SINUMERIK 840D sl Pulse Enable Missing During Axis Commissioning

1. Problem Overview and Symptom Set

During the first power-up of a mono-axis table controlled by a SINUMERIK 840D sl NCU, the operator can observe a fully stalled enable chain: the drive status indicates that pulses are not released, the SINAMICS line module reports that the infeed is not in operation, and the position controller never becomes active. The drive ready bit in the NCK interface and the pulse enable bit in the axis interface remain cleared even though the machine control panel (MCP) shows no error indication and the HMI reports "Ready to run". This condition is typical when the SINAMICS S120 drive stack has been wired and the PLC project has been loaded, but the NCU digital I/O has not yet been wired to the enable logic and the encoder interface has not been assigned.

The technical symptom cluster is defined by the following interface bits:

• DB10.DBX108.6 — Drive ready (collective, NCK → PLC) — stuck at 0.
• DB31.DBX21.7 — Pulse enable for axis 1 (NCK → PLC) — stuck at 0.
• DB31.DBX61.6 — Speed controller enable for axis 1 (NCK → PLC) — stuck at 0.
• DB31.DBX1.5 / DB31.DBX1.6 — Position measuring system 1 / 2 selected — both 0.
• NCU digital outputs assigned to OFF2 / OFF3 — stuck at 0 (drive sees coast-stop and quick-stop active).
• NCU digital input assigned to "Infeed active" (Line Module ready) — reads 0.

When all of the above are simultaneously low, the controller will refuse to release any axis and the service screen will display the alarm class "Axis disable active" with cross-reference to the missing enable sources.

2. SINUMERIK 840D sl Enable Signal Architecture

The 840D sl enable chain is a strictly hierarchical signal flow. A failure at any node prevents the next node from becoming active, so a single low pulse enable bit often masks several simultaneous upstream faults.

1. NC control enable — produced by the NCK after RESET, mode-group ready, and reference-point check have all returned 1.
2. Axis enable (per axis) — produced from DB31.DBX2.1 (controller enable request from PLC), feed hold release, and follow-up mode state.
3. Drive enable — OFF1, OFF2, OFF3 must all be released by the PLC on the SINAMICS terminals / telegram.
4. Infeed enable — the Smart Line Module (SLM) or Active Line Module (ALM) must report Run state via the cyclic PROFIdrive / PROFINET telegram (STW2 / ZSW2).
5. Pulse enable — generated by the SINAMICS Control Unit when OFF1/OFF2/OFF3 are released and infeed is OK; reflected back as

   DB31.DBX21.7.

6. Speed controller enable — produced after pulses are released; reflected as DB31.DBX61.6.
7. Position controller enable — produced after the encoder has been selected and the position controller has been closed; reflected as DB31.DBX25.0 (position reached with exact stop coarse is NOT a substitute).

The PLC therefore cannot "force" the pulse enable bit — it must release every preceding level by either driving a digital output, populating a PLC flag, or letting the NCK and drive close their own internal checks (encoder, brake, terminal model, motor ID).

3. Axis Diagnosis Bit Map (DB10 / DB31 Reference)

The following table consolidates the exact bit addresses used during the axis diagnosis screen on the SINUMERIK Operate HMI. These bit positions are stable across the SINUMERIK 840D sl software versions 4.x and 6.x for the named functions.

4. Root Cause: OFF2 and OFF3 Active at NCU

The most common root cause for a permanently low DB31.DBX21.7 pulse enable bit on a fresh SINUMERIK 840D sl install is that the NCU digital outputs reserved for OFF2 and OFF3 of axis 1 are not being driven by the PLC program. In the default PLC basic program (FB1, FB2, FC1…) the OFF signals are sourced from machine-specific flags such as LBP_Axis1.OFF2 / LBP_Axis1.OFF3, which are usually ganged to the Emergency Stop chain (NCU terminal X122 / X132 input) and the drive contactor feedback. If the project was generated from the toolbox without wiring the MCP and E-Stop chain, those flags remain at the safety default value of 1 (i.e. "OFF active").

Verification steps:

1. Open the online PLC project in STEP 7 (Classic) for the S7-300 integrated in the NCU.
2. Force-watch the data block in which the axis enable bits are mapped — typical candidates are DB31 mirror DBs created by the toolbox basic program.
3. Check the source flag for LBP_NC.A_OFF1[1], LBP_NC.A_OFF2[1], LBP_NC.A_OFF3[1] — if any of these is 1 while the machine is in a safe state, the basic program has not been parameterised for the connected E-Stop topology.
4. Inspect the NCU's onboard digital output assigned to OFF2/OFF3 (commonly X122.6 / X122.7) using a voltmeter or by inserting a "Watch" cross-reference in the HMI diagnostic screen.

5. Root Cause: Infeed Active Feedback Missing

The SINAMICS S120 line module must report "Infeed active" before the drive will accept the controller enable. This feedback is generated internally by the SINAMICS CU320-2 (or CU310-2 for single-axis modules) and is propagated through the standard telegram (typically Siemens telegram 111 or 116 for SINUMERIK) as bit 0 of the ZSW2 word. The NCK mirrors this bit into DB10.DBX106.1 (infeed ready, collective) and the per-axis infeed OK is part of the axis diagnosis screen.

If the line module has been powered up but no DRIVE-CLiQ connection has been established — for example because the DRIVE-CLiQ cables between the Line Module and the Motor Modules are not yet inserted, or the topology rules have been violated (port 0 → port 1 in a single ring) — the line module will remain in "Commissioning" state and the infeed active bit will not be set.

Verification steps:

1. From the HMI navigate to Diagnostics → Drive system → Topology and confirm that the line module and all motor modules appear with their expected component numbers.
2. Check the LED states on the SINAMICS CU: green flashing at 2 Hz = commissioning required; green solid = topology OK; red flashing = fault.
3. Use the SINAMICS Startdrive / Starter commissioning tool to read r0002 (drive operating display) on the infeed — the expected value at standstill is 0 (commissioning) or 4 (run ready); anything in the 6xx range indicates a topology / configuration fault.
4. Verify that the 24 V supply on the SINAMICS X124 terminal is present — loss of 24 V at the CU is the single most common reason for "no infeed active" on a new cabinet.

6. Root Cause: Measuring System Not Selected

The 840D sl requires that exactly one position measuring system is declared as "active" before any motion command is accepted. The selection is made by the PLC via DB31.DBX1.5 and DB31.DBX1.6:

• DBX1.5 = 1, DBX1.6 = 0 — measuring system 1 active (motor encoder by convention).
• DBX1.5 = 0, DBX1.6 = 1 — measuring system 2 active (load-side direct encoder).
• DBX1.5 = 1, DBX1.6 = 1 — both encoders active (e.g. for dual-loop control on a linear motor). Both encoder bits must also be configured in the machine data MD30200 $MA_NUM_ENCS with a value of 2.

If neither bit is set, the NCK cannot close the position loop and the pulse enable is suppressed. In the toolbox default, the basic program selects the encoder through LBP_Axis1.Meas1Active / LBP_Axis1.Meas2Active, which in turn is derived from the machine data MD30240 $MA_ENC_TYPE[AX1] and the parameter set selected by the user via HMI.

Verification steps:

1. Open Commissioning → Axes → Drive/Encoder configuration and verify the encoder assignment against the machine data MD30220 $MA_ENC_MODULE_NR[AX1, ENC].
2. Force the relevant PLC flag to 1 through a temporary watch table and confirm that DB31.DBX1.5 / DB31.DBX1.6 follow.
3. On the axis diagnosis screen, the encoder status field will indicate "Encoder not configured" if the encoder table has not yet been saved.

7. NCU Digital Output Configuration Procedure

The NCU 710.3B / 720.3B / 730.3B provide 8 onboard digital outputs on terminal X122 (8 outputs, 24 V 0.5 A each) and 8 digital inputs on terminal X122. The mapping of these physical channels to logical function names is performed in the PLC program rather than in the HMI, but the assignment must be documented in the project so that the wiring diagram matches.

1. Open the PLC project (STEP 7 Classic, V5.5 SPx or V5.6 SPx) corresponding to the SINUMERIK 840D sl target.
2. In HW Config, confirm that the NCU has been inserted with its correct order number, e.g. 6FC5371-0AA00-0AA0 for NCU 720.3B PN. The digital I/O are part of the NCU submodule, so no additional DI/DO module is required for OFF signals.
3. Open the FC for the axis enable logic — in the standard toolbox this is FC100 "Axis enable" or the equivalent machine-specific block.
4. Wire the OFF2 / OFF3 outputs so that they are released (drive convention: 0 V on terminal) when the E-Stop chain is closed, the drive contactor is healthy, and the controller enable request is true.
5. Save, compile, and download to the NCU. The download must include both the System Data (SDB) and the program blocks; an SDB-only download leaves the I/O mapping untouched.

8. PROFIBUS MCP Hardware Configuration and GSD Setup

The Machine Control Panel (MCP) connects to the NCU via PROFIBUS DP and is described to the S7 hardware configuration through a GSD file provided by the SINUMERIK Toolbox. The exact GSD depends on the MCP variant:

If the GSD entry does not appear in the HW Catalog under "SINUMERIK", the toolbox has not been installed on the engineering station. The procedure to recover is:

1. Close STEP 7 entirely.
2. Insert the SINUMERIK 840D sl toolbox DVD. The installer adds the "SINUMERIK" entry to the HW Catalog and registers the GSD files with the S7 install.
3. Re-launch HW Config; the MCP variants will now appear under PROFIBUS DP → Additional Field Devices → SINUMERIK → MCP.
4. If using a project that was created before the toolbox was installed, run Options → Install HW Updates on the GSD folder.

After installation, drag the MCP onto the PROFIBUS subnet that is connected to the NCU. The I/O addresses (typically EB0..EB31 for inputs and AB0..AB31 for outputs) must match the addresses expected by the basic program FC100..FC104 — if the toolbox placed the MCP at 0 by default, do not move it.

9. SINUMERIK Toolbox Installation Requirements

The Toolbox is mandatory because it ships:

• The "basic program" PLC library containing the standard FC1 / FB1 / FB2 / FC100..FC120 blocks that translate the NCK interface DBs into the HMI screens.
• The MCP, HHU, and handheld terminal GSD files.
• OEM-specific machine templates for S7 project generation.
• Operator panel runtime for the HMI.

A typical install path on a Windows 7 / 10 engineering station is C:\Siemens\Automation\SINUMERIK\840D sl\Toolbox\Vxx.x\.... After installation, confirm that the toolbox has registered by opening STEP 7 and checking Tools → Toolbox Integration → SINUMERIK 840D sl. The presence of the "All NC/RC" option under the "Insert Object" menu is the single most reliable check that the toolbox is fully integrated — if it is missing, the MCP GSDs will not appear either.

For new PLC projects, the recommended workflow is:

1. Open the SIMATIC Manager.
2. File → New Project from Template → SINUMERIK 840D sl.
3. Select the NCU order number that matches the physical hardware.
4. The template will pre-load all required DP slaves, basic program blocks, and machine data.
5. Download to the NCU, then connect with the HMI to verify the enable chain.

10. Commissioning Verification Sequence

Once the OFF signals, infeed feedback, and encoder selection are corrected, run the following verification sequence before attempting any motion command.

1. Power up the line module and confirm r0002 = 4 (run ready) on the SINAMICS Startdrive/Starter online view.
2. From the HMI, navigate to Diagnostics → Service axes → Axis 1. Confirm DB10.DBX108.6 = 1 (drive ready).
3. Toggle the controller enable from the MCP: the pulse enable DB31.DBX21.7 should follow within 100 ms.
4. Confirm DB31.DBX61.6 (speed controller enable) rises; this typically follows within 50 ms after pulse enable on a 1FK7 / 1FT7 motor.
5. Reference the axis: the NCK will issue DB31.DBX1.5 = 1 (encoder 1 active) automatically during referencing. Confirm that the actual position updates in the axis monitor.
6. Issue a low-speed jog in JOG mode: DB31.DBX25.0 should toggle, and the actual value should match the commanded value within the contour tolerance.
7. Repeat for all configured axes; if more than one axis is on the bus, the order of enable checks does not matter, but every axis must show all bits green.

11. Field-Tested Diagnostic Checklist

12. Frequently Asked Questions