Resolving Masterdrive MC AC Drive Not Running from S7-300

1. Problem Overview

A mixed-drive Profibus installation pairs a Siemens SIMOREG DC 6RA70 with a Siemens SIMOVERT Masterdrives MC (6SE70 series) AC/AC drive on the same Profibus-DP network, controlled by a Siemens SIMATIC S7-300 CPU. In the field scenario described, the DC 6RA70 starts, references, and runs normally from the PLC, while the Masterdrive MC AC drive controlling a traverse motor refuses to run. No fault or alarm is visible on either the PLC diagnostic buffer or the drive PMU (Parameterization Unit). The mechanical side is verified intact. This article isolates that exact failure mode and walks through every layer of the diagnostic stack from the physical copper up to the application control word.

2. System Topology and Signal Flow

The configuration under test is a single-master Profibus-DP segment with three nodes: the S7-300 CPU acting as Class-1 master, the SIMOREG DC 6RA70 with a CBP2 communication board (running normally), and the Masterdrive MC 6SE70 AC/AC unit fitted with its own CBP2 (Communication Board Profibus) option. Both drives exchange Process Data Objects (PZD) and Parameter Data (PKW) over Profibus-DP V0/V1. The PLC issues a 16-bit control word (STW1) and main setpoint (HSW) in the outgoing PZD area and reads a 16-bit status word (ZSW1) and actual value (HIW) in the incoming PZD area.

The drive accepts commands only when its parameterization layer (the source for control-word bits) routes those bits to the CBP PZD words. On Masterdrive MC the parameterization is enabled by P060 = 8 (parameter menu selection: parameter set) and the relevant control-word source parameters must point at the CBP channel.

3. Safety and Prerequisites Before Diagnosis

1. Lock out and tag out the AC drive's main contactor; observe the five safety rules (disconnect, secure against re-energizing, verify absence of voltage, ground and short, cover adjacent live parts) before opening the drive cabinet.
2. Wait the manufacturer-specified discharge time on the DC link (typically 5 minutes for a Masterdrive MC, 6 minutes for a 6RA70) before removing any module.
3. Confirm you have a DriveMonitor / SIMOVIS or Starter commissioning cable to go online with the Masterdrive MC.
4. Confirm you have the STEP 7 project (or at minimum the GSD file SIEM804D.GSD for Masterdrive MC PPO3 or PPO1, and SIMOREG.GSD or SIEM808F.GSD for the 6RA70) loaded in HW Config.
5. Bring a PG/PC with STEP 7 V5.x or TIA Portal plus the SIMATIC USB PC adapter or CP5512/CP571 ready.

4. Step 1 — Verify Profibus Hardware Integrity

Even though the 6RA70 is operating, segment integrity for the Masterdrive MC slave should still be confirmed, because a single slave with a poor connection or wrong address does not necessarily kill the bus.

4.1 Physical layer

• Inspect the Profibus D-sub connector on the Masterdrive MC CBP2: verify pins 3 (B-line) and 8 (A-line) are intact, shield is bonded, and the connector's terminating resistor switch is OFF if a downstream node is present.
• Confirm the segment has exactly two terminating resistors enabled (master end and last slave end). With three nodes including the 6RA70 and Masterdrive MC, only the physical ends are terminated.
• Measure cable length: Profibus-DP at 1.5 Mbaud supports up to 200 m of cable per segment.
• Check shield bonding at each cabinet entry — shield must be clamped to PE on both ends, ideally through 360-degree EMC backshells.

4.2 CBP2 LED status

The CBP2 board on a Masterdrive MC has three diagnostic LEDs whose state tells you immediately whether the slave is alive on the bus:

5. Step 2 — Validate PLC Master Configuration and PPO Type

5.1 Slave address and slot

Open the STEP 7 HW Config and locate the Masterdrive MC slave on the Profibus subnet. Verify:

• Profibus address matches the DIP switch setting on the CBP2 board (parameter P918 on the drive side must agree).
• The slave is not disabled (the "Activate" checkbox is ticked in HW Config).
• The GSD file imported for the Masterdrive MC corresponds to the actual firmware revision. The classic files used for 6SE70 are SIEM804D.GSD and SIEM804F.GSD; using the wrong one causes parameterization to fail silently with F0191 or F0192 on the drive.

5.2 PPO type selection

Masterdrive MC supports PPO1 (4 PKW + 2 PZD), PPO3 (0 PKW + 2 PZD), and PPO4 (0 PKW + 6 PZD). The most common production configuration is PPO3 for two-word cyclical exchange without parameter channel. PPO1 is used when the application needs read/write of drive parameters at runtime. The 6RA70 typically uses PPO3 or PPO5 depending on configuration.

5.3 I address mapping in the S7 program

For a PPO3 Masterdrive MC at address 6, the inputs and outputs in the STEP 7 symbol table typically appear as:

• QW 256 — STW1 (control word 1) to Masterdrive MC
• QW 258 — HSW (main setpoint, N-setpoint) to Masterdrive MC
• IW 256 — ZSW1 (status word 1) from Masterdrive MC
• IW 258 — HIW (main actual value) from Masterdrive MC

Compare these addresses against the actual MOVE / SFC14 / SFC15 / DPRD_DAT / DPWR_DAT calls in OB1 or OB35. A swapped slave address between the 6RA70 (e.g., address 5) and the Masterdrive MC (e.g., address 6) at this stage is a classic cause of "command sent to wrong drive, drive runs the wrong motion". If addresses are accidentally duplicated in HW Config, the PLC will report a duplicate address at startup.

6. Step 3 — Inspect CBP LED Status and Drive-Side Parameters

6.1 Drive-side Profibus parameters

With DriveMonitor connected to the Masterdrive MC via the serial commissioning interface, navigate to the parameter set and confirm:

6.2 Setpoint routing

Even when control-word routing is correct, the main setpoint must also be wired to the CBP word. If P443 = 0 (analog input) and the analog input is open, the drive will run at 0 Hz and appear "not running". Confirm P443 = 5002 for the standard profile configuration with a CBP2 PZD2 setpoint, or use the profile-specific setpoint P443 = 5005 when following the Profidrive profile mapping.

7. Step 4 — Monitor Control Word and Status Word Live

Use a VAT (Variable Table) in STEP 7 online, or the online monitoring in TIA Portal, to force and observe the four PZD words of the Masterdrive MC. The objective is to confirm that the bytes the PLC sends are actually arriving at the drive and that the drive's response status word reflects the command sequence.

7.1 Reference STW1 sequence

Per the Profidrive profile adopted by Siemens drives (and replicated in the SIMOVERT Masterdrives MC sequencer), a 6RA70 and a Masterdrive MC start command is typically the sequence shown below. Bit numbers are 0-based:

7.2 Status word interpretation

With the control word forced to 0x047F, observe the ZSW1 response from the drive. If ZSW1 remains at 0x0000 or 0x0031 ("Switch-on inhibit"), the sequencer has not accepted any of the control bits — almost always a control-word source mismatch (Section 6).

7.3 PMU observation

The PMU (front-panel display) on a Masterdrive MC shows the active parameter number and value. Cycle to r011 (operating display 1) and r012 for current setpoint/actual in Hz, and to r060 / r065 for active control word and status word bits. This is the fastest method to confirm that the CBP PZD are actually arriving at the drive when you do not have a VAT handy.

8. Step 5 — Common Root Causes Ranked

9. Step-by-Step Resolution Procedure

1. Open STEP 7 online with the S7-300 CPU. Use VAT1 to force the Masterdrive MC's STW1 to 0x047F and HSW to 0x2000 (50% of normalized reference). Read back ZSW1 and HIW. Note whether the CBP LED on the Masterdrive MC turns green and stays green.
2. Check the PLC diagnostic buffer for entries tagged to the Masterdrive MC slave address. Look for "Slave failure", "Parameterization error", "Configuration error", or "Static diagnostics". Each entry points to a specific cause.
3. Connect DriveMonitor to the Masterdrive MC X300 serial port with the SSK2 cable. Go online, select parameter set, and verify parameters P918, P927, P554–P560, P443, P590, P572.
4. Inspect the live control word on the PMU at r060. If it does not reflect the forced value from the PLC, PZD are not being received; revisit bus and slave address. If it does reflect 0x047F, the bus is healthy; the issue is internal to the drive sequencer.
5. Inspect live status word at r065. If bit 6 = 1 (Switch-on inhibit), one of the OFF conditions is active. If bit 0 = 0, STW1 bit 0 (ON) has not been accepted; control-word routing is wrong.
6. Read the fault buffer by accessing r947 (active fault), r949 (fault history). Common faults: F001 overcurrent, F002 overvoltage, F006 heatsink overtemperature, F011 overload, F015 motor stalled, F023 power section overtemperature, F030 commutation, F031 encoder. Acknowledge with a 0→1 edge on STW bit 7.
7. Re-route the control-word source parameters if needed. Save the parameter set with the PMU sequence: P052 = 1 (save in EEPROM), then P053 = exit.
8. Re-run the start sequence from the PLC program. Confirm ZSW1 transitions: 0x0031 (Ready to switch on) → 0x0033 (Ready to operate) → 0x0037 (Operation enabled) → 0x0237 (Setpoint reached).
9. Verify rotation direction with a low setpoint (e.g., 0x0500 = 5% of nmax). Use the local emergency stop at the cabinet. Confirm that the traverse motor accelerates smoothly. Reverse test with 0x0000 setpoint and toggling bit 11 of STW1.
10. Capture a DriveMonitor trace of P443 input, actual frequency r003, control word r060, status word r065 for 10 seconds at full speed. Archive for warranty / reference.

10. Verification and Commissioning Checks

Before returning the line to production, complete a structured commissioning checklist:

• Steady-state run test: With ZSW1 = 0x0237, hold setpoint for 10 minutes. Confirm no fault escalation, no DC link overcurrent, no thermistor trip.
• Direction reversal test: Alternate STW bit 11 at zero setpoint, verify ZSW reflects "setpoint-actual deviation" only during ramps.
• Stop categories: Issue OFF1 (STW bit 0 = 0), confirm ramp down with P464 deceleration. Issue OFF2 (STW bit 1 = 0), confirm coast stop within P258. Issue OFF3 (STW bit 2 = 0), confirm quick stop with P466. Each must transition ZSW1 to the corresponding state within one cycle.
• Fault injection test: Briefly enable the encoder-fail simulation or motor-stall test (with motor uncoupled if possible). Confirm PLC receives ZSW1 bit 3 = 1 (fault present) and the fault code over PKW or status bits.
• Parameter consistency check: Use DriveMonitor "Compare parameter set" between offline project and drive EEPROM. Resolve any deviation with P052 = 1 to save or with P053 = 4 to restore from drive.
• Documentation: Update the plant asset record with the CBP2 firmware version, drive firmware version, PPO type, Profibus address, and the saved parameter-set checksum.

11. Profibus-Specific Pitfalls with Mixed DC/AC Drive Populations

When the DC 6RA70 and the Masterdrive MC coexist on the same Profibus-DP master, several subtle issues emerge that single-drive installations never expose:

12. Frequently Asked Questions