Configure SINAMICS G120 CU250S-2 PROFINET Load Sharing on S7-1500 ET200SP

This reference covers load distribution between two SINAMICS G120 drives controlled by an ET200SP distributed I/O station on an S7-1500 F-CPU over PROFINET. The application mirrors a classic PROFIBUS two-drive load-sharing topology originally documented for an S7-300 with a CU250S control unit, but adapted to current-generation PROFINET hardware, the SIMATIC Technology Object "Positioning axis," and the EPOS (basic positioner) integrated in the CU250S-2.

The target application class is non-time-critical motion: large mechanical platforms translated at low speed, where modest communication latency between the master and slave drives is acceptable. The article documents the architectural shift from PROFIBUS "Querverkehr" (Direct Data Exchange) to PROFINET-based torque routing, the three practical torque-sharing strategies, telegram 111 selection, the droop function, and a complete commissioning procedure.

1. System Architecture and Topology

The migration from the original PROFIBUS two-drive load-sharing example (Siemens Application Note 60602336) to a PROFINET ET200SP configuration keeps the same mechanical principle but moves the cross-coupling intelligence from the bus (Querverkehr) into the controller.

1.1 Hardware Components

1.2 Network Topology

Use a line topology with the S7-1500 F-CPU as the IO controller and both G120 drives as IO devices on the same PROFINET subnet. The IO controller port and the two drive ports can be chained in a line; the G120 CU250S-2 PN has an integrated 2-port switch, so no additional switch hardware is required for two drives.

+-------------------+      PROFINET (RT, not IRT)      +-------------------+
| S7-1515F / 1516F  |-----------------------------------| G120 Master       |
| (IO Controller)   |---[ IM 155-6 PN HF ]---[ ET200SP F-I/O ]            |
|                   |---------------------------------------| G120 Slave        |
+-------------------+                                   +-------------------+
       |
       | PROFINET/PN to HMI, SCADA, or engineering station
       v
   TP900 Comfort / TIA Portal

1.3 Mechanical Considerations

Two G120 drives driving a common load (gearbox, conveyor, platform) must be coupled through a rigid or semi-rigid mechanical link. The original application note used a shared gearbox output shaft. For low-speed platforms, a chain or rigid coupling is sufficient; the droop function (Section 6.3) will absorb small speed offsets and prevent fighting between the two motors.

2. PROFIBUS Querverkehr vs PROFINET: Why the Migration Is Not Transparent

2.1 What Querverkehr Solved on PROFIBUS

On PROFIBUS DP, the Querverkehr (also called Direct Data Exchange, DDE, or slave-to-slave communication) lets a slave publish a subset of its process data to other slaves on the same DP segment without routing through the DP master. In a two-drive load-sharing application, the master drive's actual torque value (r0079, smoothed torque) could be consumed by the slave drive in a single PROFIBUS cycle, with deterministic cycle time on the order of 1-4 ms depending on baud rate and DP cycle configuration.

2.2 Why PROFINET Does Not Offer an Equivalent

PROFINET does not provide a standardized slave-to-slave broadcast mechanism equivalent to Querverkehr. The data flow is strictly IO controller → IO device (and the reverse for inputs). For two drives to share process data, the values must transit the IO controller.

This introduces three new constraints:

1. Round-trip latency. The torque value must travel: drive → controller → second drive. Each leg is a PROFINET RT cycle plus the controller's OB processing time.
2. Non-deterministic asymmetry. The two legs use the same update time, but the controller's scan (OB1) and the PROFINET send/receive clocks are not phase-locked. The total latency varies and cannot be tightened to a single bus cycle.
3. No isochronous option. Because G120 CU250S-2 PN does not support IRT with isochronous application, the system has no mechanism to lock the bus cycle to a fixed phase within the controller's OB.

For low-speed platforms with cycle times on the order of 100-500 ms, this is acceptable. For high-dynamic load sharing, retain PROFIBUS, or switch to a controller topology with an S120 / SIMOTION backbone.

3. Telegram Selection: Why Telegram 111

The CU250S-2 supports the standard Siemens telegrams 1-5, 7, 9, 20, 110-112, 116, and the free configurable telegrams 999. For a load-sharing application that also requires positioning, select telegram 111 on both drives. Telegram 111 is the Siemens standard telegram for the basic positioner (EPOS) and provides 12 PZD words in each direction, which is enough to carry the EPOS control and status words plus the torque setpoint channel that load sharing needs.

3.1 Telegram 111 PZD Structure (Reference)

The exact assignment of each PZD word to control words, status words, setpoints, and actuals is documented in the SINAMICS G120/G120C List Manual, function diagrams 2415 and 2420. Refer to the list manual for the current firmware revision. The relevant signals for load sharing are summarized below.

Configure the telegram in TIA Portal under Device configuration → G120 → Telegram configuration, or in Startdrive / STARTER under Communication → Telegram configuration.

3.2 EPOS Operating Modes

EPOS inside the CU250S-2 supports the following modes, switched via the EPOS control word bits:

• Jogging (continuous and incremental) - manual positioning at low speed.
• Homing / Reference point approach - reference the drive to a homing mark or encoder zero.
• Traversing blocks - up to 64 fixed-position blocks stored inside the drive.
• MDI (Manual Data Input) - target position and velocity sent as setpoints from the PLC.
• Set-up / Setup mode - low-speed manual movement for commissioning.

For platform motion at low speed, jogging plus MDI typically covers all operational needs. Traversing blocks are useful for fixed-position cycles that must run without PLC intervention (e.g., a daily positioning sequence).

4. Two Ways to Do Positioning: Drive-Integrated EPOS vs SIMATIC Technology Object

The CU250S-2 supports positioning either inside the drive (EPOS via telegram 111) or inside the controller (SIMATIC Technology Object "Positioning axis" via telegram 1, 2, 3, or 5). The choice affects where the motion-control state machine lives and how load sharing interacts with positioning.

4.1 Drive-Integrated EPOS

• Pros: Drive continues to run if the PLC scan slows; the position controller is closed inside the drive at high update rate; less bus bandwidth consumed by position data.
• Cons: Motion blocks and ramps are configured inside the drive (less convenient than TO axes); the controller must still arbitrate the torque-sharing layer above the drive's speed controller.
• Recommended for: Standalone positioning where the drive continues without PLC.

4.2 SIMATIC Technology Object "Positioning Axis"

• Pros: Centralized motion engineering in TIA Portal; standard S7-1500 motion control blocks (MC_MoveAbsolute, MC_MoveJog, MC_Home); excellent diagnostic and trace integration.
• Cons: More PROFINET bandwidth; TO axes assume closed-loop control via the drive's speed/torque interface, which still requires the slave to switch into torque control for the load-sharing layer.
• Recommended for: Coordinated multi-axis motion, synchronized axes, and projects where the S7-1500 already runs motion elsewhere.

For the load-sharing application in this article, the most common pattern is:

1. Master drive runs EPOS internally (telegram 111) and outputs its actual torque on the bus.
2. Slave drive runs as a torque follower (telegram 111 with torque control bits, or telegram 1 with torque precontrol).
3. The PLC reads the master's torque, applies scaling, and writes it to the slave.

5. Torque Sharing Strategies

Three practical topologies for the master → slave torque link are available. Each has different latency, determinism, and complexity.

5.1 PLC-Mediated Routing (Recommended Baseline)

The torque setpoint is published by the master drive as a PROFINET input, read by the S7-1500 F-CPU in the cyclic OB (OB1) or an interrupt OB, scaled/limited, and written as a PROFINET output to the slave drive's torque setpoint. The slave runs in torque-controlled mode (STW1 bit 14 / bit 15 configured for torque control, or p1503 set to the appropriate free interconnect).

[ G120 Master ] --(PROFINET r0079 torque actual)--> [ S7-1500 F ] --(PROFINET torque setpoint)--> [ G120 Slave ]
     ^                                                                                                   |
     | speed control                                                                                    v
     +---- EPOS position controller inside the drive                                       torque follower + speed limiter

Latency: 1-3 PROFINET update times plus OB processing. With a 1 ms PROFINET update and 1 ms OB1, expect 3-5 ms total round-trip.

Determinism: Non-isochronous. Jitter depends on PROFINET class and the controller's scan time. For low-speed platforms, this is acceptable.

Advantages: Centralized, scalable, and easy to add safety logic, scaling, clamping, or ramp on the torque signal in the PLC.

5.2 Analog I/O Fast Path (Lowest Latency)

The CU250S-2 PN has two analog inputs (AI0, AI1) and two analog outputs (AO0, AO1) on the control unit itself. Use the master's analog output (e.g., AO0) to output its actual torque as a 0-10 V or 4-20 mA signal, and feed the same signal into the slave's analog input (AI0) as its torque setpoint. Configure p1503 (torque setpoint source) on the slave to read from the analog input.

Latency: Analog signal chain is a few hundred microseconds at most. Negligible compared to a PROFINET round-trip.

Determinism: Excellent. No bus cycle, no PLC scan dependency.

Trade-offs:

• Loses the ability to add PLC-side clamps, ramps, or safety scaling on the torque signal.
• Analog noise and drift must be considered for long cable runs.
• Resolution is 12 bits nominal on the CU250S-2 analog I/O, which is fine for torque following but not for tight speed matching.

5.3 Droop Function (Mechanical Soft-Coupling Layer)

The G120 supports a droop function that lets the drive's speed setpoint droop in proportion to its own torque output. When two drives are mechanically coupled and both have droop enabled, the system reaches a natural equilibrium: the drive carrying less load speeds up slightly until it shares the load with the other. This is a soft mechanical coupling and is the recommended approach for permanent load sharing on rigid shafts.

The droop parameters live in the SINAMICS G120 parameter set:

Droop scaling rule of thumb: Set the droop scaling so that the full load torque of one drive produces a speed change of no more than 1-2 % of rated speed. This keeps the speed mismatch between the two drives small enough to prevent friction losses while still giving the droop loop authority to balance the load.

5.4 Hybrid: Droop + PLC Torque Trim

For installations where the mechanical coupling is not perfectly rigid (e.g., two motors coupled by a chain that stretches slightly), combine the droop function with a slow PLC trim. The droop handles the fast load sharing, and the PLC adds a small offset based on the long-term average torque difference to keep both drives from drifting apart.

6. TIA Portal Programming Implementation

6.1 Hardware Configuration in TIA Portal

1. Add the S7-1500 F-CPU to the project. Select an F-CPU that supports PROFINET IO with at least two PROFINET interfaces or one interface with sufficient ports.
2. Add the ET200SP station with IM 155-6 PN HF. Add at least one digital input module (e.g., 6ES7131-6BH01-0BA0) and one digital output module (6ES7132-6BH01-0BA0). For F-I/O, add F-DI and F-DQ modules such as 6ES7136-6BA00-0CA0.
3. Add both G120 drives with CU250S-2 PN. Assign PROFINET device names and IP addresses. Select telegram 111 on both.
4. Assign the drives to the IO controller. Configure the PROFINET update time (1 ms minimum for the G120 with RT; 0.5 ms is supported on some G120 firmware versions but check the list manual for the firmware on your hardware).
5. Configure fail-safe I/O if needed for STO via PROFIsafe.

6.2 OB Selection and Cycle Time

The torque-routing logic can run in OB1 (the main cyclic OB) or in a time-of-day / cyclic interrupt OB (OB30-OB38). The choice is a trade-off between determinism and CPU load.

For a low-speed platform (target speed well under 100 rpm at the load), OB1 at a 4-10 ms cycle is fully adequate. For higher speeds or more dynamic load sharing, move the torque routing into OB35 set to a 1-2 ms phase, and ensure the OB execution time plus PROFINET update time fits inside the OB phase plus margin.

6.3 Sample Data Block Structure

Define a shared data block (DB) that the routing code uses to pass the master torque to the slave. Example layout:

DATA_BLOCK "dbTorqueShare"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
NON_RETAIN
   STRUCT
      rMasterTorque_Nm      : REAL;    // Master actual torque from r0079
      rSlaveTorqueSet_Nm   : REAL;    // Scaled torque setpoint to slave
      rTorqueLimit_Nm      : REAL;    // Clamp for safety
      rTorqueScaleFactor   : REAL;    // Ratio between master and slave motor torque
      bEnable              : BOOL;    // Enable torque sharing
      bSlaveTorqueMode     : BOOL;    // Switch slave to torque-controlled mode
      bFault               : BOOL;    // Drive fault or watchdog timeout
      iWatchdogCount       : INT;     // Increments if master torque not updated
   END_STRUCT;
END_DATA_BLOCK

6.4 Sample Routing Logic (SCL)

Drop the following into OB1 or OB35. It reads the master's actual torque, scales it, clamps it, and writes it to the slave. Adjust I/O addresses to match your hardware configuration.

// Master torque read (telegram 111, slot 1 input word offset depends on telegram)
#rMasterTorque_Nm := "G120_Master".InputWord[6];   // Map to r0079 mapped PZD word

// Scale: if motors are identical, factor = 1.0
// If slave motor is half the size, factor = 0.5
#rSlaveTorqueSet_Nm := #rMasterTorque_Nm * "dbTorqueShare".rTorqueScaleFactor;

// Clamp to safe range
IF #rSlaveTorqueSet_Nm > "dbTorqueShare".rTorqueLimit_Nm THEN
    #rSlaveTorqueSet_Nm := "dbTorqueShare".rTorqueLimit_Nm;
END_IF;
IF #rSlaveTorqueSet_Nm < -"dbTorqueShare".rTorqueLimit_Nm THEN
    #rSlaveTorqueSet_Nm := -"dbTorqueShare".rTorqueLimit_Nm;
END_IF;

// Watchdog: count if no new master torque has arrived
IF NOT "dbTorqueShare".bEnable THEN
    "dbTorqueShare".iWatchdogCount := 0;
ELSE
    "dbTorqueShare".iWatchdogCount := "dbTorqueShare".iWatchdogCount + 1;
    IF "dbTorqueShare".iWatchdogCount > 50 THEN
        "dbTorqueShare".bFault := TRUE;
    END_IF;
END_IF;

// Write to slave (telegram 111, output word offset depends on telegram)
IF "dbTorqueShare".bEnable AND NOT "dbTorqueShare".bFault THEN
    "G120_Slave".OutputWord[5] := REAL_TO_INT(#rSlaveTorqueSet_Nm * 10.0); // scaling to 0.1 Nm units
END_IF;

6.5 Switching the Slave to Torque Control

The slave drive must be in torque-controlled mode. Two options:

1. Use telegram 111 with the torque-control bit pattern. Set STW1 bit 14 = 1 to request torque control. The drive switches from speed control to torque control without ramp on the speed setpoint.
2. Use telegram 1 with torque precontrol. Send the torque value as the supplementary setpoint in telegram 1 word 3, and let the drive's closed-loop speed controller use it as feed-forward. The drive then operates in speed-controlled mode with the master torque pre-applied. This is closer to a droop-like behavior and is less stiff than pure torque control.

For a rigid mechanical link, pure torque control is preferred. For a chain or belt, torque precontrol with the droop function gives the system a small amount of mechanical give.

7. Isochronous Mode Limitations

The G120 CU250S-2 PN does not support isochronous PROFINET IRT. The drive runs in PROFINET RT (Real-Time) class 1 only. The implications are:

• No phase-locked data exchange with the controller's OB. The PROFINET update and the OB1 / OB35 cycle run independently.
• No synchronous PROFIsafe timestamps from the drive side; PROFIsafe runs on its own watchdog.
• Jitter on the bus update can be on the order of 100-500 microseconds at 1 ms update time, and larger at faster update times.

This is the central reason the original PROFIBUS load-sharing application chose a tightly-deterministic topology. On PROFINET with the G120, accept the jitter and design the torque routing to be tolerant of a few milliseconds of round-trip. For a low-speed platform, this is normally invisible at the mechanical level.

8. Commissioning Procedure

8.1 Prerequisites

• TIA Portal V17 or newer with Startdrive V17 (or matching versions).
• STARTER or Startdrive connected to each drive via PROFINET (online access).
• Motor data entered for each drive, with motor identification performed (p1900 = 2 for asynchronous motors).
• Encoder configured on each drive (HTL/TTL on SMC30) and referenced if used for position feedback.
• EPOS configured on the master drive (mechanical system, limits, software limit switches, homing mode).

8.2 Step-by-Step Commissioning

1. Wire and address. Assign PROFINET device names, IP addresses, and topology. Verify both drives are online and reachable from the controller.
2. Configure telegrams. Set telegram 111 on both drives. Verify in Startdrive that the I/O words are mapped as expected.
3. Run motor identification. With the motor uncoupled mechanically (load disconnected), run p1900 = 2 on each drive. Let the drive complete the measurement and save the results (copy RAM to ROM).
4. Configure EPOS on the master. Set p2580, p2581, p2582 for EPOS software limit switches. Set p2596 (mechanical system) for the gear ratio between motor and load. Configure homing mode (p2595).
5. Test the master in speed mode. Issue a slow speed setpoint from the controller via the TO axis or via EPOS MDI. Verify direction, encoder feedback, and that the drive reaches setpoint speed.
6. Test the slave in torque mode. With the mechanical link reconnected, command a small torque setpoint directly from Startdrive (control panel). Verify that the slave produces torque and the load moves.
7. Enable the torque-routing logic. Activate the PLC code that reads the master torque and writes the slave torque. Use the S7-1500 trace to record both torques and the bus latency.
8. Optimize. Adjust the droop scaling on both drives, the torque scale factor, and the OB cycle to minimize the speed mismatch and torque oscillation.
9. Test EPOS positioning on the master. Run a homing cycle, then an MDI move. Verify position accuracy at the load.
10. Save and back up. Copy RAM to ROM on both drives, save the TIA Portal project, save the Startdrive / STARTER project for each drive, and export the G120 parameter set to a backup file.

8.3 Verification Checklist

9. Troubleshooting Matrix

10. References for Further Configuration

For parameter-by-parameter configuration of the droop function, telegram structure, and EPOS settings, refer to the SINAMICS G120 with CU250S-2 Control Units operating instructions (Siemens, 04/2014 edition), which documents load distribution with the droop function. The current List Manual for the G120 / G120C family and the Startdrive online help should be the primary reference for the firmware version running on your CU250S-2.

The original PROFIBUS application note Application Note 60602336 remains valid for the mechanical principle and the load-sharing control structure, even though the bus has changed from PROFIBUS DP to PROFINET.