MICROMASTER 420 PROFIBUS GSD: Resolving SF and BF Errors

Problem Summary: MICROMASTER 420 PROFIBUS Faults on SIMATIC S7

A SIMATIC S7-300 station running STEP 7 V5.5 (Simatic Manager) is configured with 32 PROFIBUS DP nodes: nine IM 151-1 distributed I/O stations and twenty-three Siemens MICROMASTER 420 inverters (e.g., 6SE6420-2UD21-5AA0). When the project is downloaded and the bus is brought online, every MM420 reports a BF (Bus Fault) and the DP master reports SF (System Fault) on the affected slaves. The IM 151 stations all come up clean. Re-importing the GSD file SI0280B5.GSD from the Siemens support entry 6567719 still produces a save/compile error when the MM4 is inserted at PROFIBUS address 36, and a phantom DP slave at station 36 has to be deleted before the new slave can be added.

This article documents the root cause (an address overlap between the MM420 and an existing phantom slave), the correct MICROMASTER 4 GSD selection for STEP 7 V5.5, and the full STEP 7 V5.5 configuration and verification procedure.

Identifying the MICROMASTER 420 Variant

Before touching the GSD file, confirm the inverter's exact order number. The MM420 uses Siemens' MLFB (order) code, which encodes the voltage class, power rating, and filter option. Example: 6SE6420-2UD21-5AA0 decodes as:

Correct GSD File for MICROMASTER 4 on STEP 7 V5.5

There is exactly one currently supported GSD for the MICROMASTER 4 family (MM411/420/430/440) under PROFIBUS DP-V0/V1: SI0280B5.GSD, supplied in the archive si0280b5-gsd.zip via Siemens support entry 6567719 — MICROMASTER 4: Amended PROFIBUS GSD files. The "B5" suffix indicates revision 5, which is the amended/corrected variant required for STEP 7 V5.4 SP5 and later, including V5.5.

GSD specification summary:

Install the GSD into STEP 7 V5.5 via HW Config → Options → Install GSD File… and point to the extracted SI0280B5.GSD plus the bitmap file SI0280B5.BMP. After installation, restart HW Config. The MM4 will appear under PROFIBUS DP → SIMOVERT → MICROMASTER 4. If only MICROMASTER 420 is shown (not MICROMASTER 4), an older GSD revision is loaded; delete it from Options → Install GSD File… → Remove and reinstall the B5 file.

Hardware Configuration in STEP 7 V5.5

Bus topology and termination

PROFIBUS DP supports 32 nodes per segment without repeater. The reported network has 32 nodes plus the master, exceeding the 32-node segment limit and therefore requires a repeater (RS-485) or OLM to extend to a second segment. The user reports the first and last physical devices are IM 151 stations and the bus terminator is on at both ends — this is correct, but verify each PROFIBUS connector has its terminator switch OFF for the intermediate nodes and ON for the two end nodes.

Slot configuration for the MM420

Drag the MICROMASTER 4 onto the PROFIBUS line at the correct station address (set the same value on the drive's rotary switches). Open the slave's slot table and select a PZD telegram:

For most MM420 applications, PPO 3 (4 PKW / 2 PZD words = 0/2) is the smallest and fastest. PPO 1 is the default selected by the GSD. Reserve I/O addresses that do not overlap with the IM 151 stations. The STEP 7 I/O address assignment in the user program must match what is configured in HW Config — for example, MM420 at address 36 would reserve input bytes 36–39 and output bytes 36–39 for PPO 3.

Diagnostic Interpretation: BF and SF

STEP 7's Module Information (online → DP slave → Module Information) decodes PROFIBUS diagnostics. For an MM420 returning BF and SF simultaneously, the most relevant standard DP diagnostic bytes are:

If the SF lamp on the master is on and the BF lamp on the slave is on, byte 0 bit 2 is set: the slave is configured but not in Data_Exchange. The most common reasons in this scenario are (a) station address mismatch between HW Config and the drive's rotary switches, (b) two slaves configured with the same PROFIBUS address, or (c) a phantom/stale DP slave entry in the S7 project blocking the address slot.

Root Cause: Address Conflict at Station 36

The reported fault — every MM420 in BF, IM 151 in Data_Exchange, and a diagnostic snapshot showing station 36 — is a classic PROFIBUS address conflict. The GSD import did not fail; rather, the MM420 was added at the same station number (36) as an existing or orphaned DP slave in the HW Config tree. STEP 7 V5.5 will accept the second insertion silently until the project is saved and compiled, at which point it returns the dialog "The selected DP slave cannot be inserted; address already in use."

When the user opened the hardware catalog and selected PROFIBUS DP → SIMOVERT → MICROMASTER 4, the slave appeared on the bus only after the conflicting entry at address 36 was removed. The fix is therefore not the GSD file itself; the B5 GSD is the correct revision. The fix is removing the duplicate address.

Step-by-Step Resolution

1. Verify the GSD is the amended B5 revision. Open Options → Install GSD File…; if more than one MICROMASTER entry exists, remove the older ones and reinstall from support entry 6567719.
2. Go online and read every DP station's actual address. In HW Config, PLC → PROFIBUS → Diagnose. Record every address that responds, including the MM420s (which will appear with BF).
3. Compare against the configured addresses in HW Config. Locate any configured slave whose station number matches an address already used by a physical device or by another configured slave. In this case, station 36 is occupied by a stale or duplicate entry.
4. Delete the stale/duplicate DP slave at address 36 from the bus line in HW Config. Use Right-click → Delete.
5. Insert the MICROMASTER 4 from the catalog PROFIBUS DP → SIMOVERT → MICROMASTER 4 at the now-free address. Assign a unique address that matches the rotary switches on the drive (e.g., 3, 4, 5 … 25 for 23 drives on a separate sub-network).
6. Configure the slot (PPO type, PKW/PZD lengths). Defaults from the GSD are PPO 1; select PPO 3 if you do not need parameter channel access.
7. Save and compile the HW Config. The save/compile error will be gone once the duplicate is removed.
8. Download the hardware configuration to the CPU (target system → CPU 31x → Download). Bring the CPU to RUN with all drives powered and PROFIBUS terminated correctly at both ends.
9. Verify every MM420 transitions out of BF by checking Module Information → Operating Mode → Data Exchange for each slave.
10. Address the second reported fault (a single slave still showing SF): check the Details button on the save/compile error for that slave. The user's follow-up confirmed this was also an address error (a different slave, duplicate address). Apply steps 3–8 to that node as well.

Drive-Side PROFIBUS Parameters on the MM420

To allow the S7 master to control the drive, the following MM420 parameters (accessed via the basic operator panel, BOP, or Starter/STARTER) must match the HW Config telegram:

Verification Procedure

1. All MM420 stations show Data Exchange in Module Information → Operating Mode; BF LEDs off on every drive.
2. CPU diagnostic buffer (online → CPU → Diagnostic Buffer) shows no PROFIBUS-related events after the download.
3. A short S7 program toggles control word bit 0 (OFF1) and writes a setpoint; the drive runs and reports actual speed on the read-back PZD. Example ST snippet:

   // Inputs from MM420 at IB 36 (status word + actual value for PPO 3)
   L IB 36       // Status word (r0052)
   T MW 100      // Copy to flag word for diagnostics
   L IB 38       // Actual frequency (r0021, HIWORD)

4. Use Starter or DriveMonitor to read parameter r0021 (output frequency) and confirm it follows the setpoint written by the S7 program.
5. Cold-restart the CPU and re-power the DP segment; all 32 nodes (9 × IM 151 + 23 × MM420) must come up in Data_Exchange within ~5 seconds at 1.5 Mbit/s.

Common Field Faults and Fixes

Firmware and Compatibility Notes

The MICROMASTER 420 firmware versions known to operate correctly with SI0280B5.GSD are V1.10 through V3.20. Earlier firmware (V1.05 and below) used the obsolete GSD SI_02804.GSD (revision 4) and exhibits the same PPO mismatch faults even when correctly addressed. Use Starter or the BOP to read drive parameter r0018 for the firmware version. If the drive is below V1.10, upgrade with the appropriate firmware update tool (e.g., Siemens support firmware delivery via RS-232 and DriveMonitor). The amended B5 GSD is backward compatible with all MM420 firmware revisions that support PROFIBUS (i.e., have the PROFIBUS module fitted).

For PROFIBUS conformance and the standard meaning of the diagnostic bytes above, refer to IEC 61158 / EN 50170 (PROFIBUS standard), and for the GSD specification language itself refer to profibus.com GSD specification.

Safety and Commissioning Checks

• Confirm the Safe Stop (STO) function on the MM420 is not being used over PROFIBUS — the MM420 does not support PROFIsafe. The drive is therefore not suitable for SIL 2/3 safety functions; use a SINAMICS drive with PROFIsafe for those requirements.
• Set drive parameter P2040 (process data monitoring time) to a value ≤ the PLC's PROFIBUS watchdog (typically 1000 ms). If the PLC scan stalls longer than P2040, the drive will fault on F0072.
• After configuration, perform a one-time RAM-to-ROM save on the drive (P0971 = 1) to persist the bus parameters across power cycles.
• For the final commissioning, cycle the DC bus on the drive after the S7 has been put in RUN, to ensure the drive's process data handshake aligns with the master's first Data_Exchange cycle.