right-click the parameter and select Change value…, or double-click the value field.
9. Enter 1 to reverse the direction, then confirm with OK.
10. Save the project to the PG (Project → Save) and download to the drive (Online → Download to target device).
11. Perform a power-on reset of the drive object (POWER ON or Commissioning → Restart drive object) for the change to take effect.

5. Locating p1821 in SINAMICS Startdrive (TIA Portal)

Startdrive embeds the SINAMICS S120 commissioning inside TIA Portal. The function view in Startdrive intentionally hides expert parameters, so you must switch to the parameter view.

5.1 Step-by-step in Startdrive / TIA Portal

1. Open the TIA Portal project that contains the S120 drive device.
2. Select the SINAMICS S120 device in the project tree and open the device view.
3. Expand the drive object's tree node. Right-click the Drive_1 (Servo/Vector) and open Commissioning → Parameter view (not the function view).
4. In the parameter view, the filter bar defaults to Standard. Open the filter and switch it to Expert to reveal all access-level-3 parameters.
5. Use the search field to type p1821. The row appears with the title Direction of rotation reversal.
6. Click the value cell and enter 1. The cell turns red and a small icon indicates the change has not been downloaded yet.
7. Right-click the parameter and choose Download to device, or use the toolbar Online → Download to device to push the entire parameter set.
8. Perform a restart of the drive object (right-click → Restart drive object) so the new rotating field direction becomes active.

6. Locating p1821 in SCOUT

SCOUT is the SIMOTION-coupled variant of STARTER. The expert list workflow is identical to STARTER, but the navigation differs.

1. Open SCOUT and go online with the SIMOTION D4xx or T-CPU containing the CU320-2 PN.
2. In the project navigator, drill into the SINAMICS drive object — for example, Drive axis_1 or TO axis_1 / Drive_1.
3. Double-click Expert list on that drive object.
4. Set the access filter to Expert (level 3) using the filter dropdown.
5. Search for p1821 and change the value to 1.
6. Save (Project → Save and recompile all if SIMOTION coupling requires it).
7. Download to the target.
8. Cycle power on the drive object or use the Restart drive object command.

7. Effect of p1821 on Speed, Torque, and Position

Direction reversal with p1821 is a transparent mapping change inside the drive. After the change, the following relationships hold:

Higher-level controller programs (SIMATIC S7, SIMOTION, T-CPU, third-party PLC) do not need to be modified. The drive internally re-maps the rotating field so that a positive speed setpoint command still accelerates the motor, just in the new direction. Position-controller programs that home to a reference mark also continue to work because the position counter direction is corrected automatically.

8. Alternative Methods of Direction Reversal

When p1821 is not available or not the preferred solution, three alternative methods are available. Each has engineering trade-offs.

8.1 Negative speed setpoint

Many control architectures (PROFINET, PROFIdrive, SINAMICS telegram 1/2/3/5) accept a signed speed setpoint. Driving the drive with a negative setpoint achieves the same shaft rotation as p1821 = 1 but inverts the sign at the controller level. The drawback is that every consumer of the speed/position value (HMI, SCADA, safety logic) must also be configured for signed values.

8.2 Hardware phase swap

Swapping any two of the three motor feeder phases (U-V, V-W, or U-W) at the Motor Module output terminals reverses the rotating field. The change is purely mechanical and does not require a software parameter. Drawbacks:

• Asymmetric terminal torque on the Motor Module terminal block if the swap is not performed uniformly across all axes in a multi-motor cabinet.
• Documentation drift: the wiring diagram must be updated and re-validated against the as-built drawing.
• On motors with integrated holding brakes or third-party cable assemblies, a phase swap may invalidate the brake wiring convention used by the OEM harness.

8.3 Encoder inversion via p0410 / p0430

For Servo axes, the encoder evaluation direction can be inverted independently through p0410 (encoder inversion actual value) or p0430.13 (invert signal A/B) without touching p1821. This is sometimes used when the encoder was mounted in a reversed orientation. Do not combine p1821 with manual encoder inversion unless you understand the resulting two inversions — they cancel out.

9. Firmware Requirements and Update Path

The CU320-2 PN has been in production since 2012. The original and current firmware branches are summarized below.

To verify the firmware currently running on the CU320-2 PN, read r0019.0 (Control Unit firmware version) or check the device label in STARTER/Startdrive under Properties → Diagnostics → Version. The firmware version string is formatted as Vx.y.z.f.

If a firmware update is required (typically only on the very early V2.5.x units or where a new feature is needed), use one of the following Siemens tools:

• SIMATIC S7-STARTER: Online → Target system → Firmware update
• SIMATIC Automation Tool (AT): batch update of multiple CU320-2 PN units
• Web server of the CU320-2 PN (firmware ≥ V4.6): Update & Backup page

10. Safety Considerations

Direction reversal affects the safety chain in several ways that must be reviewed before commissioning.

10.1 Safe Torque Off (STO) and Safe Stop 1 (SS1)

STO via the X132 terminal strip on the CU320-2 PN is direction-agnostic — it removes drive enable regardless of rotation. The wiring does not need to change when p1821 is modified. Safe direction selection (SDI), if configured, is direction-aware and must be re-validated for the new rotating field direction.

10.2 Mechanical limit switches and software end limits

After p1821 = 1, the convention that was used for limit switches must be reviewed. A positive end-limit switch in the old mapping becomes a negative end-limit switch in the new mapping. The drive's software limit switches in p2540/p2541 (or p2570/p2571 for EPOS) must be re-checked.

10.3 Regenerative energy and DC link

Direction reversal does not change the brake chopper or Active Line Module behavior. A motor that was overhauling (regenerative) when running in the original direction will also overhaul when running in the reversed direction, provided the torque sign is preserved. Verify that the Line Module (Smart Line, Basic Line, Active Line) is sized for the worst-case continuous regenerative energy.

11. Troubleshooting Matrix

12. Verification Procedure

After modifying p1821, perform the following verification before returning the machine to production:

1. Parameter readback: in STARTER/Startdrive, navigate to p1821 on the drive object and confirm the online value is 1.
2. Run the drive in jog: with the enable on, jog the motor in the positive direction using the control panel. Confirm the motor rotates in the desired mechanical direction.
3. Speed sign check: in the trace, capture r0061 (speed setpoint smoothed) and r0063 (speed actual smoothed). Both must be positive when the operator commands positive direction.
4. Position sign check: in the trace, capture r0482 or r2521[0] (position actual). Move the axis in the positive direction. The position actual must increase.
5. Limit switch test: manually drive the axis into each end limit. Confirm that the positive software end limit (p2540) trips the drive before the mechanical end stop in the new positive direction.
6. Safety function test: trigger STO via the X132 input. The drive must coast to a stop independent of direction. Trigger SS1 and confirm the controlled stop honors the new direction.
7. Save and back up: upload the project to the PG, archive the drive's parameter set to the memory card (function "Backup to memory card"), and save the project.

13. Commissioning Checklist

14. Field-Proven Tips

• Two-axis machines: if two axes share a CU320-2 PN, set p1821 on each drive object individually. The parameter is per drive object, not per Control Unit.
• EPOS positioning: when using basic positioner (EPOS), the mechanical direction is set by p1821, while the traversing block direction is set in the program. The two must be consistent — a "POS" block with positive target must physically move in the positive direction.
• Telegram 1/2/3/5: the PROFIdrive control word and speed setpoint signs are unaffected by p1821. The PLC program does not need to invert the setpoint.
• Safety integrated: if Safety Integrated is configured (SLS, SDI, SLP), the safety direction limits are calculated from the same encoder. The Safety Commissioning Wizard must be re-run after changing p1821 to regenerate the safe direction values.
• Offline/online discrepancy: in STARTER, the project file (offline) and the drive (online) can drift after a p1821 change. Use Online → Target system → Load to PG to re-synchronize the project after the change.

15. Frequently Asked Questions