Configuring SINAMICS G120 CU240E-2PN Brake via Digital Output

Problem Overview

A SINAMICS G120 drive package — specifically a PM240-2 Power Module paired with a CU240E-2PN Control Unit — is being commissioned against a 10 kW motor fitted with a 415 V AC spring-applied holding brake. The brake is the type with a built-in rectifier block mounted on the motor terminal box, which converts the 415 V AC supply to the DC voltage the brake coil actually consumes. According to the SINAMICS G120 operating instructions, the canonical hardware path is the dedicated Brake Control Module with article number 6SL3252-0BB00-0AA0. That module sits between the Control Unit and the brake rectifier and handles the brake contactor switching, suppression, and diagnostic feedback.

When the Brake Control Module (BCM) cannot be sourced in time for commissioning, the question is whether the relay contacts available directly on the Control Unit can be used as a substitute. The answer from Siemens field practice is yes — with caveats. The CU240E-2PN exposes a digital status bit in its brake control word (r899.12) that mirrors exactly what the BCM would have switched, and that bit can be routed via BICO to any available relay or transistor output on the Control Unit. The only hardware that the BCM provides and the Control Unit does not is a recommended surge-suppression element across the brake coil; that must be re-introduced externally.

Prerequisites

Before reconfiguring the drive, verify the following:

• CU240E-2PN firmware version V4.7 SP10 or later (required for stable BICO brake control behavior; older FW4.6 builds have intermittent r899 bit-update timing under heavy load).
• STARTER commissioning tool (V5.4 or later) or SINAMICS Startdrive in TIA Portal V16+ for online parameterization.
• Knowledge of the current access level — write access to p1215 requires at least expert level (p0003 = 3).
• Motor data sheet values: rated current, magnetizing time, brake opening time, brake closing time.
• Brake rectifier voltage rating matches the 415 V AC supply (verify with motor nameplate; some rectifiers are 400 V only and require de-rating or a series dropper resistor).
• External 24 V DC relay with contact rating adequate for the inrush current of the brake rectifier capacitor bank. A typical 10 kW motor brake draws 0.5 to 1.5 A DC continuous, with inrush peaks of 3 to 5× rated current at rectifier turn-on.

Hardware: CU240E-2PN Digital Output Inventory

The CU240E-2PN provides four programmable digital outputs. Two are electromechanical relay contacts; two are solid-state transistor outputs rated at 24 V DC only. The choice between them matters when switching a relay coil.

Because the external relay coil is a 24 V DC load, any of the four outputs is electrically suitable. However, the relay outputs (DO 0 / DO 1) provide galvanic isolation between the Control Unit's internal logic and the relay coil circuit, which is the preferred engineering practice. Most field implementations use DO 0 with its parameter source index p0730.

Hardware Substitution: External Relay and Surge Suppressor

The BCM does three things that a bare Control Unit digital output does not:

1. Switches the brake rectifier AC line.
2. Provides an RC snubber or varistor across the brake coil to suppress the back-EMF when the coil is de-energized.
3. Provides a single diagnostic wire (BR+/BR−) that the drive monitors for brake feedback.

Without the BCM, items 1 and 2 must be replicated by an external relay-and-snubber assembly; item 3 is dropped, which means the drive runs the brake in no-feedback mode and relies entirely on the configured brake timing parameters.

Recommended external wiring

   CU240E-2PN                      24 VDC              Brake Rectifier
   ┌─────────┐                    Relay Coil          (415 VAC input)
DO0│NO    20 ├────┐      ┌───[ Coil ]─── +24 V
   │COM   19 ├────┼──────┤                  │
   │        │    │     ┌─┴─┐                │
   │        │    │     │RC │  Snubber       │
   │        │    │     │ ~ │  (across coil) │
   │        │    │     └─┬─┘                │
   │        │    │       │                  │
   └─────────┘    │      GND                │
                  │                          │
                  │   ┌──────── NO ──────────┤ Brake coil
                  └───┤ Relay common         │ (415 VAC via rectifier)
                      └──────── NC (unused) ─┘

Snubber recommendations:

• For a 24 V DC relay coil: a 1N4007 diode in reverse parallel across the coil is acceptable; a 100 Ω + 0.1 µF RC snubber is preferred if the relay release time must be tight.
• For the brake rectifier AC input side, the BCM normally contains a varistor (typically a 275 V or 320 V rated MOV) between L and N. Mount an equivalent varistor at the rectifier input terminals.
• Wire the 415 V AC brake feed through the relay contact on its own circuit breaker (separate from the drive's 400 V feed to the PM240-2 input).

Parameter Configuration: Enabling Brake Control via BICO

The default configuration of p1215 in the G120 commissioning wizard is 0 (brake control inactive). To re-engage the brake logic but route its trigger through the Control Unit BICO table rather than the BCM hardware, set:

p1215 = 3       ; Brake control via BICO, no feedback signal

Setting p1215 = 3 activates the integrated brake sequencer in the Control Unit. The sequencer watches the drive state machine (OFF1, OFF2, OFF3, pulse enable, magnetization complete) and produces the brake-open and brake-close commands internally. What it does not do is physically drive the BCM — instead, it exposes the brake state on the status word r899.

r899.12 is the connector output to wire to a digital output. It integrates the brake opening time p1216 and the brake closing time p1217 into a clean on/off signal that drops when an OFF command is detected and rises only after the magnetization ramp is complete plus the configured brake-open delay.

Parameter Configuration: Mapping r899.12 to a Digital Output

Each digital output on the CU240E-2PN has a corresponding BICO source parameter that selects which signal drives the output:

To route the brake signal to DO 0:

p0730 = 899.12    ; route brake status bit to DO 0 (relay contact 19-20)

Format of the index is parameter_number.bit_number. The parameter value 899.12 therefore selects the twelfth bit of status word r899. After writing this value, the next step is to copy RAM to ROM with p0977 = 1 so the assignment survives a power cycle.

Optional: invert the output logic

If the relay you have chosen requires a HIGH signal to release the brake rather than engage it (i.e., failsafe-engaged design), invert the signal at the output by setting:

p0748.0 = 1      ; invert DO 0

Verify the inversion is appropriate for your brake — a 415 V AC brake fed through a rectifier and energized to release is normally driven by an energize-to-release relay, so a non-inverted output is correct. Failsafe (de-energize-to-apply) brakes, which are common on vertical-axis applications, require the inversion and an independent safety path that is beyond the scope of BICO-only control.

Brake Timing Parameters

The brake sequencer in the G120 requires three time parameters to be set to match the actual mechanical brake behavior. Incorrect values cause either the motor to fight a still-engaged brake (premature wear) or the load to coast down a vertical axis with the brake open (drop hazard).

The relationship enforced by the firmware is:

• The inverter output stage is enabled.
• The motor is magnetized (p0346 elapses).
• The brake opening delay p1216 elapses.
• r899.12 transitions from 0 to 1 → DO 0 closes → relay energizes → 415 V AC reaches brake rectifier → brake releases.
• On OFF1: r899.12 transitions from 1 to 0 → DO 0 opens → relay drops → 415 V AC removed → brake applies.
• After p1217 elapses → inverter output stage is disabled.

Brake Sequencer Timing Diagram

Step-by-Step Commissioning Procedure

1. Wire the external relay assembly. Mount the 24 V DC relay on a DIN rail inside the cabinet. Wire DO 0 (terminals 19 and 20) across the relay coil. Wire the relay common and NO contact across the 415 V AC feed to the brake rectifier. Install the varistor or RC snubber directly at the rectifier input terminals.
2. Apply 24 V DC control power to the Control Unit but leave the PM240-2 in a no-pulse state (do not issue ON).
3. Connect via STARTER or Startdrive. Establish an online connection. Confirm firmware version with r0018. Navigate to Drive → Functions → Brake control or directly to the parameter list.
4. Set expert access. p0003 = 3.
5. Activate BICO brake control. p1215 = 3. Acknowledge the safety warning that pops up — without the BCM, the drive cannot diagnose the brake hardware.
6. Set brake timing. Enter values from the motor data sheet into p1216 and p1217. Verify p0346 < p1216; if not, the firmware throws F07902.
7. Route r899.12 to DO 0. p0730 = 899.12.
8. Save to ROM. p0977 = 1 and wait for completion.
9. Run a no-load test. Issue ON with the motor uncoupled. Measure the 24 V DC coil voltage with a multimeter at the relay. Listen for the brake click within p1216 ms of pulse enable. Listen for the second click on OFF1.
10. Run a load test. Couple the motor to its load. Jog at low speed. Verify that the brake holds against any preload (vertical axis). Verify that the motor shaft is free to rotate after the brake releases.
11. Monitor fault buffer. Watch r0947[0..7] during the test. The most common fault with this configuration is F07902 "Motor holding brake defective", which will appear if p1215 was set to a value that expects BCM feedback. Confirm p1215 = 3 was actually accepted.

Verification and Diagnostics

After commissioning, verify that the brake state and the digital output are consistent:

Troubleshooting matrix

Safety Considerations

Additional precautions:

• The 415 V AC feed to the brake rectifier must be on a dedicated breaker with its own overcurrent protection.
• Do not route the 415 V brake cable and the 24 V relay coil cable in the same conduit without segregation — cross-talk and insulation breakdown are real hazards.
• When the drive enters safe stop or STO, the digital output follows r899.12 which transitions to 0 (brake close). This is the correct behavior for a non-vertical load, but on a vertical axis you must verify that the brake can hold against gravity in the absence of motor torque. Most 10 kW AC motor brakes are rated for static holding only at the motor rated torque.
• If the SINAMICS G120 trips off-line (F-class fault), p1215 = 3 drives r899.12 to 0 immediately. The brake applies. This is normally what you want, but if the application requires brake to stay released during fault, you must implement that externally — the drive cannot guarantee it through BICO alone.

BCM versus BICO Output: Practical Comparison

The BICO approach is the right answer when the BCM is unavailable and the application is a horizontal axis or non-safety-critical vertical axis with a mechanical backstop. The BCM is the right answer when the application is safety-rated, when the brake must be monitored for stuck-pad or wire-break faults, or when the long-term maintenance plan requires Siemens field technicians to recognize the standard configuration.

Migrating Back to BCM Once Stock Arrives

Once the 6SL3252-0BB00-0AA0 BCM is in hand, revert to the canonical configuration without rewriting the rest of the drive logic:

1. Remove the external relay and the DO 0 → relay wiring.
2. Install the BCM on the Control Unit's DCP/BR+/BR− terminals (the BCM clips directly to the CU on PM240-2 systems).
3. Wire the 415 V AC feed to the BCM output and through to the brake rectifier.
4. Set p1215 = 1 (brake with feedback) or p1215 = 2 (brake without feedback).
5. Reset p0730 to its original fault-output value or your desired status function.
6. Reset p0748.0 if it had been set to invert.
7. Run a full fault-injection test — pull the brake wire and confirm the drive detects it within the configured number of cycles.

FAQ

Refer to the Siemens Industry Online Support portal for the SINAMICS G120 CU240E-2 PN List Manual, parameter list, and firmware release notes. The BCM article number 6SL3252-0BB00-0AA0 and its integration steps are documented in the SINAMICS G120 product documentation.