Implementing S7-300 Master-Slave VFD Speed Control via PROFIdrive

Overview

Coordinated multi-drive systems — such as a four-motor conveyor feeding a compactor — require a central "master" reference that caps or scales the speed of every Variable Frequency Drive (VFD) simultaneously, while each individual drive still accepts its own ratio setpoint. On a Siemens SIMATIC S7-300 controller communicating to PROFIdrive-compliant drives over PROFIBUS DP, the cleanest implementation places the master/individual scaling math in the PLC, sends the resulting normalized setpoint in a Standard Telegram 1 PZD frame, and lets the drive perform its own ramp acceleration and deceleration per its parameter set.

This reference documents the architecture, the PROFIdrive PZD structure, the TIA Portal hardware configuration, the LAD and SCL code for the master multiplier, drive-side parameter assignments, HMI integration, and the commissioning procedure required to bring the system online and validate it.

Architecture and Operating Principle

The application has four VFDs feeding material to a compactor. Each VFD has its own HMI-entered setpoint expressed as a percentage of rated speed (0–100 %), which is mapped by the drive to 0–60 Hz internally. A master percentage (also 0–100 %) limits the entire feeding system. The final per-drive frequency reference is:

Drive_Setpoint_Hz = (Master_% ÷ 100) × (Drive_Individual_% ÷ 100) × Max_Hz

For example, with master = 50 %, drive 1 individual = 80 %, and a 60 Hz base, drive 1 runs at 24 Hz. Drive ramping (accel/decel ramps) is handled inside the drive; the PLC only changes the target reference and the drive's internal ramp generator produces a controlled transition.

Why Centralize the Math in the PLC

Three reasons favor calculating the final reference in the PLC rather than cascading analog signals between drives:

• Determinism: PROFIBUS DP cycles every few milliseconds with a fixed update rate per slave; analog cascade bandwidth depends on the slowest drive and wiring.
• Diagnostics: Each drive returns ZSW1 and NIST_A so the HMI can show the actual master-scaled speed per drive and raise alarms if a drive is not tracking.
• Engineering flexibility: The master/individual formula can be modified, inverted, or replaced with a ramped profile without rewiring.

Prerequisites

PROFIdrive Profile and PZD Telegram Structure

The PROFIdrive profile defines the cyclic I/O between the PROFIBUS master and the drive. The drive's GSD exposes standard telegrams the master selects.

Standard Telegram 1 PZD Layout

Refer to the PROFIdrive Profile Drive Technology (PNO) specification for the complete bit-level definition.

Control Word 1 (STW1) Key Bits

Speed Setpoint / Actual Value Normalization

NSOLL_A and NIST_A are 16-bit signed values normalized as:

Normalized_Value = (Percent_of_Rated_Speed) × 163.84

Because 100 % = 4000h = 16384 decimal, 50 % maps to 2000h = 8192 decimal. The valid range is C000h to 4000h (representing −200 % to +200 % of rated speed); values outside are clipped by the drive.

TIA Portal Hardware Configuration

1. Add the S7-300 CPU with a DP master interface in the project view. Place the rack in slot 1 and the CPU in slot 2.
2. Configure the DP master port: right-click the DP interface → Properties → PROFIBUS settings. Set the master address (default 2), baud rate (1.5 Mbit/s recommended for ≤ 200 m cable runs).
3. Install the GSD for each VFD: Options → Manage device description files (GSD). Select the .gsd or .gsdml file shipped with the drive. Restart TIA Portal if prompted.
4. Add drives to the DP master system: drag the VFD from the hardware catalog onto the DP master system. Assign PROFIBUS addresses 3, 4, 5, 6 (one per drive). Match these with the drive's hardware DIP switches.
5. Select the telegram: in the device view of each drive, slot 0 typically shows the telegram selection. For Standard Telegram 1, the slot configuration becomes 2 PZD words in / 2 PZD words out.
6. Note the I/O addresses the wizard assigns (e.g., Drive 1 = IW 256 / QW 256; Drive 2 = IW 258 / QW 258). Record these for the program.

PLC Program Structure

Recommended structure for a four-drive master-slave system:

Data Block Layout (DB 100)

DATA_BLOCK "DB_VFD_Coord"
TITLE = VFD Coordination Data
VERSION : 0.1
  STRUCT
    MasterPct : REAL;        // 0.0 - 100.0, from HMI
    Drive1_IndividualPct : REAL;  // 0.0 - 100.0, from HMI
    Drive2_IndividualPct : REAL;
    Drive3_IndividualPct : REAL;
    Drive4_IndividualPct : REAL;
    Drive1_CombinedPct : REAL;    // computed
    Drive2_CombinedPct : REAL;
    Drive3_CombinedPct : REAL;
    Drive4_CombinedPct : REAL;
    Drive1_NSOLL : INT;       // normalized 0 - 16384 (0x0000 - 0x4000)
    Drive2_NSOLL : INT;
    Drive3_NSOLL : INT;
    Drive4_NSOLL : INT;
    Drive1_STW : WORD;        // control word to drive 1
    Drive2_STW : WORD;
    Drive3_STW : WORD;
    Drive4_STW : WORD;
    Drive1_ZSW : WORD;        // status word from drive 1
    Drive2_ZSW : WORD;
    Drive3_ZSW : WORD;
    Drive4_ZSW : WORD;
    Drive1_NIST : INT;        // actual speed normalized
    Drive2_NIST : INT;
    Drive3_NIST : INT;
    Drive4_NIST : INT;
  END_STRUCT;
END_DATA_BLOCK

Master-Slave Setpoint Calculation Logic

The PLC multiplies the master percentage by the per-drive individual percentage, producing a combined setpoint. This is then normalized to the PROFIdrive range and written to the NSOLL_A word.

SCL Implementation (FC 10)

FUNCTION "FC_CalcCombinedSetpoint" : VOID
TITLE = Calculate combined setpoint for one drive
VAR_INPUT
    iMasterPct        : REAL;   // 0.0 - 100.0
    iIndividualPct    : REAL;   // 0.0 - 100.0
END_VAR
VAR_OUTPUT
    oCombinedPct      : REAL;   // 0.0 - 100.0
    oNSOLL_Normalized : INT;    // 0 - 16384 (0x4000h = 100 %)
END_VAR
BEGIN
    // Clamp inputs
    IF iMasterPct < 0.0   THEN iMasterPct := 0.0;   END_IF;
    IF iMasterPct > 100.0 THEN iMasterPct := 100.0; END_IF;
    IF iIndividualPct < 0.0   THEN iIndividualPct := 0.0;   END_IF;
    IF iIndividualPct > 100.0 THEN iIndividualPct := 100.0; END_IF;

    // Combined percentage = master × individual / 100
    oCombinedPct := (iMasterPct * iIndividualPct) / 100.0;

    // Convert to PROFIdrive normalized: 100 % = 16384
    oNSOLL_Normalized := REAL_TO_INT(oCombinedPct * 163.84);

    // Clamp to valid range
    IF oNSOLL_Normalized < 0      THEN oNSOLL_Normalized := 0;      END_IF;
    IF oNSOLL_Normalized > 16384  THEN oNSOLL_Normalized := 16384;  END_IF;
END_FUNCTION

LAD Equivalent (One Drive)

For engineers who prefer ladder, the equivalent ladder diagram is shown below as an inline styled SVG. Inputs are DB100.DBX0.0 (Master INT), DB100.DBW2 (Drive1 individual INT). Output is DB100.DBW20 (Drive1 NSOLL INT).

Drive Control Word Packing and Dispatch (FC 11)

Each drive requires its STW1 to be written to QW 256, QW 258, QW 260, QW 262 respectively. A typical STW1 word for "Drive running, controlled by PLC, ramp enabled, setpoint enabled" is:

STW1 = 0x047F = 0000 0100 0111 1111b

(Bit 10 = control by PLC, bit 6 = setpoint enable, bit 3 = enable operation, bit 2 = OFF3 inactive, bit 1 = OFF2 inactive, bit 0 = ON.)

LAD Example: Write Control Word and Setpoint to One Drive

NETWORK 1  // Drive 1 STW
      A   "Start_Cmd"          // global run command
      =   L 0.0
      AN  "Fault_Active"
      =   L 0.1
      O   "Fault_Ack_PB"
      =   L 0.2

      SET
      =   L 0.3

      // Pack STW1 = 0x047F when running, 0x047E when stopped
      L   W#16#047F
      T   "DB_VFD_Coord".Drive1_STW

NETWORK 2  // Drive 1 NSOLL dispatch
      L   "DB_VFD_Coord".Drive1_NSOLL
      T   PQW 256

Reading Back Status and Actual Speed (FC 12)

NETWORK 1  // Drive 1 ZSW read
      L   PIW 256        // status word from drive 1
      T   "DB_VFD_Coord".Drive1_ZSW

NETWORK 2  // Drive 1 NIST actual speed
      L   PIW 258
      T   "DB_VFD_Coord".Drive1_NIST

NETWORK 3  // Convert NIST to percent
      L   "DB_VFD_Coord".Drive1_NIST
      ITD
      DTR
      L   1.6384e+04
      /R
      T   "DB_VFD_Coord".Drive1_ActualPct

Drive-Side Parameter Configuration

Each drive must be told to expect PROFIdrive telegrams. The exact parameter depends on the drive vendor:

HMI Integration

Tags to Expose

Use WinCC Comfort or WinCC Professional with standard Siemens HMI Operating Manual conventions. Map REAL tags to a slider with limit value 100.0 and a process value limit indicator; tag scaling is direct.

Commissioning Procedure

1. Hardware pre-check. With power OFF, measure PROFIBUS terminator resistance across the cable end pins: should read 220 Ω at each end with no device connected in the middle.
2. Bring up the PLC. Connect STEP 7 to the CPU via MPI/PN. Download hardware configuration only — do not download program yet.
3. Verify PROFIBUS in TIA Portal → Online → Accessible nodes. Confirm all four VFDs appear with their assigned addresses.
4. Set drive PROFIBUS address via p0918 (Siemens) or equivalent parameter, then cycle drive power.
5. Configure telegram. Select Standard Telegram 1 in each drive's p0922 (Siemens) or equivalent.
6. Force an STW1 of 0x047E (OFF1 true, no run) from a watch table and verify the drive echoes ZSW1 with bit 0 = "ready to switch on" and bit 1 = "ready" set.
7. Force NSOLL = 0x1000 (25 %) in the same watch table and confirm NIST_A echoes a value close to 0x1000 when the motor is rotating.
8. Disable drive enable (STW1 bit 3 = 0), download the full PLC program, then go online and toggle bit 3 — motor should run only when STW1.3 = 1.
9. Test the master multiplier: set master = 25 %, drive 1 individual = 50 %. Verify drive 1 frequency = 7.5 Hz on a clamp meter or in the drive's monitoring parameter (e.g., r0021 on SINAMICS).
10. Test ramp behavior: change master from 0 % to 100 % in steps. Confirm drive internal ramp time (p1120 on Siemens) governs the transition, not the PLC cycle.
11. Test fault propagation: force a fault on drive 3 via a drive-side test command. Verify ZSW1 bit 3 sets in DB100 and OB 86 / OB 82 report the diagnostic event in the PLC diagnostic buffer.
12. Validate HMI: confirm all sliders move the corresponding DB values and the read-back frequencies match clamp-meter readings within ±2 %.

Verification and Diagnostics

Online Watch Tables

Create two watch tables in TIA Portal:

• VT_Setpoints: Monitor DB100.DBD0 (Master), DB100.DBD8 (Drive1 ind), DB100.DBD24 (Drive1 combined), DB100.DBW20 (Drive1 NSOLL hex). Expected: NSOLL ≈ combined × 163.84.
• VT_DriveStatus: Monitor PIW 256/258/260/262 (ZSW1) and PIW 258/260/262/264 (NIST_A). Expected: ZSW1 bit 0 = 1 when running; NIST_A ≈ NSOLL when at setpoint.

Diagnostic Buffer

The PLC diagnostic buffer (STEP 7 → Online → Diagnostics → Diagnostic Buffer) will log PROFIBUS slave diagnostics as OB 82 events and rack failures as OB 86. The drive's fault code (e.g., F07900, F08501) is typically surfaced through the drive's parameter r0947 and can be read by adding parameter access via PROFIBUS DPV0 acyclic calls or via PROFIdrive parameter channel (PKE/IND/PWE in Telegram 4 or 7).

Troubleshooting Matrix

Edge Cases and Field Tips

Master reduced below individual setpoint: If the master falls below 50 % while a drive's individual setpoint is 80 %, the combined reference becomes 40 %. Because the drive's internal ramp generator follows the new reference, the frequency transitions smoothly. No special PLC logic is required.

Reverse direction: If reverse is required, drive direction is encoded as the sign of NSOLL_A (negative values). Extend the calculation:

NSOLL_signed := CombinedPct × Direction (-1 or +1) × 163.84

The direction signal comes from a separate bit in STW1 (bit 11 in some implementations) or from a separate output word. Verify with the drive's PROFIdrive mapping.

Different base frequencies per drive: If drives 3 and 4 are 50 Hz while drives 1 and 2 are 60 Hz, replace the single combined percentage with a per-drive scaled REAL using the drive's base:

Drive1_Hz := CombinedPct × 60 / 100

Normalize to NSOLL using each drive's own p2000 / base frequency. This keeps the math in the PLC consistent while letting the drive apply its own ramp and limits.

Fault handling across the group: Tie STW1 bit 0 (ON/OFF1) for all four drives to a single system-start bit in the PLC. Drive-specific faults can be read from ZSW1 bit 3; on detection, clear STW1.0 for that drive alone while letting the others run, or stop the entire system depending on process requirements.

PROFIBUS cycle time vs. PLC scan time: PROFIBUS DP cycles are configured per slave (typically 2–4 ms). The PLC OB 1 cycle must be faster than the PROFIBUS cycle, or output PZDs may be overwritten by stale values. Confirm OB 1 priority and cycle in TIA Portal diagnostics.

Replacing the S7-300 with S7-1500: The same PROFIdrive logic applies; the S7-1500 can use the same Telegram 1 PZD structure. The block library changes (DB/FB) but the control word and setpoint normalization remain identical. Consult the S7-1500 Motion Control Function Manual for migration.

Frequently Asked Questions