Siemens V90 EPOS Position Direction Reversal via P1821

Problem Overview

A common commissioning symptom on the Siemens SINAMICS V90 servo drive is a mismatch between the commanded mechanical direction and the position counter polarity in EPOS (Basic Positioner) mode. The motor shaft rotates in the correct (positive) direction, the speed setpoint matches the actual speed, no drive alarm is raised, yet the position actual value r2521 decrements when the axis should be incrementing. When the same hardware family and Telegram configuration is used on a machine with three or more identical axes, the fault typically isolates to a single drive where the encoder zero pulse sits on the opposite side of the sine/cosine window, or the encoder was wired in a way that the drive's internal direction detection latched to "negative".

This article documents the field-proven method to reverse the position counting direction of a V90 with a 2500 ppr TTL incremental encoder without rewiring the motor phases, without swapping the encoder cable, and without inverting the setpoint in the PLC application. The recommended path is to write parameter p1821 (direction of rotation reversal) from the S7-1500 controller through SinaPos or SinaPara, since V-Assistant does not expose p1821, p0410, or p29030 in its standard view.

Affected Hardware and Firmware

Why V-Assistant Hides the Solution

The V90 commissioning tool V-Assistant exposes only the simplified parameter set (the p290xx range) that maps onto the SINAMICS V90 user interface. The following BICO-style and encoder parameters exist on the drive firmware but are intentionally hidden from V-Assistant to prevent novice users from corrupting the closed-loop structure:

• p0410[0] – Encoder inversion (rotates the encoder evaluation; affects position counter)
• p1821 – Direction of rotation reversal (rotates the sign of speed and position actual values)
• p29030 – Selection of direction of rotation reversal (the V90-specific alias of p1821, but with limited range)

Of these, p1821 is the parameter that the S7-1500 / SinaPos environment writes when you toggle the "Positive" / "Negative" input at the function block. This is the only sanctioned method to invert the sign convention inside the drive while leaving the motor phases and the encoder cable untouched.

Step-by-Step: Reverse Position Direction on a Single V90 Axis

Step 1 – Verify Mechanical Wiring First

1. Set p29001 = 0 (default direction of rotation) on the affected drive.
2. Using the BOP (Basic Operator Panel) on the V90, navigate to the jog screen and command a slow positive setpoint.
3. Observe the motor shaft from the flange (load side) – clockwise (CW) rotation must correspond to a positive speed setpoint. If CW corresponds to negative speed, the motor phases or encoder A/B assignment are reversed mechanically, and the wiring must be corrected before continuing. The rest of this procedure assumes CW = positive speed.
4. Switch the drive to "Position control loop" via the control panel (use the V-Assistant control panel, NOT the BOP jog, since the BOP jog is open-loop and does not exercise EPOS).
5. Command a small positive relative jog and observe the position actual value. In V-Assistant, monitor r2521 (actual position value, LU) and r2556 (position setpoint, LU). If both increment with CW rotation, the drive is correctly configured and the problem is at the PLC application level (skip to Step 5). If r2521 decrements with CW rotation, the position counter polarity is reversed and Step 2 must be applied.

Step 2 – Write p1821 = 1 from SinaPara (Direct Method)

SinaPara (FB286) is the low-level parameter access block. It does not need a configured axis; it simply reads or writes a single parameter index over the cyclic PROFIdrive channel or via the acyclic DP-V1 slot. Use this method when the application must persist the direction reversal across power cycles and you want the drive itself to remember the sign.

Drop LAcycCom (PROFINET name) and the SinaPara instance into a separate OB (for example OB1 cyclic at low priority) and call it as follows:

// TIA Portal - Structured Text (SCL) example
// SinaPara DB: drives[affectedAxis]
#drives[1].AxisNo      := 1;            // logical drive number
#drives[1].ParameterNo := 1821;         // p1821
#drives[1].SubIndex    := 0;            // no subindex
#drives[1].WriteRead   := TRUE;         // write mode
#drives[1].ReqValue    := 1;            // 0 = no reversal, 1 = reversal
#drives[1].Execute     := TRUE;

// wait for Done or Error
IF #drives[1].Done THEN
    // p1821 accepted
    #drives[1].Execute := FALSE;
END_IF;
IF #drives[1].Error THEN
    // inspect Status / ErrorID for drive-side reject
    #drives[1].Execute := FALSE;
END_IF;

Resulting drive-side state: p1821 = 1. The drive inverts the sign of r0061 (speed actual), r2521 (position actual), and the internal setpoint/actual evaluation used by EPOS. The change is volatile until saved with p0971 = 1 on the drive (or by pressing the SAVE button on the BOP for V90 firmware < 1.05). If the drive loses power and reloads factory defaults, the change is lost.

Step 3 – Persist the Change

Trigger the RAM-to-ROM copy so that p1821 survives power cycle. Use SinaPara with ParameterNo = 971, ReqValue = 1, WriteRead = TRUE. Do not write p971 while the drive is in motion, while traversing to a position, or while EPOS is active. The save operation takes 3 – 8 seconds. Read p0971 afterwards to confirm r0971 = 0 which means "save completed".

Step 4 – Alternative: Toggle the SinaPos "Positive" / "Negative" Inputs

SinaPos (FB285) exposes two Boolean inputs that map to p1821 internally. Wiring them is the preferred way when the application already uses SinaPos and a runtime selector is required (for example, a HMI button to swap direction for mirror-image axes on a gantry). The relevant block inputs are:

Toggling the Positive / Negative input at SinaPos issues a SinaPara-equivalent write to p1821. Therefore the same persistence (p971) step applies if the change must survive a power cycle.

Step 5 – Verify

1. Power cycle the drive (or pulse OFF/ON at the 24 V control) so the new p1821 is loaded into the active parameter set.
2. In V-Assistant, switch to the control panel. Select "Position control" and "Jog in positive direction".
3. Command a small relative positioning motion. Monitor r2521 – the LU value must increment.
4. From the PLC, perform a relative positioning of 1000 LU using SinaPos ModePos = 2 (relative positioning with setpoint set). Confirm that the actual position is +1000 LU at standstill, with no F07491 (drive: speed controller output limited), no F07492 (drive: position controller output limited), and no following error F07451.

Parameter Reference

Drive-Side Error Codes When Writing p1821

The following fault / alarm codes are commonly returned if the write is issued under the wrong drive state. SinaPara surfaces them as ErrorID and Status at the FB output.

Troubleshooting Matrix

Edge Cases and Field Notes

Reference / Homing Interference

Do not change p1821 while the drive is in "Active referencing" state (r2684.0 = 1). The encoder is captured during the reference cam; if p1821 changes between two referencing operations, the absolute offset stored in p2599 is invalidated and the axis will start from a random mechanical position on the next referencing. Always complete the homing, then change p1821, then re-reference.

Modulo Axes

For modulo axes (rotary tables, tool turrets) with p29245 = 360000 LU/revolution, p1821 = 1 inverts the modulo direction. The drive still rolls over correctly; only the sign convention changes. Verify that your PLC application uses the same modulo convention.

Spindle Synchronism

If the affected V90 is part of a synchronized group (SinaPos with "FollowingAxis" mode or a cam from SIMOTION), changing p1821 on one axis will desynchronize the group. Re-establish coupling and re-synchronize after the change.

Brake Control

Setting p1821 = 1 inverts the holding brake release direction. If the axis uses an internal holding brake (p1215 = 3 or 4), confirm that the brake still opens at the correct state; otherwise the brake may chatter. Use p1216 (brake release time) and p1217 (brake closing time) to debounce.

Alternative Methods (Not Recommended for This Use Case)

1. Invert the encoder A and B traces in the cable. This is mechanically equivalent to p1821 = 1 but requires physical intervention. Acceptable on a single cabinet; not acceptable when the encoder cable is shared with a downstream safety encoder or routed through a field junction box.
2. Swap two motor phases (U and V). This swaps the torque direction, not the encoder direction. It does not fix r2521.
3. Apply a "−1" multiplier in the PLC on every setpoint and actual value. This is the worst option: it propagates the sign error into the HMI, the trace, the cam, and the PLC arithmetic. It is forbidden in safety-relevant applications because the HMI display no longer matches the actual machine geometry.

Commissioning Checklist After p1821 Change

• [ ] Encoder pulses in CW direction (r0061 positive) verified with trace in V-Assistant
• [ ] r2521 increments in CW direction verified with trace in V-Assistant
• [ ] SinaPos absolute encoder adjustment (homing) re-executed
• [ ] Position controller Kv (p29120) checked; re-tune if needed
• [ ] Following-error window (p2544) checked
• [ ] Standstill window (p2542) checked
• [ ] Brake timing (p1216 / p1217) checked
• [ ] Drive alarm buffer cleared (p0952 = 0)
• [ ] Drive state machine reflects S1 (run) and ZSW1.2 (ready) on HMI

FAQ