MICROMASTER 4 PROFIBUS LED Status: Interpreting Drive State

Problem Description

When commissioning or operating a Siemens MICROMASTER 4 (MM4) drive over a PROFIBUS-DP network, the green LED on the PROFIBUS communication board (6SE6400-1PB00-0AA0) flashes at a slow, regular cadence instead of lighting continuously. Many engineers interpret this as a communication fault, but the flashing state has a precise meaning that is documented in the Siemens PROFIBUS diagnostics reference. The flash is not a fault; it is a deterministic operating state.

The three-color LED on the front of the PROFIBUS-DP communication board is the primary visual indicator of bus state. A slow green flash means the cyclic process-data (PZD) exchange is active, the bus is healthy, and the drive is waiting for a non-zero control word from the PROFIBUS master. Once the master issues an ON / Start command, the LED transitions to a continuous green state and the drive ramps to the commanded setpoint.

Three-Color LED States on the PROFIBUS Communication Board

Reference: Siemens Support entry 6586565 – MICROMASTER 4 (MM4): PROFIBUS Diagnostics using LED display.

The flashing cadence of approximately 0.5 Hz is documented in the Siemens PROFIBUS diagnostics reference. The cadence is one pulse per two seconds, which is distinct from the faster parameterization flash used in some MM4 firmware revisions. Engineers should always observe the LED for at least five seconds to distinguish 0.5 Hz from 2 Hz before drawing a conclusion.

Why the Green LED Flashes: Control Word = 0 Explained

The MM4 PROFIBUS communication board decodes the first PZD output word from the master as control word 1 (STW 1) per the PROFIdrive profile. The control word is a 16-bit field; bit 0 is the ON/OFF1 command. The LED control logic on the board classifies the drive state as follows:

• If STW1 = 0x0000 (all bits cleared, master is in Stop or has not written the PZD output) and cyclic exchange is active, the LED flashes green at 0.5 Hz.
• If STW1 ≠ 0x0000 and at least the run-enable bits are set, the LED is solid green.

When the PLC is in Stop state, SIMATIC masters typically write zero to all PZD outputs as part of the peripheral-output reset behavior. The MM4 communication board detects this and reports "setpoints invalid" to the LED driver. The bus is healthy, the slave is exchanging diagnostics frames, but the drive is not being commanded to run. The flashing green LED is the correct visual state for this condition. It does not indicate a broken cable, a wrong node address, or a bad GSD file.

Control Word Bit Definitions (PROFIdrive Profile for MM4)

The MM4 implements the PROFIdrive profile subset that is standard for low-voltage inverters. The control word bits that the drive evaluates for run state are documented in the MM4 Operating Instructions and the PROFIBUS Communication Module manual.

To transition the LED from flashing to solid, the master must transmit a control word with bits 0, 1, 2, 3, 4, 5, 6, and 10 set. A typical "start" value is 0x047F (bits 0–6 plus 10). A common fault-acknowledge + start sequence is 0x0480 (bit 7 + 10) followed by 0x047F. The setpoint is transmitted in the second PZD word (HSW) as a normalized value where 0x4000 = 100% of P2000 (default 50 Hz).

BOP Display Correlation

The Basic Operator Panel (BOP) mirrors the LED state in its decimal-frequency readout. When the drive is in the "setpoints invalid" condition, the BOP displays the last commanded setpoint value with flashing digits at the configured refresh rate. For example, if the master has written a setpoint of 30.00 Hz in HSW but the control word is still zero, the BOP shows 30.00 with the value flashing.

Once the master sends a valid control word, the BOP stops flashing and displays the actual output frequency as the drive ramps. The flashing BOP and the flashing LED both clear simultaneously when the run command is accepted. The flashing condition is a deliberate UI behavior: the BOP signals that a setpoint is pending but the drive is not being commanded to run.

Required Configuration Parameters for PROFIBUS Control

Before the LED will go solid green and the drive will actually run, the MM4 must be configured to accept PROFIBUS as the command source and setpoint source. Verify the following parameters from the BOP, STARTER, or DriveMonitor:

If P0700 is set to 2 (terminal strip) or another value, the drive will ignore PROFIBUS commands even when the bus is healthy and the LED is solid green. The LED reflects bus state, not command-source selection. A solid green LED with the drive not running is a classic symptom of P0700 ≠ 6.

PKW Parameter Channel and Diagnostic Access

The MM4 PROFIBUS module supports both the PZD channel (cyclic, fast process data) and the PKW channel (acyclic, parameter access). The PKW channel is four words long and allows the master to read and write any drive parameter over PROFIBUS. This is the recommended method for commissioning verification.

Typical PKW read sequence in SIMATIC:

1. Write the parameter number to the PKW request word (PKE) and the parameter index to the index word (IND).
2. Wait one bus cycle for the slave to populate the response.
3. Read the response PWE (parameter value) from the input PKW area.

Use the PKW channel to confirm P0918, P0700, P1000, and the actual control word value during operation. Reading r0052 (CO/BO: actual status word 1) over PKW is the fastest way to confirm the drive has accepted the run command. Bit 2 of r0052 reports "Operation enabled" and should be set when the LED is solid green.

PZD Configuration (P2040, P2041, P2051)

The default PZD mapping for the MM4 with the standard GSD file is two words in each direction: control word + main setpoint (outputs) and status word + actual value (inputs). Engineers who need additional status information can extend the PZD length using the "MM4 PZD 4/4" or "MM4 PZD 6/6" GSD variants.

For the default 2/2 mapping, leave P2051.0 = 52 and P2051.1 = 21. If P2040 (process data monitoring time) is set to a non-zero value, the drive will trip with fault F0070 if no PZD telegram is received within that time. Setting P2040 = 0 disables the watchdog and is acceptable for commissioning but not for production operation.

For GSD file selection, use the file that matches the MM4 variant:

• MM420: siem808d.gsd (or the Siemens naming convention used in STEP 7 HW Config)
• MM430: siem80d0.gsd
• MM440: siem80b0.gsd

Mismatched GSD files cause the yellow LED state (parameterization phase) and the drive will not enter cyclic exchange.

Verification Procedure

Use this sequence to confirm the LED state is normal and the drive is ready to run from the PROFIBUS master:

1. Connect the BOP (or use STARTER / DriveMonitor via the RS485 port).
2. Confirm r0000 displays the current drive state (e.g., 7 = Run, 1 = Ready).
3. Verify the PROFIBUS node address in P0918 matches the SIMATIC hardware configuration.
4. Verify P0700 = 6 and P1000 = 6 if the master is to control both command and setpoint.
5. From the PLC, place the PROFIBUS master in RUN mode (not Stop).
6. Watch the green LED: it should remain solid green when the master writes a non-zero control word.
7. Use the PLC to write a setpoint (e.g., 30.00 Hz) and a control word of 0x047F to the PZD output words.
8. Confirm the BOP stops flashing and shows the actual output frequency.
9. Confirm r0052 bit 2 (Operation enabled) is set.
10. Confirm r0021 (actual output frequency) matches the commanded setpoint after ramp completion.

Troubleshooting Matrix

PLC-Side Configuration Notes

Common reasons a SIMATIC master keeps the MM4 in the "setpoints invalid" state even with the master in RUN:

• The PZD output words are mapped to a peripheral output (e.g., PQW) but the program does not write to them in a cyclic OB. Check that the output words are in the process-image partition assigned to OB1 or a cyclic interrupt OB.
• The control word is loaded once during startup but never updated. The MM4 communication board can interpret a stale zero as a stop command if the master clears the outputs on Stop → Run transition.
• The slot configuration in the SIMATIC hardware catalog does not match the MM4 GSD file. Mismatched PZD lengths can cause the slave to ignore the data or the master to drop the slave.
• The slave is in "DPV1" mode but the master is configured for DPV0. The MM4 supports DPV0 by default; DPV1 requires explicit configuration in the slot properties.
• The PLC is in Stop or the CPU is in fault. The MM4 LED will continue flashing green in this case because the bus is electrically healthy.

Field-Commissioning Best Practice

1. During bench commissioning, leave the master in Stop. Confirm the green LED is slow-flashing before connecting to the production bus. This confirms the cable, termination, and node address are correct.
2. Bring the master to RUN with a known control word (e.g., 0x047F) and a 5 Hz setpoint. Confirm the LED is solid green and the drive is ramping.
3. Use a PROFIBUS tracer (e.g., Softing PROFIusb, Siemens BT200) to capture the PZD exchange during start-up. The first PZD should transition from 0x0000 to 0x047F within one bus cycle.
4. Document the LED state in the commissioning checklist so that the same reference point exists for every drive in the cabinet.
5. Verify the watchdog behavior: with P2040 = 100 ms, pull the PROFIBUS connector and confirm the drive trips with F0070 within the configured time.
6. Test the fault-acknowledge sequence from the PLC: write 0x0480, then 0x047F, and confirm the drive restarts without a power cycle.

Safety Considerations

The PROFIBUS control word contains the OFF2 (bit 1) and OFF3 (bit 2) commands that the drive evaluates on every bus cycle. If the PLC program clears these bits for any reason (programmer error, watchdog trip, scan-time overflow), the drive will execute a coast-to-stop (OFF2) or quick stop (OFF3) without warning. Always implement a default-zero safety policy: the PZD outputs default to zero on PLC Stop, which will coast the drive to stop.

The MM4 OFF2 response is category 0 (immediate removal of inverter pulses, motor coasts). OFF3 is category 1 (controlled quick stop on the configured ramp). Verify the application can tolerate coast-to-stop on PROFIBUS loss. For applications that require a controlled stop on bus loss, use P2040 to trip the drive on a watchdog timeout and configure an external safety relay for the final disconnect.

Module Replacement and Firmware Considerations

The MM4 PROFIBUS communication module (6SE6400-1PB00-0AA0) is hot-swappable in most installations, but the node address is stored on the module itself, not on the drive. When replacing a module, set the address switches (DIP switches on the module, or P0918 in software) to match the original configuration. If the address is set in software (P0918), the new module will default to address 126 (DPV0 default) until the drive is re-parameterized.

MM4 firmware revisions below V1.05 may not support the full PROFIdrive profile. Confirm the firmware version with r0018 (firmware version) before assuming the LED state table applies. The Siemens diagnostics reference for the LED states applies to all MM4 firmware revisions that support the PROFIBUS module.

FAQ