Configuring PROFIBUS Between S7-300 and SINAMICS G120 CU240E-2

Overview

PROFIBUS DP (Decentralized Periphery) is the dominant fieldbus for connecting Siemens drives, I/O, and third-party actuators to SIMATIC controllers. This reference documents the complete commissioning of a PROFIBUS DP link between a SIMATIC S7-300 (CPU 31x-2 DP / CPU 31x-2 PN/DP) and a SINAMICS G120 inverter built around a CU240E-2 DP Control Unit. The article covers cable and connector requirements, telegram selection, control and status word bit semantics, the drive parameter set, GSD installation in STEP 7 V5.5, HW Config wiring, and STL sample code that calls SFC14 / SFC15 to read current and speed feedback and to issue start, stop, and speed setpoint commands.

The S7-300 master must be equipped with an integrated DP port or a CP 342-5 communications processor. The CU240E-2 DP variant carries a 9-pin sub-D female PROFIBUS connector and supports baud rates from 9.6 kbit/s up to 12 Mbit/s with auto-detect. SINAMICS G120 with firmware V4.4 or higher (see parameter r0018) supports the Standard Telegrams 1, 2, 3, 4, 5, 6, 7, 9, 20, 220, 352, and 999 (free configuration). Standard Telegram 3 is the smallest standard telegram that exposes motor current feedback to the PLC and is the recommended minimum for a closed-loop speed control application with current monitoring.

Network Topology and Physical Layer

PROFIBUS DP is a shielded, two-wire RS-485 network terminated at both ends with 220 Ω bus terminators (built into the 9-pin Siemens PROFIBUS connector, switch ON at the first and last device). The maximum bus length depends on the configured baud rate:

Use PROFIBUS cable type A (Siemens 6XV1830-0EH10 or equivalent) with characteristic impedance 135 to 165 Ω, loop resistance ≤ 110 Ω/km, and capacitance ≤ 30 pF/m. Strip back the outer sheath and braid only over the length needed for the strain relief and shield clamp; never coil surplus cable. Maximum 32 stations per segment without repeater, maximum 126 stations overall with repeaters (e.g., 6GK1 500-3AA00 or 6GK1 500-0DA00). The CU240E-2 DP does not provide a repeater function; the integrated DP port of the S7-300 is a non-repeating master interface. For background on the protocol itself refer to the PI PROFIBUS technology page and the ISA Basics of PROFIBUS Operation (Chapter 1).

Hardware Requirements

PROFIBUS Address and Telegram Selection

The drive PROFIBUS node address is parameter P0918 (default 3, valid 1…125, with 0 reserved for service). Telegram type is P0922 (default 1 = Standard Telegram 1). The PLC programmer must select exactly the same telegram length in HW Config; mismatches are reported by SFC14 / SFC15 with RET_VAL W#16#80A2 (range error) or W#16#80B1 (length error).

For an application that requires start command, speed setpoint, speed feedback, and current feedback, the recommended selection is:

Standard Telegram 3 exposes ZSW1 (status word 1), NIST_A (speed actual, 16-bit), IAIST (current actual, 16-bit), and MIST (torque actual, 16-bit). IAIST is scaled with reference current P2002 (factory default = motor rated current r0305). One IAIST unit = P2002 / 4000h. MIST is scaled with reference torque P2003. The full parameter dictionary is in the SINAMICS G120 CU240E-2 List Manual.

Process Data Encoding

All PZD values are 16-bit signed integers transmitted in big-endian order on PROFIBUS. The reference normalization is documented in the SINAMICS G120 List Manual. The fundamental formula for setpoint and actual value interpretation is:

The PLC programmer treats the raw words as INT. P2000 is the reference speed in RPM (default = motor rated speed, p2000[0] = r0313 or 1500 RPM for a 50 Hz 4-pole motor). P2002 is the reference current in A (default = motor rated current r0305). P2003 is the reference torque in Nm (default = r0334 × r0335 = kT × I_rated).

Direction handling: NSOLL_A is a 16-bit two's-complement value where 4000h (16384 dec) = +100 % reference speed. Range 0000h … 7FFFh = positive direction, 8000h … FFFFh = negative direction. To prevent reverse operation, clamp the value to 0…7FFFh in the PLC. To enable reverse operation, set P1035 / P1036 bit 11 to allow STW1.14 = direction inversion, or set P1113 / P1114 direction enable bits to permit negative setpoints.

Control Word 1 (STW1) Bit Definitions

The minimum control word to run the drive is W#16#047E (bits 1, 2, 3, 4, 5, 6, 10 = 1 → drive ready to run, OFF1 active). To start, write W#16#047F (set bit 0). To acknowledge faults, set bit 7 to 1 momentarily then back to 0 (edge-triggered). The command to coast to stop is W#16#047C (bit 1 = 0). Quick stop (OFF3) is W#16#047A (bit 2 = 0).

Status Word 1 (ZSW1) Bit Definitions

State machine interpretation: ZSW1.0 = 1 means the drive is in "Ready to switch on". With STW1 bits 0, 1, 2, 3 = 1 and STW1.10 = 1, the drive transitions through "Switched on" (ZSW1.0 + ZSW1.1) to "Operation enabled" (ZSW1.0 + ZSW1.1 + ZSW1.2). The PLC must monitor ZSW1.3 for faults and ZSW1.6 for switching-on-inhibited (start-inhibit) before issuing a new start. The full state machine is documented in the SINAMICS G: Speed control of a G120 with S7-300 application example.

Drive Parameterization

Configure the G120 with the BOP-2 / IOP-2 or by writing parameters via PROFIBUS. The minimum parameter set for PROFIBUS speed control with current feedback is:

For commissioning details refer to the SINAMICS G120 Operating Instructions.

Installing the GSD File in STEP 7 V5.5

1. Download the latest GSD file from Siemens Product Support. For the CU240E-2 DP use the file SIEM8110.GSE (or the hardware-specific GSD for firmware V4.7 SP10 and later, e.g. SI01810B.GSE). The filename prefix indicates the manufacturer ID (SI = Siemens = 0x8110).
2. Open SIMATIC Manager → Options → Install GSD File. Point to the directory containing the GSD / GSE files. Click Install.
3. Open HW Config. The G120 appears under PROFIBUS DP → Additional Field Devices → Drives → SINAMICS → SINAMICS G120 CU240E-2 DP.
4. Drag the device into the PROFIBUS subnet. Set its PROFIBUS address to match P0918 (default 3). Open the device properties → DP slave properties → Standard / user-defined to confirm the slot assignment matches P0922.
5. Insert the configured telegram into slot 2 of the slave. STEP 7 V5.5 automatically derives the input / output length and aligns it with P2050 / P2051.
6. Save and compile (Station → Save and Compile). Download the HW Config to the CPU (PLC → Download).

PLC Programming with SFC14 / SFC15

SFC14 reads a defined length of consistent data from a DP slave into a data block. SFC15 writes a defined length of consistent data from a data block to a DP slave. The return parameter RET_VAL contains the DP error code (see Diagnostics section).

Data block for the G120 with Standard Telegram 3 (4 words input, 4 words output):

DATA_BLOCK DB100
  STRUCT
    // Output: PLC → drive
    STW1   : WORD;     // W#16#047E ready, W#16#047F start
    NSOLL_A: INT;      // 0…16384 = 0…+100 % P2000
    RES_OUT1: WORD;    // 0 (unused with Tel 3)
    RES_OUT2: WORD;    // 0
  END_STRUCT
END_DATA_BLOCK

DATA_BLOCK DB101
  STRUCT
    // Input: drive → PLC
    ZSW1   : WORD;
    NIST_A : INT;      // raw, scale by P2000/16384
    IAIST  : INT;      // raw, scale by P2002/16384
    MIST   : INT;      // raw, scale by P2003/16384
  END_STRUCT
END_DATA_BLOCK

Symbolic I/O addresses (per the HW Config slot start address, e.g. PE 256 / PA 256) and the call in OB1 (or OB35 for cyclic 100 ms task):

// Read drive telegram (Standard Telegram 3, 8 bytes input)
CALL "DPRD_DAT"   // SFC14
     LADDR  := W#16#100    // logical address of input slot 2 = 256
     RET_VAL := MW200      // error code
     RECORD  := P#DB101.DBX0 BYTE 8

// Write drive telegram (Standard Telegram 3, 8 bytes output)
CALL "DPWR_DAT"   // SFC15
     LADDR  := W#16#100    // logical address of output slot 2 = 256
     RECORD  := P#DB100.DBX0 BYTE 8
     RET_VAL := MW202      // error code

// Compose STW1
// bits required: 1, 2, 3, 4, 5, 6, 10 = 1
// STW1_base = W#16#047E
A   "DB_Start_Cmd"
S   "DB_Start_Bit"
R   "DB_Start_Bit" after 1 scan  // one-shot

// Speed setpoint: convert engineering RPM → INT
L   "DB_Setpoint_RPM"      // REAL, RPM
L   "DB_P2000"             // REAL, P2000
/R   
L   16384.0
*R   
T   "DB100".NSOLL_A        // INT

For one-shot starting, generate a rising edge on STW1.0 from a pushbutton or operator tag, OR-merge the start bit into the base STW1, and avoid leaving STW1.7 = 1 indefinitely (the fault acknowledge bit must be a 0 → 1 edge, not a level).

Reading speed in RPM and current in A:

L   "DB101".NIST_A        // INT
ITD
DTR
L   "DB_P2000"             // REAL
*R   
L   16384.0
/R   
T   "DB_Speed_Act_RPM"     // REAL

L   "DB101".IAIST          // INT
ITD
DTR
L   "DB_P2002"             // REAL
*R   
L   16384.0
/R   
T   "DB_Current_Act_A"     // REAL

Watchdog: call SFC14 / SFC15 only from OB1 if the process I/O update time is sufficient, or from OB35 at 100 ms. PROFIBUS DP cycle time at 1.5 Mbit/s for one slave with 8 bytes in / 8 bytes out is approximately 1 ms. Going below 1 ms of PLC cycle can starve the DP bus of access time, especially when multiple slaves share the segment.

Commissioning Procedure

1. Mechanical and electrical pre-checks. Verify the cabinet earth, the motor insulation (1 kV megger for a 400 V motor, all phases to earth), and the line voltage at the PM240-2 line-side terminals. Measure 24 V at the CU240E-2 X12 terminals 1 (+24 V) and 2 (GND); voltage must be 20.4 V … 28.8 V.
2. PROFIBUS wiring. Wire both 9-pin connectors; turn on the termination switch on the first and last device. Verify the shield is clamped at every cabinet entry. Connect the PLC PG cable only when the system is de-energised.
3. Drive quick commissioning (local). Set P0010 = 1, motor data, P1900 = 2, P0010 = 0. Run the motor once locally via the BOP-2 / IOP-2 jog to confirm direction of rotation. Set P0015 = 7, P0918 = 3, P0922 = 3, P2040 = 100, P0970 = 1 (save). Cycle 24 V off / on.
4. PLC configuration. Install the GSD, configure the slave at address 3 with Standard Telegram 3, compile, download HW Config.
5. PROFIBUS diagnostics. In STEP 7: PLC → PROFIBUS → Diagnose. Confirm the G120 is online (green). Open the slave diagnostic buffer to read any DP errors (see Diagnostics table).
6. First PLC control. In OB1, write STW1 = W#16#047E, NSOLL_A = 0. Read ZSW1 and verify bit 1 = 1 (Ready). Set STW1 = W#16#047F (bit 0 = 1) and ZSW1.2 should go to 1 within 5–10 ms. Ramp NSOLL_A from 0 to 4000h over P1120 seconds.
7. Verify current feedback. Read IAIST, scale to A. At rated speed and no load, IAIST × P2002 / 16384 should be ≤ 30 % of r0305. Under load, current should rise to rated within P1522 / P1523 torque limit envelope.
8. Safety checks. Wire the hardware STO inputs (CU240E-2 DP-F variant) or use the hardwired OFF2 / OFF3 terminals for non-safe emergency stop. PROFIBUS-controlled OFF2 / OFF3 is not safety-rated; route the safety chain through the dedicated terminals. The full safety chain is documented in the SINAMICS G120 Function Manual Safety Integrated.

Diagnostics and Common Fault Codes

When a fault occurs, SINAMICS G120 stores it in parameter r0947[0…7] (current fault code at index 0, history at higher indices) and r2131 (fault number). The PLC reads the fault code via the parameter channel (acyclic DP-V1 read of r0947) or, in telegram 20, the dedicated FAULT_CODE word. PROFIBUS-related faults use the F19xx range.

PROFIBUS diagnostic block in SFC14 / SFC15 RET_VAL (hex):

Troubleshooting matrix:

FAQ