Resolving Sinamics G120C PROFIBUS DP Communication with S7-300

Resolving Sinamics G120C PROFIBUS DP Communication with S7-300

Problem Overview

Engineers commissioning a SINAMICS G120C with a PROFIBUS DP Control Unit (CU240S DP or CU240E DP) against a SIMATIC S7-300 DP master (CPU 31x-2 DP or CP 342-5) frequently report that the drive does not appear in Accessible Nodes, that the cyclic I/O handshake never starts, or that the bus fault LED (BF) on the Control Unit lights steady red even though the GSD file was installed. The drive responds at the BOP/IOP panel but rejects all PLC write attempts to the process data. In every documented case of this symptom the root cause is a mismatch between the HW Config telegram selection, the DIP-set PROFIBUS node address, and the drive's own expert-list parameters (p0922, p2042, p2043).

This reference walks through the diagnostic path, the correct parameter set, and the verification steps that produce a stable cyclic PROFIBUS link between an S7-300 and a SINAMICS G120C.

Affected Components and Firmware

Root Cause Analysis

Five independent failure modes account for almost every G120C ↔ S7-300 PROFIBUS problem encountered in field commissioning. Diagnose in this order before swapping hardware.

1. Drive-side fieldbus not enabled. Parameter p2043 (activate fieldbus interface) defaults to 0 on a factory-fresh G120C. The DP port is physically present but logically inert. The PLC sees no answer to its FDL_Diag requests, even though the bus terminator is correct and the cable is good.
2. Telegram mismatch. The HW Config slot is set to Universal Module (no fixed structure) or to a different telegram than what p0922 requests inside the drive. The PLC opens the connection but every value-word handshake fails the consistency check on the first cycle.
3. PROFIBUS node address conflict. DIP switches on the CU240 were not changed from default (0), or were set to the same value as another slave on the segment. p2042 in the expert list and the DIP switches must agree; the drive honors whichever was set last via parameter write.
4. Drive not commissioned. p0010 is still in commissioning state (1, 4, 5, 29, etc.) so the drive has not loaded its application settings. p0099 shows an unknown configuration. The starter project cannot upload, and the PLC handshake cannot start because the drive is in first commissioning state.
5. Baud-rate auto-negotiation failure. On long segments with low-quality connectors or missing termination, the drive settles at 1.5 Mbaud but the master pushes 12 Mbaud. The BF LED flickers red instead of staying lit.

Prerequisites

• GSD file for the exact CU variant installed; import via Options → Install GSD File in HW Config. Restart HW Config so the catalog refreshes.
• Starter (V4.x or V5.x) or Startdrive in TIA Portal V13+ available on the engineering PC with a USB or PROFIBUS-to-Ethernet routing path.
• PROFIBUS cable terminated at both ends with 220 Ω + 390 Ω resistor network, shield clamped at the connector backshell.
• Drive and PLC powered up together; the G120C accepts fieldbus configuration only after the basic commissioning (motor data, application class) has been performed.

Step-by-Step Configuration

Step 1 - Set the PROFIBUS node address

The G120C supports PROFIBUS addresses 1 through 126. Address 0 is reserved for temporary wiring checks.

1. Power down the drive.
2. Set the DIP switches on the front of the CU240 to the desired address. Switch positions are read left-to-right as binary with the leftmost switch = bit 0 (value 1). For address 5: switches 1 and 3 ON, all others OFF.
3. Power up. The active address is reported by r2050[0] at the BOP as "5" in the fieldbus status screen.

Step 2 - Activate the PROFIBUS interface

Navigate from the BOP or via Starter to the parameter list and set:

Value 3 means fieldbus interface active, PROFIBUS DP protocol. Without this assignment the drive ignores every telegram and the BF LED stays steady red.

Step 3 - Select the telegram

Apply with p0922 = 1 for Standard Telegram 1 or p0922 = 352 for the 6/6 vector telegram. Save with p0971 = 1; the drive reboots once and comes back with the new telegram active.

Step 4 - Configure HW Config in STEP 7

1. Open the S7 project, insert the G120C from the catalog: PROFIBUS-DP → SINAMICS → SINAMICS G120C → CU240S DP. Do not select the entry under Other field devices → Drives; that entry does not honor p0922.
2. Assign the PROFIBUS address you set on the DIP switches (here: 5).
3. Drag the appropriate telegram from the right catalog pane into slot 0 of the slave. For Telegram 1 the catalog label is SINAMICS G120C → Telegram 1 (PZD-2/2).
4. Double-click the slot and verify the PZD lengths match. STEP 7 will show 2 words input / 2 words output.
5. Compile and download to the CPU.

Step 5 - Commission the drive with Starter

1. Connect Starter to the drive via the PC connection cable or over PROFIBUS using the CPU as gateway.
2. Open Accessible Nodes. The drive should appear at the configured PROFIBUS address.
3. If the drive shows "first commissioning required", run the commissioning wizard: enter motor data from the rating plate, choose application class Standard Drive Control (Speed) for V/f, or Dynamic Drive Control for vector without encoder.
4. Set p0010 = 0 and confirm the drive leaves commissioning mode (RDY LED green, no warning).
5. Save with p0971 = 1 or via the toolbar icon. The parameter set is now retained across power cycles.

Step 6 - Establish the cyclic link

From the S7 program:

// Example: write control word 1 (047E hex = OFF1 inhibit, ready)
L  W#16#047E
T  PAW 256      // First PZD out, depends on HW Config base address

// Setpoint speed: 1500 rpm = 4000 hex on NSOLL_A reference
L  W#16#4000
T  PAW 258      // Second PZD out

The drive should respond in the same cycle on the input PAW range with ZSW1 = 0x4331 (operation enabled, no fault) and a feedback speed value. Watch the diagnostic buffer of the CPU for PROFIBUS events.

Verification Checklist

• BF LED off on the G120C Control Unit. A steady-red BF means telegram mismatch or address conflict; a flickering BF means cable/baud-rate problems.
• RDY LED green continuously (no orange/yellow indicating first commissioning pending).
• PROFIBUS monitor in Starter shows the drive as online and the cyclic I/O words update in the IF1 trace window at the configured bus cycle.
• r2050[0] reports the correct node address (BOP display).
• STW1 transitions through the correct bit pattern: 047E → 047F (on) → 0x0F7F (acknowledged). A stuck 047E means the drive never received the OFF1 enable.
• STEP 7 diagnostic buffer: Slave OK for the G120C node, no Station failure entries.
• Save the project, perform a power cycle, and confirm the link re-establishes without manual intervention.

Troubleshooting Matrix

Common Fault Codes Related to PROFIBUS

Parameter Reference for Quick Setup

Performance and Timing Notes

• At a 12 Mbaud PROFIBUS rate and Telegram 1 (2/2 words) the typical bus cycle on the G120C is approximately 1.6 ms with default p2040 = 100 ms monitoring.
• The PLC scan time must be substantially shorter than p2040. For an S7-300 with a 50 ms OB1 cycle, raise p2040 to 200 ms or shorten the OB1 time.
• If multiple G120C drives share a segment, use a 9.6 kbaud minimum segment line and keep the total segment length below the value defined by the baud rate (e.g. 100 m at 12 Mbaud).
• The G120C DP Control Unit does not support isochronous PROFIBUS; for synchronized motion use the PROFINET variant (CU240PN or CU240E PN) and IRT instead.

Safety Considerations

Standard Telegram 1 with STW1 OFF1 (bit 0) follows the STO function category per IEC 61800-5-2 when wired through the safety terminals of the G120C. If the application requires SIL 2 or SIL 3, the CU must be a Failsafe variant (CU240S DP-F, order code 6SL3244-0BA21-1PA0 with the -F suffix in the firmware) and PROFIsafe must be mapped in addition to PROFIdrive. This reference covers the standard PROFIdrive handshake only.

Documentation and Standards

Cross-reference these official Siemens and IEC sources during acceptance testing:

• SINAMICS G120 with PROFIBUS application example (Siemens Support)
• SINAMICS G120C operating instructions (Siemens Support)
• SINAMICS G120C parameter manual / List Manual (Siemens Support)
• S7-300 automation system, CPU 31xC and CPU 31x manual (Siemens Support)
• PROFIdrive Profile V1.1.1 (PI / PROFIBUS & PROFINET International)
• IEC 61800-7 Adjustable speed electrical power drive systems, Part 7-304 (PROFIdrive profile)

FAQ