Troubleshooting SIMODRIVE 611 X304 Pinout and Z-Axis 300608 Fault

Overview

The Siemens SIMODRIVE 611 universal series powers the spindle and feed drives on a large installed base of CNC machine tools, including the EMCO VMC 300 machining center described in the field case. The system is built from a CCU (Computerized Numerical Control Unit, in this case a 6FC5410-0AY01-0AA0 810DE), an ER module (Einspeisung/Rück-speisung infeed/regen unit), and one or more LT modules (Leistungsteil power modules) for each axis. The LT module is the axis-specific intelligent power stage and exposes its control, setpoint, and status signals through a fixed family of front-panel connectors: X304, X305, X411, X412, X413, X414.

On the EMCO VMC 300 in the source case, the operator reported:

• Power supply OK, NC and PLC report ready.
• On referencing, the Z axis display increments roughly 0.3 mm but the motor never moves.
• Alarms 20005, 21612, 300608, 25201 from NCK and 700032 AUX OFF from the PLC.
• After a full IGBT, EUPEC pack, diode, opto-coupler, and current-hybrid check on the Z LT module, every power component tested healthy.
• Burned resistors and diodes were later identified on the CPU board around the F312709GFT host-IC.

This article consolidates the X304 / X305 pinout data with the alarm-by-alarm diagnostic procedure required to localize this class of fault, and provides the signal-level checks for a CPU-side root cause.

System Topology and Identified Hardware

The SIMODRIVE 611 universal in the field case is a two-axis configuration (X, Y) plus a spindle fed from a common DC link. Hardware identified by the operator:

Each LT module is a self-contained intelligent drive that receives its velocity/torque command from the NCU over the backplane and from the SIMATIC S7 PLC over hard-wired enable/ready signals at the X304 connector. The setpoint and enable path through X304 is therefore the first place to confirm when the LT module reports healthy power electronics but unhealthy control.

X304 Connector — Setpoint and Enable Interface

X304 is the 12-pin setpoint / enable / status connector on the front of every SIMODRIVE 611 universal LT module. The pinout below is taken from the SIMODRIVE 611 universal Function Manual (FBU_0510). Pin 1 is at the top right of the connector as drawn, with the keying tab on the left.

The pulse-enable path (pins 1-2) is sourced from the NCU's terminal 663 rail through the PLC's NC-Ready and axis-specific enable chain. The controller enable (pins 9-10) is sourced from the PLC output that drives the axis-specific enable relay contact; the PLC tears this open in response to fault 700032 AUX OFF or to a stop category from the operator panel.

X305 Connector — Incremental Encoder and Auxiliary I/O

X305 is the 25-pin sub-D used for incremental encoder emulation, axis-specific digital I/O, and the second channel of the analog I/O. Relevant signals for the VMC 300 case:

Alarm Decoding for the EMCO VMC 300

Each alarm must be interpreted in conjunction with the others. None of these five by itself points unambiguously to a power-stage failure; the combination points to a control-stage or setpoint-stage failure.

Alarm 21612 is the key. It tells you the PLC pulled controller enable (X304 pin 9) while the axis was in motion. If the operator sees 0.3 mm of position change but no motor rotation, the sequence is:

1. NC commands reference move on Z1.
2. NCU issues n_set on X304 pins 3-4 and asserts pulse enable on pins 1-2.
3. PLC, seeing a fault condition (700032 AUX OFF or 25201 in the NCK), de-asserts controller enable on X304 pin 9 mid-traverse.
4. Drive stops torque production, NC drops into follow-up mode, position display "creeps" from encoder counts that were already latched.

Alarm 300608 with a healthy LT module is the most diagnostic clue: the speed controller is at output limit, meaning n_set is being demanded but no motor current is being produced, which is the textbook signature of a torn enable path or a CPU-side setpoint collapse.

Root Cause Hypothesis — CPU Board Defect

The field case shows the symptom chain healthy LT power electronics → controller enable drops mid-motion → n_set collapses → 300608 → 25201 → 700032. After full component-level verification of the Z LT module, the next suspect is the host CPU / NCU board. The user identified burned resistors and diodes on the F312709GFT host IC, which is consistent with a logic-supply overcurrent on the chip's I/O ring — typically caused by a downstream opto-coupler, line-receiver, or setpoint-driver failure that dragged the F312709's output stage to ground.

The X304 signals that the CPU board drives or receives — and that can therefore damage it — are:

• Pin 1-2: pulse enable (NCU source) — high-side switch +24 V at up to 50 mA per axis.
• Pin 3-4: n_set differential driver — ±10 V at 5 mA, short-to-ground destroys the op-amp stage.
• Pin 5-6: n_act receiver — high-Z, but exposed to field wiring transients.
• Pin 7-8: ready contact — inductive load on PLC side, flyback voltage on release.
• Pin 9-10: controller enable from PLC opto — can back-drive the receiver if mis-wired.

Step-by-Step Diagnostic Procedure

1. Verify DC link. Measure the ER module DC bus test points: 600 VDC ±10 % when the line contactor is closed, decay below 50 V within 5 minutes of opening. Do not proceed until DC bus is verified safe.
2. Confirm the alarm set. From the HMI, navigate to Diagnosis → Alarms and record the full alarm log, not just the active row. The full log distinguishes an alarm generated by a transient PLC drop from a permanent drive fault.
3. Measure X304 quiescent states with the drive disabled. With the NCU powered but no controller enable, expected states:
   • Pin 1: 0 V (pulse enable off)
   • Pin 2: 0 V
   • Pin 3, 4: 0 V differential
   • Pin 5, 6: 0 V
   • Pin 7 vs. 8: contact open (high-Z)
   • Pin 9 vs. 10: 0 V (controller enable not asserted)
4. Enable pulse enable only. From the PLC force table, assert the pulse enable for axis Z1 (terminal 663 should appear on X304 pin 1 at +24 V). Measure pin 1 to pin 2: should be +24 VDC. If pin 1 stays at 0 V, the NCU-side 663 driver or the ribbon back to the CCU is open.
5. Read drive ready on pins 7-8. After pulse enable, the LT module should close the ready contact within ~200 ms. Use a multimeter on continuity: open collector type, expect <1 Ω when ready.
6. Force a 0 V n_set on pins 3-4. With the controller enable still off, verify 0 V differential at pins 3-4 and 0 V at pins 5-6. This isolates the CPU's D/A stage from the LT module's A/D stage. Any offset > ±50 mV here indicates a damaged op-amp on either end.
7. Issue a controlled 5 % n_set from the NCU. Through the NC MD (machine data) for the test, force a +0.5 V differential on pins 3-4. The current command should rise proportionally on the LT module's internal I* monitor. If the drive reports 300608 (output limit) at < 5 % n_set, the current feedback path is open (most likely the current-sense hybrid or the CPU's I_act receiver).
8. Assert controller enable on pin 9. With everything else healthy, apply +24 V to X304 pin 9 referenced to pin 10. The motor should develop holding torque within 50 ms. If 21612 fires immediately on the next reference attempt, the enable line is bouncing — check the PLC output module and the opto-isolator on the LT module side.
9. Scope the X304 waveform during the fault. Capture pin 1, pin 3, pin 5, and pin 9 simultaneously during a reference attempt. Look for the moment pin 9 drops — correlate against the PLC scan time and the rising edge of 700032. The 0.3 mm of creep is the number of encoder counts latched between the pin-9 drop and the NC's follow-up detection.
10. Inspect the CPU board around F312709GFT. With the LT module removed, ring out the F312709's I/O pins for shorts to +5 V, +15 V, and ground. The companion chips (typically the setpoint DAC, the enable driver, and the ready receiver) sit in the same I/O ring and should be checked for the same signature.

Field-Proven Repair Strategy for the CPU Board

Once the F312709GFT has been reflowed (or replaced as a reballed BGA), the four steps below restore the system to a known-good state without re-introducing collateral damage:

1. Power the NCU on the bench with no LT module connected. Measure the +5 V, +15 V, −15 V, and +24 V rails on the CPU connector. Verify F312709's Vcc and Vccio pins match the rail spec.
2. Loopback test on the backplane. With the LT module still removed, drive a software n_set from the NCU and verify the analog setpoint at the backplane edge connector before it reaches the LT module's X304. This confirms the CPU's setpoint DAC is alive after repair.
3. Hot-plug a single LT module (the lowest-current one — the 2 × 15 A unit on the VMC 300). Run a 5 % n_set torque test with the motor uncoupled. This catches any remaining solder shorts on F312709 before reconnecting all axes.
4. Re-couple and re-reference with current limits reduced to 25 % via the axis MD (e.g., MD 1105 / MD 1107 on 810DE). Increase in 25 % steps over four cold cycles to confirm thermal stability.

Verification Checklist

• All five field alarms (20005, 21612, 300608, 25201, 700032) clear after the controller enable and n_set paths are restored.
• Holding torque is verified at 0 % and 25 % n_set on the Z axis with the motor uncoupled.
• Reference move completes in < 5 s end-to-end with no 21612 reset event.
• After 30 minutes of thermal soak, the LT module's IGBT case temperature is < 70 °C at 50 % n_set.
• Alarm log captured post-repair for the machine file. Siemens service expects a clean alarm log for warranty sign-off.

Troubleshooting Matrix — Fast Triage

FAQ