Troubleshooting LED Errors on SIMODRIVE 610 6SC6901 Drives

1. System Identification: SIMODRIVE 610 versus 650/690

The Siemens SIMODRIVE 610 family (Siemens order designation 6SC610 series) is a digital three-phase servo drive system intended for use with feed and main spindle motors in CNC machine tools. It is commonly paired with SINUMERIK 810, SINUMERIK 820 and SINUMERIK 840C controls, and accepts resolver or encoder feedback from 1FT5, 1FT6, and 1PH series motors.

The order number 6SC6901 reported by the user is, however, ambiguous. The order-number root 6SC6901 belongs to the SIMODRIVE 650/SIMODRIVE 690 families used with the SINUMERIK 805 control, NOT the SIMODRIVE 610 line. The 610 line uses roots such as 6SC6101, 6SC6102, and 6SC6108. Before any LED diagnosis, verify the data plate MLFB on the side of the module to determine which family is actually in the cabinet.

2. LED Panel Layout on SIMODRIVE 6SC6901 Modules

The front panel of the 6SC6901 module exposes a vertical column of status LEDs. The exact count depends on the variant suffix (for example 6SC6901-0AA02 versus 6SC6901-2AB01), but the canonical five-LED layout from top to bottom is:

The figure above shows the typical LED arrangement on a 6SC6901 front panel. The two green LEDs dedicated to encoder tracks must change state when the motor shaft is rotated by hand with the drive disabled; if they remain dark or solid, the encoder cable, the encoder itself, or the receiver circuit on the module is at fault.

3. Encoder Track LED Diagnostics (H2 / H3)

The middle LEDs indicate activity on the two quadrature channels (typically channel A and channel B, sometimes labelled SpA / SpB or Track I / Track II) coming from the motor's incremental encoder. Normal behaviour:

• Motor stationary: both H2 and H3 dark (logic-low state).
• Motor rotated by hand, drive disabled: H2 and H3 alternate flickering.
• Frequency of flicker is proportional to shaft speed; at very slow rotation the LEDs appear to blink alternately.

Abnormal patterns and their likely causes:

4. Write-Protection LED (H5, Red, Lower Position)

The red LED at the lower position of the LED column indicates the state of the internal parameter write-protection flip-flop. It is also used as a fault latch indicator on certain firmware revisions.

On older 6SC6901-0AAxx modules the write-protect LED shares its driver with the watchdog output. A continuously-on H5 LED combined with a dark H1 LED is, in many cases, the watchdog having tripped — which can be caused by:

1. Loss of one phase on the 3-phase 400 V supply (the module powers down its internal logic but the DC bus capacitor retains charge).
2. Overtemperature on the heatsink (PTC > 110 °C) latched.
3. I²C / serial EEPROM error during parameter load.

5. Power and DC-Bus Status LEDs (H1)

The top green LED reflects the health of the internal 5 V logic supply derived from the DC bus. The drive generates its own DC link from the 3-phase 400 V AC input; nominal DC bus voltage is approximately 540 V DC for a 400 V AC line (peak × √2 × rectified, minus diode drop), reaching about 565 V DC under no-load conditions.

Voltage expectations during commissioning:

6. Common LED Error Patterns and Root Causes

The following matrix aggregates the most frequent LED combinations observed on 6SC6901 modules in CNC retrofit projects. Use it as the first decision tree before opening PCIN.

7. Step-by-Step Diagnostic Procedure

Use the following sequence to isolate an LED fault on a 6SC6901 module. Always work with the main breaker open and confirm zero energy at the bus capacitors before touching connectors.

1. Confirm the data plate. Record the full MLFB (for example 6SC6901-2AB01-Z), serial number, and firmware sticker. The firmware suffix matters: a 6SC6901 with FW V3.x reports different LED behaviour than a V5.x unit.
2. Power up with no enable. Apply 3-phase 400 V AC and 24 V DC control. Observe H1. If H1 is OFF, measure DC bus voltage at P600 / N600 before assuming a module failure.
3. Rotate motor shaft by hand. H2 and H3 must alternate-flicker. If they do not, lift the encoder connector X411 (or X412 depending on variant) and measure 5 V supply between pins 1 and 2. Replace cable if the supply is missing at the motor end.
4. Check SINUMERIK alarm history. On the 805 operator panel press MENU SELECT > ALARM. Note any alarm numbers such as 6020, 6040, 6042, 16830, or 300613. Cross-reference with the SINUMERIK 805 diagnostics manual.
5. Establish PCIN 4.7 link. Use a Siemens RS-232 cable (6FC9341-1AA) on COM1 of the 805 and the operator PC. Start PCIN 4.7, set 9600 8N1, and request READ OUT DATA. If the link fails, the module's red H5 LED may be latched and require a 24 V control-power cycle.
6. Clear latched faults. Cycle only the 24 V DC control supply (leave the 400 V AC main breaker on) for at least 10 seconds, then re-apply. Watch the LED pattern transition during power-up.
7. Verify enable chain. The drive will refuse to enable if terminals 63 and 64 (drive enable) are not at 24 V and 9 (NE) is referenced to 19 (NE). Verify at the terminal block before suspecting the module.
8. Run a no-load reference move. In PCIN 4.7, load the original backup file (*.TEA), set MD bit 0 (write-protect) to 0, and perform a 1 mm reference move. Confirm H4 yellow lights at standstill and H2/H3 flicker in proportion to commanded velocity.

8. PCIN 4.7 Backup, Restore and Parameter Handling

Siemens PCIN 4.7 is the standard Windows tool to read and write parameters of SIMODRIVE 6SC6901 modules via the SINUMERIK 805 RS-232 interface. The procedure below is the field-validated workflow.

PCIN 4.7 reads *.TEA ASCII files. Each parameter line is of the form P<number> = <value>;. Do not edit the file in a non-ASCII-safe editor such as Notepad++ with UTF-8 BOM enabled — the BOM character causes the module to reject the block.

9. Field Commissioning Checks

After replacing or re-flashing a 6SC6901 module, perform the following verification sequence before returning the machine to production.

1. Insulation test. With the module isolated, megger between P600 and PE at 500 V. Reading must be > 5 MΩ.
2. Phase rotation. Confirm L1-L2-L3 sequence at the line contactor matches the marking on the module. Reverse sequence will not damage the module but will spin the motor backwards at first command.
3. DC bus rise time. From 0 V to 540 V DC must occur in < 1.5 s under soft-charge. A slow rise indicates a soft-charge resistor or contactor fault.
4. Encoder cable shield. Shield must be bonded to cabinet ground at the entry plate only — not at both ends. Use Siemens pre-fabricated cable 6FX5002 series where possible.
5. Enable response time. From 24 V at terminal 63 to H4 yellow LED ON must be < 50 ms. A delay > 100 ms indicates parameter loading not yet complete.
6. Speed-loop bandwidth. Run a step response in PCIN 4.7 diagnostic mode. The current loop bandwidth must be > 1 kHz and velocity loop bandwidth > 100 Hz for a healthy 6SC6901.

10. Safety and Replacement Notes

The 6SC6901 module weighs between 18 and 32 kg depending on the variant. The DC bus capacitors retain dangerous voltage for up to 5 minutes after the 400 V AC line is removed. Observe the warning label on the front panel: "DC bus dangerous — wait 5 minutes before opening".

Use only Siemens original spare parts or formally re-qualified substitutes. Compatible Siemens order numbers include:

11. Frequently Asked Questions