Troubleshooting Siemens POSMO A Motor Synchronization on PROFIBUS DP

The Siemens SIMODRIVE POSMO A is a distributed intelligent positioning motor that participates as a slave on a PROFIBUS-DP network. A recurring field issue is positional drift between two POSMO A drives of identical part number when one unit carries firmware version F and the other carries firmware version J. This article documents the root cause, the part-number decoding that disambiguates gearbox options, the PROFIBUS DP timing behavior that produces a multi-millimeter start delay, and the verified remediation path using a hard-wired digital input trigger.

1. Affected Product Identification

The motor in question is identified by Siemens part number 6SN2155-0AA10-1BA1. This is a SIMODRIVE POSMO A 300 Watt intelligent positioning motor. The designation pattern is consistent with the SIMODRIVE POSMO A catalog family documented in the Siemens product finder and the PROFIBUS product registry.

Where two motors carry the same string 6SN2155-0AA10-1BA1, Siemens manufacturing records confirm the gear ratio is identical and the resolver feedback resolution is identical. The letters F and J appearing on the unit label or in the SIMATIC Manager / commissioning tool are firmware release identifiers for the integrated POSMO A control section and do not modify the mechanical transmission ratio.

1.1 Gearbox Option Reference

When the middle field of the POSMO A part number is anything other than AA, an external gearbox is fitted at the factory. The Siemens POSMO A catalog defines the following gear ratios by the same digit position:

Confirm the AA field on both nameplates before continuing. If the codes differ between the two units, the mechanical ratio difference is the root cause and firmware synchronization will not resolve the drift.

2. Problem Details: Multi-Millimeter Start Offset

Symptom reported in the field:

• Two POSMO A 6SN2155-0AA10-1BA1 motors mechanically coupled through the driven load.
• One unit reports firmware F, the other firmware J.
• Both receive the start command from the same PROFIBUS DP master (typically a SIMATIC S7-300/400 or SIMOTION controller).
• On command issuance, the F-version motor begins the programmed motion immediately, while the J-version motor trails by a constant offset of a few millimeters at the load.
• The offset is repeatable, not random, and grows as a function of traverse velocity.

The drift is small but consistent, which is what rules out a gross mechanical ratio error and points instead at a deterministic timing differential between the two PROFIBUS DP slaves.

3. Root Cause: PROFIBUS DP Is Not Isochronous by Default

POSMO A slaves communicate on PROFIBUS DP using standard DP-V0/V1 cyclic I/O. By default this is a non-isochronous, acyclic or cyclic-but-non-time-synchronized channel. The Siemens POSMO A documentation and the PROFIBUS product registry both note that the POSMO A is intended as a distributed intelligent actuator on PROFIBUS-DP, with the application program executed locally in the drive. See the PROFIBUS product entry for SIMODRIVE POSMO A.

The implication for synchronous operation is fundamental:

1. The DP master (PLC) runs its OB1 cycle and writes the control word to each slave in turn.
2. Each POSMO A consumes its telegram at a slightly different phase of the PROFIBUS token rotation, determined by its bus address, cable distance, and any repeaters between it and the master.
3. When the control word is latched inside the POSMO A, the drive begins trajectory generation from the current actual position at that latching instant.
4. For two slaves at different bus addresses, the latching instant differs by one or more DP scan cycles, producing the observed start-time offset.

Firmware revisions F and J do not change this behavior; the timing differential is inherent to the PROFIBUS DP telegram exchange and to the bus topology. The change from F to J firmware may, however, have changed the internal trajectory start latency by a small but non-zero amount (for example a different interpolation clock prescaler or a different ramp-start routine), making the existing scan-time jitter visible where it was previously masked.

4. Diagnostic Procedure

Run the following checks in order. Each step eliminates one hypothesis before moving to the next.

4.1 Confirm Mechanical Identity

1. Read the full 14-digit part number on each motor nameplate and verify they match exactly, including the AA gearbox code and the 1BA1 winding code.
2. Inspect the mechanical coupling between the two motor shafts (or between each motor and the common load). Confirm no external gear reducer has been added that is not reflected in the nameplate code.
3. Hand-rotate the shafts with power removed and verify the mechanical ratio at the load is 1:1 between the two motors.

4.2 Quantify the Start Offset

1. Command a slow positioning move (10% of rated velocity) from a known reference point to a known target.
2. Record the actual position trace of each drive with a sampling rate at least 10x the PROFIBUS DP cycle time, using the drive's own actual-position word in the DP receive buffer or an external encoder on the load.
3. Measure the time delta between the two drives' departure from the start position. This delta, multiplied by the commanded velocity, equals the observed start offset in mm.

4.3 Measure the PROFIBUS DP Scan Time

1. In the SIMATIC Manager or TIA Portal, read the configured DP cycle time for the POSMO A slaves.
2. Use a PROFIBUS diagnostic tool (for example a PROFIBUS tracer or the diagnostic buffer in STEP 7) to record the actual telegram phase for each slave address.
3. Compute the phase difference between the two slaves' control-word latching events.

5. Solution: Trigger Synchronous Start via a Hard-Wired Digital Input

The field-proven fix for the F-vs-J start offset on POSMO A is to remove the start command from the PROFIBUS DP telegram and instead route it through a single digital input that is wired in parallel to both drives. The drives are configured to begin trajectory generation on the rising edge of this common hardware input. Because the input is sampled at the drive's local control clock (typically 1 ms or faster), and both drives see the same electrical edge within nanoseconds of each other, the start-time offset collapses to a level set by cable propagation delay only.

5.1 Prerequisites

• One free digital input on each POSMO A, configured as a fast start input (refer to the POSMO A configuration manual for the assignment of DI functions).
• A shielded twisted pair from a single PLC digital output module, or a 24 VDC source, fanned out to both drives' start inputs in a star or daisy-chain topology.
• Cable length between the two start inputs kept within 5 m to limit propagation delay differential below 50 ns.

5.2 Configuration Steps

1. Open the POSMO A commissioning tool (SIMODRIVE POSMO A Configurator or equivalent) for each drive.
2. Assign the chosen digital input to the Start Positioning function.
3. Pre-position each drive at the start of the move by issuing a Jog or Reference command over PROFIBUS DP and waiting for the In-Position status bit.
4. Disable the DP-triggered start bit in the POSMO A control word, or leave it as a Pre-Enable only.
5. Configure the PLC program so that the DP telegram sets the target position, velocity, and acceleration, but the actual start is fired by the common digital input pulse.

5.3 Verification

1. Re-run the diagnostic move from Section 4.2.
2. The start offset should drop below 0.5 mm at 100 mm/s.
3. Repeat the move 100 times and confirm the offset distribution is tight (standard deviation < 0.2 mm).

6. Alternative Solution: PROFIBUS DP Isochronous Mode

Where the application can tolerate the engineering cost of isochronous PROFIBUS DP, the POSMO A can be configured for synchronized operation using DP-V2 with the equidistance guarantee. In this mode, the master broadcasts a global control telegram with an isochronous tick, and all slaves sample their inputs and latch their outputs at the same instant. This eliminates the scan-time jitter and allows multi-axis coordinated motion at the cost of:

• A SIMOTION or SIMATIC S7-300/400 with isochronous PROFIBUS DP master capability.
• Recabling with a clock-sync-capable PROFIBUS DP network.
• Configuration of the equidistance time in HW Config.

This is overkill for a simple two-axis start coincidence but is the correct path for full multi-axis interpolation.

7. Mechanical Verification (When Software Path Fails)

If the digital-input start trigger and the isochronous mode both fail to remove the offset, the cause is mechanical. Run the following checks:

1. Decouple the two motors from the load and run a closed-loop position profile on each drive independently. Confirm both drives hit the commanded position to within the resolver resolution (typically a few encoder counts).
2. Inspect the load coupling for backlash, torsional wind-up, or compliance. A compliant coupling will absorb the start-time jitter and release it as a position error after a few hundred milliseconds.
3. Verify the load itself is not binding. A binding load can present a different effective inertia to each motor, causing one drive to take longer to break static friction.
4. Check the holding brake (if fitted) for equal release time. The 10 in the part number indicates without holding brake on this variant, but verify against the actual nameplate.

8. Firmware Considerations

POSMO A firmware revisions F and J are field-released by Siemens to address bug fixes and feature additions. The change history is documented in the POSMO A firmware release notes shipped with each firmware package. The user-visible effect of an F-to-J upgrade on this specific motion application is that the drive's start-of-motion latency changed by a small but deterministic amount. This is the most likely reason the existing PLC program, written for firmware F, now produces a visible offset on a J-version unit.

Siemens policy on POSMO A firmware is to keep functional compatibility across revisions for the standard PROFIBUS DP interface, but timing parameters are not guaranteed bit-exact across major revision letters. If a fleet mix of F and J units is unavoidable, use the digital-input start method in Section 5 to remove the firmware-version dependence entirely.

9. Commissioning Checklist

10. Troubleshooting Matrix

11. Safety Notes

12. When to Escalate to Siemens Support

Escalate to Siemens Technical Support if any of the following are true after the Section 5 fix is applied:

• Start offset remains above 0.5 mm at 100 mm/s with the digital-input trigger.
• Either POSMO A reports a firmware-version-specific fault on power-up.
• The mechanical ratio at the load is verified to be 1:1 but the offset grows non-linearly with traverse distance.
• Both drives lose reference simultaneously when the start command is fired.

When escalating, include the full part number (for example 6SN2155-0AA10-1BA1), the firmware revision letter of each unit, the PROFIBUS DP configuration, the actual-position traces from both drives, and the diagnostic buffer dump from the SIMATIC Manager.