Forcing 1FT7 Motor Brake Release on SINUMERIK 828D with S120

Problem Overview

On a SINUMERIK 828D controller running SINAMICS S120 firmware 4.8, an operator-assisted grinding process requires manually rotating the rotor of a 1FT7 synchronous servo motor. The machine builder must therefore provide a way to force the holding brake open by software without driving the axis through its normal motion program.

Several conventional approaches — setting p1215 = 2, activating the extended brake control (p1278 = 1), and commanding the NC/PLC interface signal DB380x.DBX4000.5 — all report the correct status bits in the drive (r0899.12 = 1, r0899.13 = 0), but the 24 V brake output on the Single Motor Module (SMM 6SL3120-1TE21-8AD0) never energises. The brake still opens normally when the control enable and pulse enable are active, or when 24 V is applied externally to the BR+ / BR- terminals.

The root cause is not a wiring fault or a defective brake coil — it is the SINAMICS brake-control state machine: software brake release is only authorised when the drive is in the correct operating state. This article documents the correct parameter combination and the conditions under which the drive will physically open the brake contacts.

Affected Hardware and Firmware

Brake Control Architecture in SINAMICS S120

The holding brake on a 1FT7 motor is driven from the Motor Module, not from the NC. The drive runs an internal state machine that uses the operating state, the safety state, the OFF responses, and the configuration parameters to decide whether to energise BR+ / BR-.

Two brake-control modes are available in firmware 4.8:

1. Basic brake control — selected when the drive option Extended brake control is inactive (r0108.14 = 0). The brake follows the sequence control (SCC) and is driven by the signal open holding brake derived from r0899.12 / r0899.13.
2. Extended brake control — activated by p1278 = 1 (with option module). It provides separate signals for opening and closing, plus diagnostic words such as r0899.0 … r0899.13.

In both modes the physical output on the Motor Module is not switched solely because r0899.12 = 1. The state machine must additionally see that the drive is in Operation enabled (r0899.0 = 1) or that an explicit "open brake by sequence control" path has been configured.

Parameters and Signals Reference

The parameter list above is taken from the Siemens Industry Online Support parameter documentation for the SINAMICS S120/S150 List Manual. The signals DB380x/DB390x are documented in the SINUMERIK 828D List of Interface Signals (NC variables interface).

Root Cause Analysis

Three different attempts were made on the bench:

1. Setting p1215 = 2 — this is the value "motor holding brake always released" in some firmware versions, but in S120 FW 4.8 the value 2 means "motor holding brake, connection via BICO" only when used together with p0855; the 24 V output is still gated by the state machine. Result: drive-status bits toggle, but the BR+ / BR- terminals stay de-energised.
2. Extended brake control (p1278 = 1) with the option module not licensed — the drive reports the correct sequence-control status words (r0899.12 = 1, r0899.13 = 0), but the option module is not active, so the output stage is not enabled.
3. NC/PLC command DB380x.DBX4000.5 = 1 via the SINUMERIK PLC — the NC correctly echoes the command (DB390x.DBX4000.5 = 1) and the drive-side status bits change, but the Motor Module still does not drive the brake output because the drive is in Pulse inhibit / Ready to switch on rather than Operation enabled.

The common denominator is the missing operating state: the SINAMICS S120 will not energise the internal brake output unless the drive is at least in Ready to operate with the controller enable and pulse enable both set. In other words, the sequence control must have produced a request to open the brake (r0899.12 = 1) AND the drive must be in a state where it is allowed to follow that request.

Solution Procedure

The combination that actually opens the brake contacts from software on this configuration is:

1. Set p1215 = 1 (motor holding brake like sequence control, connection via MOTOR connector — internal output on the Motor Module).
2. Set the controller enable STW1.0 = 1 for the drive object (or wire p0840 = 1 via the PLC).
3. Bring the drive to Ready to operate by clearing all faults (r2135 = 0) and confirming that the pulse enable is granted (r0899.0 = 1).
4. From the PLC user program, set DB380x.DBX4000.5 = 1 for the axis. The drive sets r0899.12 = 1 and after p1216 milliseconds the BR+ / BR- output energises the brake coil.
5. To release the brake without rotation, keep the drive in a torque-disabled state (e.g., jog speed set to zero) so the rotor can be turned manually by the operator.

Alternative: BICO Release Through a Digital Input

If you need to release the brake from a hardware key (for example, a button on the MCP) without involving the PLC, use the BICO path:

p1215 = 3         ; sequence control + BICO
p0855 = r0722.x   ; source = status of digital input x on the Control Unit
p0840 = r0722.y   ; optional: drive enable tied to the same input

As long as the digital input is high, r0899.12 = 1 and the Motor Module energises BR+ / BR- after the configured opening time p1216. The brake closes immediately when the input goes low, following the closing time p1217.

This is the same wiring path that the Siemens application example for "service / setup mode" uses when the operator must be allowed to move the motor by hand before the axis has been homed.

NC/PLC Interface Method (SINUMERIK 828D)

The signal DB380x.DBX4000.5 is the "Open motor holding brake" request from the PLC to the NC. The NC forwards it to the drive as a request; the drive then executes the brake sequence through r0899.12.

Minimal PLC example (structured text, axis index 1):

// Manual brake release request, axis 1
// DB3801 is the axis-specific request DB; bit 4000.5 is "open holding brake"
U  E 100.0;        // MCP key "release brake"
=  DB3801.DBX4000.5;

// Optional interlock to prevent the operator from moving the rotor
// while the spindle drive is at high speed
U  DB3901.DBX4000.5;  // acknowledge from NC
S  M 200.1;           // "brake released" flag for HMI

On the HMI the operator button can be wired through softkey PLC flags; the SINUMERIK Operate screen does not normally expose a "release brake" command, so this is almost always a custom screen or a user-defined key on the MCP.

Extended Brake Control with p1278 = 1

When the drive option "Extended brake control" is licensed (r0108.14 = 1), p1278 = 1 unlocks the full set of status words in r0899 and adds the diagnostic signals listed in the List Manual. With extended brake control, the brake opening request can come from:

• The sequence control (default — p1215 = 1)
• A BICO source on p0855 (with p1215 = 3)
• The drive-internal signal r0899.12 via the NC/PLC interface

The physical output on the Motor Module is identical in all three cases. The extended mode only changes which status bits are visible in r0899 and how the brake is integrated with Safety Integrated (SBC / SS1).

Safety Considerations

The discussion thread that produced this case highlights a critical point: any active Safety Integrated function will overrule the manual brake release. If SBC (Safe Brake Control) or SS1 (Safe Stop 1) is configured for the axis, neither p1215 nor p1278 nor the NC/PLC command will energise the brake output while the safety function requests "brake closed".

• Verify that no Safety Integrated function is active on the affected axis. In STARTER, the screen form Functions > Safety Integrated must show the safety option as deactivated for the axis.
• If the axis is part of a Safety Integrated group, all axes in that group must be in a safe state before the manual brake release is honoured.
• For a horizontal grinding spindle (as in this case), the rotor position is not gravity-loaded. The mechanical risk of an unintentional free rotation is low, but the application is still subject to the general safety policy of the machine.

For a deeper treatment of brake timing in vertical and horizontal applications, the Rockwell Automation motion application note "Vertical Load and Holding Brake Management" is a useful cross-vendor reference. The holding-brake opening time given in the 1FT7 data sheet (typically 60 – 180 ms) should be entered in p1216 so the drive does not command torque before the brake has physically released.

Verification

1. Connect a voltmeter between BR+ and BR- on the Motor Module X2 terminal block.
2. With the drive in Ready to operate and the controller enable asserted, set DB380x.DBX4000.5 = 1 from the PLC.
3. Read r0899.12 = 1, r0899.13 = 0 in STARTER / Startdrive.
4. After the time p1216, the voltmeter should read 24 V DC, polarity BR+ = positive.
5. Confirm the rotor can be turned by hand with the controller in this state.
6. Reset DB380x.DBX4000.5 = 0 and verify the brake re-engages after p1217.

If step 4 shows 0 V even though all status bits are correct, check the safety state of the drive with r9772 (SI diagnostics) and the drive state with r0899.0. The drive must be in Operation enabled for the BR+ / BR- output to be active.

Parameter Quick Reference

Troubleshooting Matrix