Configuring SINAMICS G120C PROFINET Speed Control via Telegram

Overview: Telegram vs. Digital I/O Control on SINAMICS G120C

The SINAMICS G120C is a compact, modular drive platform designed for wall-mount and motor-mounted installations in the 0.55 kW to 132 kW power range. When integrating the drive into an S7-1200 or S7-1500 control system over PROFINET, two fundamentally different command paths exist for commanding start, stop, coast, and speed setpoint:

• Digital I/O control: Start/stop and direction bits are wired through 24 V digital outputs on the PLC to discrete inputs on the Control Unit (CU). Speed setpoint may be analog (0–10 V) or fixed via parameter.
• PROFIdrive telegram control: Control word 1 (STW1) and a normalized speed setpoint (NSOLL) are packed into a single PROFIdrive telegram and transferred cyclically over PROFINET. The drive returns status word 1 (ZSW1) and actual speed (NIST).

Field experience on retrofit projects — particularly where legacy hard-wired systems are being migrated to PROFINET in incremental stages — strongly favors telegram-based command. The control bits are sampled by the drive on the same PROFINET cycle that delivers the setpoint, eliminating the scan-time, relay-pull-in, and field-wiring race conditions that plague multi-wire digital control. This reference documents how to configure a minimal free PROFIdrive telegram between an S7-1200 and a G120C using SINAMICS Starter, the correct STW1 / ZSW1 bit map, and the typical commissioning workflow that delivers reliable start, stop, coast, and emergency stop behavior.

Prerequisites: Hardware, Firmware, and Software Stack

Before commissioning, verify that the hardware, firmware, and software versions on site support the configuration described here.

Hardware

• SINAMICS G120C with a PROFINET-capable Control Unit (CU230P-2 PN, CU240E-2 PN, or CU240S2 PN-F). Confirm the CU type by reading the rating plate or via parameter r0001.
• SIMATIC S7-1200 CPU with PROFINET interface (CPU 1211C, 1212C, 1214C, 1215C, 1217C, or 1214FC / 1215FC for fail-safe variants).
• PROFINET cable (Cat 5e minimum, shielded) and an industrial switch if multiple PROFINET nodes are present.
• PROFINET device name labels for the G120C; the device name must be unique on the subnet.

Firmware

Software Stack

• SINAMICS Starter (or StartDrive in TIA Portal V15.1+) with the appropriate Hardware Support Package (HSP) installed.
• SinaSpeed function block library (part of the SINAMICS blocks for TIA Portal / STEP 7). Library identifiers: "SINAMICS" or "SINAMICS_S7-1200".
• GSD file for the G120C installed in TIA Portal or STEP 7 (GSDML-Vx.x-Siemens-Sinamics-G120C-xxxxxxxx.xml).

PROFIdrive Telegram Architecture on the G120C

PROFIdrive telegrams on the G120C are defined by parameter p0922 (IF1 PROFIdrive PZD telegram selection). Standard telegrams (e.g., 1, 2, 3, 20, 352) are pre-defined; for a minimal custom layout a free telegram is configured via p0922 = 999 with the PZD setpoints and actual values selected explicitly under p2071 / p2079.

Recommended Minimal Free Telegram Layout

Configuring a Free Telegram in SINAMICS Starter

1. Open the project in Starter and go online to the G120C. Accept any firmware version warning if the G120C is older than the HSP.
2. Select the telegram: Navigate to Configuration → Communication → IF1 PROFIdrive PZD telegram selection and set p0922 = 999 (free telegram configuration).
3. Configure PZD setpoints (PLC → Drive): Under Free telegram configuration → PZD setpoints (p2051):
   • Set p2051[0] = (STW1 source). By default Starter routes to the BICO source r2090.0, which is the receive word 0 from IF1. Leave at default unless you require BICO re-routing.
   • Set p2051[1] = (NSOLL source). The default target is the main setpoint connector; verify p1070[0] = 2050[1] so the received speed setpoint is fed to the ramp-function generator input.
4. Configure PZD actual values (Drive → PLC): Under Free telegram configuration → PZD actual values (p2050):
   • Set p2050[0] = 53 (status word 1, ZSW1).
   • Set p2050[1] = 21 (smoothed actual speed, NIST).
5. Set the reference speed: p2000 defines the speed value that corresponds to 100 % setpoint (4000 hex). Set p2000 to the motor nameplate rated speed in RPM (default 1500 RPM).
6. Set the bus cycle: In the PLC, configure the PROFINET send clock (typically 1 ms for motion, 4 ms for non-motion applications). Verify the G120C's PROFINET cycle matches; mismatched cycles trigger F08501 "PROFINET communication fault" or A08502 warning.
7. Copy RAM to ROM and perform a power cycle to load the new telegram configuration.

Control Word 1 (STW1) Bit Map and Sequencing Rules

Control Word 1 (STW1) is the primary command interface for PROFIdrive profile drives. The bit definitions for the G120C conform to the PROFIdrive profile V4.1 (AC1) and are summarized below.

Mandatory Run Sequence in STW1

For the drive to release the pulses and accelerate to setpoint, the following bits must be true simultaneously:

• Bit 0 (ON/OFF1) = 1
• Bit 1 (OFF2) = 1
• Bit 2 (OFF3) = 1
• Bit 3 (Enable operation) = 1
• Bit 6 (Setpoint enable) = 1
• Bit 10 (Control by PLC) = 1

The drive evaluates STW1 once per PROFINET cycle (typically every 1–4 ms) and applies the safety AND of all bits before energizing the IGBTs. Telegram delivery guarantees that all bits arrive simultaneously and on the same bus cycle.

Status Word 1 (ZSW1) Bit Map for Returned Diagnostics

ZSW1 returns the drive's response to the PLC on every PROFINET cycle. The PLC program should monitor ZSW1 to verify the drive is actually in the commanded state.

Why Telegram Control Is More Reliable Than Digital Outputs

A common retrofit pattern when migrating from legacy hard-wired controls to PROFINET is to "leave the digital wiring in place for now" and only convert the speed setpoint to PROFINET. This preserves risk but introduces a class of failure modes that are not visible at design time:

Sources of Latency in Digital I/O Control

1. PLC scan time: The PLC reads inputs, executes logic, and writes outputs at the end of the scan cycle. On an S7-1200 this can range from 1 ms (small program) to 10 ms (large program, organization blocks, and motion OB's).
2. Digital output update: On transistor outputs, the propagation delay is ~50 µs. On relay outputs (S7-1200 Rly variants), mechanical pull-in adds 5–15 ms and bounce time can extend another 2–8 ms.
3. Field wiring: Cable capacitance and noise filtering on the drive's 24 V inputs add another 0.5–3 ms of RC delay.
4. Drive input scan: The G120C scans digital inputs every 1 ms (parameter p0799 or as configured). Inputs can be debounced via p0797 / p0798.

The cumulative latency from "PLC logic asserts bit" to "drive IGBTs energize" can therefore exceed 20 ms on relay-output S7-1200 systems. During that 20 ms window, the PLC has already started the next scan and may have changed other outputs. If the PLC issues a sequence of commands that the drive interprets out of order, the drive may end up in a state the PLC did not command.

Timing Failure Modes: The OFF1/OFF2 Race Condition

Consider the following PLC logic sequence on a legacy system using digital I/O control:

Initial drive state: OFF1 = 0 (stopped), OFF2 = 0 (coast active), OFF3 = 1.

PLC scan N: Logic decides to start the drive. Sets OFF1 = 1 and OFF2 = 1 on outputs O:0 and O:1.

End of scan N: Output image is written to physical outputs. With a relay output module, the O:0 relay pulls in before the O:1 relay due to mechanical tolerance — let's say O:0 (OFF1) closes 4 ms after the command, while O:1 (OFF2) closes 9 ms after the command. The drive's input scan runs at 1 ms intervals.

The drive's digital input scanner sees the following sequence:

1. T=0 ms: OFF1 = 0, OFF2 = 0 (start of sequence — both still open)
2. T=4 ms: OFF1 = 1, OFF2 = 0 — drive logic: OFF2 active → coast command, OFF1 ignored
3. T=9 ms: OFF1 = 1, OFF2 = 1 — drive would now run, but the OFF2 trigger at T=4 ms has latched the coast inhibit; depending on parameterization, the drive may require a fresh OFF1 edge to restart.

Result: the drive fails to start. The PLC continues to assert OFF1 and OFF2 = 1, but the drive refuses to release pulses because, from its perspective, OFF2 was asserted when the run command was issued.

Why Telegram Control Avoids This

PROFIdrive telegrams deliver all 16 control word bits as a single 16-bit word on the same PROFINET cycle. The drive receives STW1 atomically. Bit 0 (OFF1) and bit 1 (OFF2) cannot physically arrive on different cycles. The PROFIdrive state machine inside the G120C evaluates the full STW1 once per bus cycle, so a PLC that puts STW1 = 0x047F (OFF1=ON, OFF2=inactive, OFF3=inactive, enable=ON, setpoint enable=ON, PLC control=ON) always results in a coherent command. There is no race condition because there is no per-bit independent delivery.

Wiring the S7-1200 to G120C over PROFINET

Even when telegram control is used, certain terminals must still be wired for hardware interlock safety (e.g., STO via terminal, hardware coast). On the G120C, the Safe Torque Off (STO) function is implemented as a hardwired safety input when using Safety Integrated via terminal; it is implemented over PROFIsafe when using a PN-F Control Unit. Verify which variant is in use before commissioning.

Minimum Wiring for Telegram Control (no PROFIsafe)

Configuration of Digital Inputs in Telegram Mode

When the PLC is in control (STW1 bit 10 = 1), the drive ignores commands from DI0/DI1 for ON/OFF/coast. However, the digital inputs are still scanned and can be BICO-routed to other functions (e.g., DI0 mapped to STO via p1012 = 722.0, or DI1 mapped to a local jog command). Verify in the function plan under Configuration → Inputs/Outputs → Digital Inputs that DI0 and DI1 are not double-assigned to ON/OFF1/OFF2 commands if you want pure telegram control.

PLC Program Skeleton: SinaSpeed Function Block

Siemens provides the SinaSpeed function block (FB 285 / "SinaSpeed" in the SINAMICS library) as a turnkey interface for the G120C. The FB handles PROFIdrive state machine sequencing, fault acknowledgement, and setpoint ramp management, eliminating the need for the PLC to manually toggle individual STW1 bits.

SinaSpeed Block Interface (STEP 7 / TIA Portal)

Example Call in Structured Text (SCL)

// G120C_Instance.scl — SinaSpeed wrapper
IF "dbG120C".ConfigRequest THEN
    "dbG120C".ConfigAxis := TRUE;        // one-shot to FB
END_IF;

"SinaSpeed_DB"(
    EnableAxis   := "dbG120C".RunReq AND NOT "dbG120C".HwStop,
    SpeedSp      := "dbG120C".SpeedRPM,
    RefSpeed     := 1500.0,
    ConfigAxis   := "dbG120C".ConfigAxis,
    Acknowledge  := "dbG120C".AckPulse,
    HwStop       := "dbG120C".HwStop OR "EStopPushbutton",
    AxisReady    => "dbG120C".Ready,
    AxisError    => "dbG120C".Faulted,
    ActSpeed     => "dbG120C".ActualRPM
);

// Pulse acknowledge on rising edge
"dbG120C".AckPulse := "dbG120C".AckCmd AND NOT "dbG120C".AckPrev;
"dbG120C".AckPrev := "dbG120C".AckCmd;

Commissioning Procedure and Verification

1. Pre-power checks: Verify mains voltage, motor nameplate data vs. p0301–p0349, and that STO hardware circuit (terminals 31/32) is wired to the safety relay.
2. Motor identification: Run p1900 = 2 (motor data identification, rotating) or p1900 = 1 (non-rotating). This calibrates the field-weakening range and the I²t model.
3. Save parameters: Copy RAM to ROM (p0971 = 1) before loading the telegram configuration; this prevents parameter loss on power cycle.
4. Configure PROFINET device name: Use the PRONETA or TIA Portal topology editor to assign the device name (e.g., "G120C-001"). The PROFINET device name must match the PLC project exactly.
5. Set the IP address: Either via DHCP server (PRONETA / TIA Topology) or statically via parameter p8921 (PN NameOfStation IP) and p8922 (subnet mask).
6. Verify PROFINET communication: In Starter, monitor r2050[0], r2050[1], r2090.0, and r2090.1 live. You should see ZSW1 and NIST updating on every PROFINET cycle.
7. Test telegram control from the Starter control panel: Set p854.0 = 0 to disable local control, then issue STW1 = 0x047F with NSOLL = 0x1000 (10 % of p2000). Verify the motor accelerates to 150 RPM.
8. Test the safety AND: Toggle STW1 bit 1 (OFF2) to 0 with the motor running. The drive should coast immediately and pulse-inhibit. Restore STW1 bit 1 to 1 and re-issue STW1 bit 0 (OFF1) rising edge to restart.
9. Hand-off to PLC: With the drive verified, set p854.0 = 1 (control from PLC) and load the SinaSpeed instance. Run the PLC program in monitor and observe the AxisReady / AxisError outputs and ActSpeed.
10. Failure-mode test: While running, force STW1 bit 1 to 0 from the PLC for 200 ms, then restore. The drive should coast, stop, and re-start on the next OFF1 edge. This is the test that proves telegram control is working correctly.

Troubleshooting Matrix

Field-Proven Notes from Legacy Retrofits

• Keep the existing hardware stop: Do not remove the hardwired STO or coast terminals when migrating to telegram control. STW1 bit 1 (OFF2) is a software command, not a safety function. Hardware STO is required for SIL 2/3 applications regardless of telegram path.
• Phase-in the speed setpoint first: On most retrofits, the first PROFINET element to migrate is the speed setpoint (NSOLL). Speed control is non-safety, and a wrong ramp or scaling is recoverable. Save the control word migration for the second phase when the field team is comfortable with the new telegram.
• Document the BICO wiring: Many legacy G120C installations have non-default BICO routing (e.g., DI0 → ON/OFF1, DI1 → OFF2). When migrating to telegram control, reset these BICO connections to defaults via p0010 = 30, p0970 = 1 (factory reset), then reapply only the parameters you intend to change.
• Use the Starter trace: Record STW1, ZSW1, NSOLL, NIST, and r0063 (smoothed actual speed) simultaneously for 5 seconds during start, run, stop, and fault scenarios. The trace file is invaluable when validating the safety AND logic against PLC scan behavior.