SINAMICS S120 X46 Brake Output: Testing, Wiring, and Fault Isolation

Diagnose the X46 brake control interface on SINAMICS S120 chassis Power Modules, isolate a Safe Brake Adapter (SBA) versus Power Module fault, and verify the 24 V control output using Siemens SCOUT / STARTER commissioning.

1. Problem Overview

On SINAMICS S120 chassis-format Power Modules, the X46 terminal block is frequently mistaken for a 24 V output capable of directly switching a motor holding brake. In reality, X46 is a low-current control output dedicated to a Safe Brake Adapter (SBA), and it is not rated to switch a brake coil. Typical field symptoms reported on chassis units such as 6SL3320-1TE32-1AA3 include:

• Holding brake never releases; the motor cannot drive a vertical / loaded axis.
• Brake closes but does not release, often with F07490 / F07491 brake diagnostic faults.
• Multimeter reading on X46 pins 1-2 floats, drops, or shows apparent "modulation" near 5 Hz instead of a stable 24 V.
• Brake releases briefly on enable, then drops, coincident with SINAMICS state transitions (OFF1, OFF2, OFF3, STO).

The proper diagnostic path is to place the drive in commissioning state, force the X46 control output from the parameter tree, and verify both the drive command and the SBA switching. The sections below walk through the hardware, the SCOUT force test, the meaning of the 5 Hz diagnostic pulse, and a fault-isolation matrix.

2. Affected Hardware

The chassis Power Module 6SL3320-1TE32-1AA3 is a 3AC 380-480 V, frame size D, internal air-cooled module from the SINAMICS S120 family. The X46 terminal on this and similar chassis units is the Safe Brake Adapter interface, not a free 24 V output. The two Safe Brake Adapter catalog numbers that mate with it are:

3. X46 Pinout and Electrical Specification

X46 is a 4-pin pluggable terminal block on the chassis Power Module, typically Phoenix MC 1.5/4-ST-3.81 or equivalent. On chassis units the pinout is:

Electrical characteristics on a healthy chassis unit:

• Output voltage: 24 V nominal, driven from the internal 24 V rail through a switching element.
• Continuous load: ≤ 0.2 A; sized for the opto-input of the SBA only.
• Switching characteristic: the output is held at 24 V with a periodic low-going diagnostic pulse at approximately 5 Hz (pulse width typically 1-2 ms) so the SBA can detect cable breaks, shorts to M, and shorts to 24 V.
• Isolation: the output shares the 24 V return (M) with the rest of the chassis module; it is not isolated.

The 5 Hz "modulation" visible on a DMM is normal. It is the diagnostic pulse used by the Safe Brake Adapter to verify wiring integrity. A DMM in DC V mode samples slowly and the input filter / autoranging can produce erratic readings; an oscilloscope on a DC-coupled channel will show a clean 24 V with brief low-going diagnostic dips.

4. Why X46 Cannot Drive the Motor Brake Directly

A motor holding brake typically draws between 0.5 A and 3 A continuous current at 24 V, with inrush peaks well above that during release. X46 is current-limited by design - it is sized to drive the opto-isolated input of the Safe Brake Adapter, not the brake coil itself. Driving a holding brake directly from X46 produces one of the following:

1. The drive's internal 24 V rail folds back or current-limits; the brake release is intermittent or absent.
2. The X46 output MOSFET overheats, producing a voltage drop (the 2 V reading reported during a SCOUT force test with the brake wired direct).
3. Brake diagnostic faults are raised because the drive never sees a healthy BR+ feedback signature.

The Safe Brake Adapter provides the high-current 24 V path from an external supply (or from a separate 24 V rail) to the brake coil through an internal contactor, plus cross-fault monitoring of the brake circuit and STO/SLS-safe integration with SINAMICS Safety Integrated. This allows the brake to drop on STO and not just on OFF1. Wiring X46 directly to a brake coil - even with the SBA bypassed - is a Safety Integrated violation and will fail the next safety acceptance test.

5. Safe Brake Adapter Hardware (6SL3355-2DX01-1AA0 / -1DX00-1AA0)

The SBA is a DIN-rail module that mounts outside the Power Module, with three wiring interfaces:

1. X46 cable (pluggable terminal / sub-D variant depending on revision): connects the SBA to the Power Module's X46.
2. Brake output (terminals on the SBA): switches the 24 V holding brake coil via an internal contactor rated for the brake inrush current.
3. 24 V supply input (terminals on the SBA): the SBA needs its own 24 V supply to drive the brake; the X46 control signal is only a few milliamps.

The SBA enforces that the brake releases only when BOTH the X46 control signal is high AND the safety chain is closed. Any mismatch drops the brake. This is why a missing or failed SBA produces a brake that "almost" works but drops out under load or during state transition.

For the 6SL3355-2DX01-1AA0 variant, the LED diagnostics are:

6. Testing X46 with Siemens SCOUT / STARTER

The recommended way to test X46 is to place the drive in commissioning state and force the brake control output from the parameter tree. Procedure:

1. Connect the commissioning PC to the CU320-2 via PROFIBUS, PROFINET, or service Ethernet (X127) depending on the CU variant.
2. Open SCOUT (TIA or V5.x) and go online with the drive object that owns the Power Module. Acknowledge any active faults before continuing.
3. Navigate to Commissioning → Motor holding brake (parameter range around p1215, p1216, p1217, p1218, p1220).
4. Set p1215 bit 0 to enable the holding brake and p1215 bit 2 to select the SBA / SBC control path appropriate to the chassis configuration.
5. Use the "Test brake release" or commissioning dialog (depending on SCOUT version) to force the brake control output high. The drive moves to a "Brake test active" state and the X46 output should be held at 24 V (modulated with the 5 Hz diagnostic pulse described in Section 3).
6. Measure X46 pin 1-2 with a DMM (DC V) or oscilloscope. Expect ≈ 24 V with brief diagnostic dips, not a stable 2 V reading.

A reading of ≈ 2 V on pins 1-2 during a forced test indicates the X46 output stage is current-limiting or shorted - usually because the load on X46 exceeds the rated output (e.g., the brake coil is wired directly across X46 without the SBA). Disconnect the X46 wiring to the SBA and re-measure open-circuit. If the open-circuit voltage is 24 V and the loaded voltage drops to 2 V, the X46 side is fine and the wiring / brake / SBA is the issue.

7. Reading the 5 Hz PWM on a Multimeter

The "modulated 24 V" observed on a DMM is normal X46 behavior. The drive intentionally injects a low-going diagnostic pulse at approximately 5 Hz so the Safe Brake Adapter can detect:

• Open circuit (cable break) - the diagnostic pulse is not echoed back.
• Short to 24 V (BR+ shorted to BR-) - the diagnostic pulse is shunted.
• Cross-fault (BR+ shorted to PE / M) - the SBA's input stage flags a fault.

On a healthy X46 the diagnostic pulse is short (typically 1-2 ms) and the duty cycle is high (≥ 90 % high). A DMM in DC V mode integrates the signal; depending on the input filter and the meter's true-RMS algorithm, the displayed value can swing between roughly 22 V and 26 V, which can look like "modulation" but is in fact a steady 24 V with a periodic test pulse.

To get a clean reading:

• Use a DMM with DC-coupled averaging, or an oscilloscope.
• Disable autorange and lock the range to 30 V DC.
• Trigger the scope on the falling edge of the diagnostic pulse to see the pulse width and amplitude.

A 2 V DC reading with the X46 force test active is NOT the diagnostic pulse - it is a failed X46 output stage or a heavily loaded output. The two conditions can be distinguished by looking at the diagnostic pulse on a scope: if the pulse is present but the high-time voltage is low (e.g., 2 V), the X46 output is sagging under load; if the pulse is absent entirely, the drive is not commanding release at all.

8. Fault Isolation Matrix

9. Wiring and Commissioning Sequence

Correct commissioning avoids most X46 problems. Use this sequence on a chassis Power Module with Safe Brake Adapter:

1. Mount the Safe Brake Adapter (6SL3355-2DX01-1AA0) on a DIN rail within the cable-length limits specified in the SBA manual (typically ≤ 5 m to the Power Module, ≤ 50 m to the brake).
2. Wire the X46 cable from the Power Module to the SBA per the SBA manual, observing the pin 1-4 assignment (BR+ / BR- / n.c. / n.c.).
3. Supply the SBA with 24 V DC from a separate, fused source - do NOT feed the SBA from the Power Module's internal 24 V (it is not sized for the brake inrush).
4. Wire the SBA's brake output to the motor holding brake terminals. Verify polarity per the brake manufacturer's datasheet; most 24 V brakes are not polarity-sensitive, but spring-engaged / fail-safe brake variants must observe the polarity marking.
5. In SCOUT, configure p1215 (motor holding brake configuration) and the safety chain integration. On Safety Integrated drives, also configure the SBA in Safety Integrated commissioning (parameter range p9601 - p9875).
6. Run a first enable with no load and verify X46 output and SBA switching using the force test described in Section 6.
7. Run a load test and verify brake release under rated torque. Monitor r0898 (sequence control word) and r9773 (safety diagnostics) during the test.

10. Parameter Reference (Motor Holding Brake and SBA)

For a full list of holding-brake, SBA, and Safety Integrated parameters, refer to the SINAMICS S120/S150 List Manual. For function and commissioning background, refer to the SINAMICS S120 Function Manual and the SINAMICS S120 Safety Integrated Function Manual. The 6SL3355-2DX01-1AA0 SBA device manual and the chassis Power Module manual contain the wiring diagram referenced by this article.

11. Verification Checklist

After any X46 / SBA / brake intervention, verify the following before returning the drive to production:

1. Drive goes online in SCOUT with no active Safety Integrated faults.
2. Brake force test in SCOUT releases the brake; X46 pin 1-2 measures 24 V DC on a scope or stable ~22-26 V DC on a DMM.
3. SBA READY LED is on, FAULT LED is off.
4. 24 V measured at SBA brake output terminals when forced.
5. 24 V measured at motor brake terminals when forced.
6. Brake releases mechanically (rotation check with drive disabled, or torque check on a coupled load).
7. No F07490, F07491, F01611, F01670 faults on enable.
8. Safety acceptance test re-run on Safety Integrated drives, with the safety signature log updated.

12. Frequently Asked Questions