Troubleshooting MM420 V1.3 DriveMonitor 5.05 USS Communication

Problem Overview

The Siemens MICROMASTER 420 (MM420) inverter with firmware release V1.3 cannot establish a reliable commissioning link to DriveMonitor V5.05 on a laptop using the legacy Micromaster /PC inverter connection kit (order number 6SE6400-1PC00-0AA0). Other MM420 drives on the same bench running firmware V1.2 connect without issue. In the DriveMonitor "Establish USS Connection" dialog, the MM420 V1.3 is detected by its USS node address but the firmware version field is left blank, and parameter upload either hangs or terminates with a protocol error.

This is a classic firmware / tool version mismatch symptom. DriveMonitor V5.05 (build 5.05.02.00) was released against the MM4 parameter set baseline that shipped with firmware V1.x through V1.2.x. The V1.3 firmware introduced object-table revisions and additional parameter scaling on the MM420 that V5.05 of DriveMonitor does not fully recognise, which is why the version is reported as empty and the parameter read returns partial / corrupted data.

Affected Products and Versions

Root Cause Analysis

Three independent factors converge in this fault signature. Each must be ruled out before replacing hardware.

1. DriveMonitor V5.05 Firmware Descriptor Limitation

DriveMonitor V5.05 ships with a parameter descriptor database (*.dva / *.ddb) that targets the MM420 parameter set as it existed at V1.0 / V1.1 / V1.2. The blank version field in the "Establish USS Connection" dialog confirms that V5.05's drive identification routine receives a firmware identifier string it cannot parse. The drive is visible because the USS broadcast poll (address 0 or 31, depending on configuration) is transport-level and does not require descriptor knowledge. The moment DriveMonitor tries to read the parameter description, the request either times out or returns an error block because the V1.3 firmware responds with a different object dictionary layout.

2. USS Protocol Parameter Mismatch

Both sides of the link must agree on baud rate, parity, number of stop bits, and the PKW / PZD frame length. MM420 firmware V1.3 defaults to:

• P2010 = 6 (baud rate 9600 bit/s) — parameter P2010[0]
• P2011 = 2 (USS PZD length = 2 words)
• P2012 = 127 (USS PKW length = variable, 4-word max)
• P2013 = 2 (USS telegram off-time = 2 ms)
• P2014 = 1000 (USS telegram failure time = 1000 ms)
• P2015 = 1 (USS PKW stop bits / parity = 1 stop bit, no parity is set by P2015; parity is fixed at none on the MM420)

If DriveMonitor is configured for 19200 bit/s (P2010 = 7) while the drive is at 9600, the drive still appears in the search dialog (because search uses a slower fallback scan) but parameter read/write fails silently.

3. PC Interface Hardware / Driver Layer

The 6SE6400-1PC00-0AA0 kit contains a USB-to-RS485 or RS232-to-RS485 isolator depending on the revision. Driver issues, COM-port renumbering after a Windows update, or a power-state change in the USB hub all produce the intermittent behaviour reported ("sometimes I have to restart the computer").

Pre-Change Compatibility Matrix

Hardware: Building the USS Link

The MM420 exposes its USS / RS485 interface on terminals 29 (P+) and 30 (N-) on the control terminal block. The 6SE6400-1PC00-0AA0 PC inverter connection kit terminates to the same two pins.

1. Power down the MM420. Verify the DC bus is discharged before opening the terminal cover.
2. Strip 8 mm of insulation from the USS conductors. The MM420 terminals are caged-clamp / screw-less; insert a 2.5 mm flat-blade screwdriver to open the cage.
3. Connect the kit's P+ lead to terminal 29 and N- to terminal 30. Maintain polarity throughout the bus.
4. For a point-to-point laptop connection, leave the kit's bus terminator (typically a jumper marked TERM or a 120 Ω resistor across 29/30) in the ON position.
5. If the drive is the last device on a multi-drop USS bus, the on-board terminator must be enabled via DIP switch / jumper (refer to the MM420 Operating Instructions for the exact location per frame size).
6. Connect the kit's USB / RS232 end to the PC. Allow Windows to enumerate the COM port; record the assigned COMx number.

Software: Configuring DriveMonitor V5.05

If you must remain on DriveMonitor V5.05 because of a legacy install base, the configuration below is the only one that can establish a usable V1.3 link. Most users should, however, migrate to STARTER.

1. Launch DriveMonitor V5.05 with administrator rights. On Windows 10 / 11, the tool requires elevated rights to access the COM port.
2. Select Online → Establish USS Connection.
3. Set the COM port to the value Windows assigned to the 6SE6400-1PC00-0AA0 kit (Device Manager → Ports).
4. Set baud rate to 9600 bit/s (matches P2010 = 6). Do not auto-scan; DriveMonitor's auto-scan can lock the bus when V1.3 firmware is present.
5. Set the USS node address to 0 for first contact, then refine to the drive's P2011 / address parameter once the drive is identified.
6. Click Read Drive Parameters. If the read returns an error, the firmware is incompatible — proceed to the STARTER section below.

Software: Configuring STARTER V4.3 or Later (Recommended)

STARTER is the supported commissioning tool for all MM4 firmware releases from V1.0 through V2.x. The recommended path for an MM420 V1.3 is STARTER V4.3 with service pack SP3 or later, or STARTER V5.x. Newer TIA Portal versions ship SINAMICS Startdrive as the equivalent plugin.

1. Install STARTER from the Siemens support portal. Accept the default installation path; do not change the starter working directory if multiple versions are present.
2. Launch STARTER. Create a new project (Project → New → with navigator).
3. Insert a new drive object: Insert → Drive → MICROMASTER 420. STARTER will prompt for the order number (for example 6SE6420-2UD17-5AA1).
4. Open Set PG/PC interface (menu: Options → Set PG/PC interface). Select the S7ONLINE access point and the correct COM-port device driver for the 6SE6400-1PC00-0AA0 adapter.
5. Set the bus address to the value stored in drive parameter P2011 (default 0 for point-to-point).
6. Click Connect to target system (the icon shows a plug and socket). The status bar should read Online.
7. Right-click the drive → Upload to PG. This populates the offline project with the V1.3 parameter set, including any V1.3-only parameters.

Parameter-by-Parameter Verification

After the link is up, verify the following parameters via STARTER or DriveMonitor. Values shown are the MM420 V1.3 factory defaults relevant to USS communication.

Fault Code Reference

The MM420 reports the following faults when the USS link is broken at runtime. The fault word is held in r0947[0..7] and the fault value in r0949[0..7].

Troubleshooting Matrix

Step-by-Step Recovery Procedure

1. Confirm drive firmware: read r0018 via the operator panel (BOP / AOP). If r0018 reads 1.30 or higher, DriveMonitor V5.05 will not reliably read parameters.
2. Back up parameters from the V1.2 drive on the same bench that does communicate. Use DriveMonitor's Parameter → Upload from drive to capture the V1.2 parameter set as a .dnx file.
3. Install STARTER V4.3 SP3 (or later) on the same laptop. The installation can coexist with DriveMonitor; they use independent working directories.
4. Configure Set PG/PC interface to the COM port assigned to the 6SE6400-1PC00-0AA0 kit. Test the interface with Diagnostics → Test.
5. Create a STARTER project, insert an MM420, enter the exact order number printed on the drive data label.
6. Click Connect to target system → Upload to PG. The project navigator should now show the V1.3 parameter set.
7. Compare against the saved V1.2 backup. New V1.3 parameters will appear, typically under P10xx extended setpoint and P20xx USS extended telegram.
8. Apply the new project to the drive with Download to target. Tick Copy RAM to ROM if the changes must survive a power cycle.
9. Cycle the drive's 24 V control supply (do not short the DC link) and re-verify r0018 shows the same firmware version.

Verification Checklist

• STARTER status bar shows Online with the drive's USS node address.
• r0018 matches the nameplate firmware.
• P0003 is reachable at expert level (3) and write-able.
• A round-trip parameter read / write completes in < 200 ms at 9600 bit/s.
• Motor accelerates to a 50 Hz reference issued from the PC control word 1 (bit 0 = ON/OFF1, bit 1 = OFF2, bit 2 = OFF3, bit 3 = enable pulse, bit 4 = ramp enable, bit 5 = ramp start, bit 6 = setpoint enable).
• Disconnecting the USS cable with the drive running produces F0071 within 1 s (P2014 default).
• Reconnecting the cable clears F0071 automatically if P1210 = 1 (auto restart enabled).

Long-Term Recommendation

The MM420 reached end of production in 2018 and the recommended successor is the SINAMICS V20 (frame sizes A–E, 1AC 230 V and 3AC 400 V). For new installations using a TIA Portal engineering environment, the V20 with SINAMICS Startdrive provides the same USS-on-RS485 connection profile plus PROFINET. The 6SE6400-1PC00-0AA0 PC inverter connection kit is mechanically compatible with the V20 and the V20 firmware descriptor is fully supported in Startdrive V15 and later.