SINAMICS SLM No LED on Power-Up: Wiring X24 24 V Supply

Problem Details

A booksize Smart Line Module (SLM), order number 6SL3130-6AE15-0AB0, is connected to the 3-phase AC line. The DC link measures approximately 570 V DC at the DCP/DCN busbars, confirming that the line contactor/fuses, the incoming supply, and the internal rectifier bridge are all functional. The signal connectors X21 and X22 have been wired to digital inputs/outputs of a TB30 terminal board. Despite this, no LED on the SLM front panel illuminates and the module never reaches a ready state.

The measured DC link value is consistent with a passive (uncontrolled) 6-pulse rectifier, which is exactly what a Smart Line Module uses for feeding:

• V_dc ≈ 1.35 × V_LL for an uncontrolled 3-phase bridge under light load
• 570 V / 1.35 ≈ 422 V line-to-line — i.e. a 400 V class supply on the high side of tolerance

So the power section is energized. The fault is entirely in the electronics (24 V) domain, which is powered separately from the line input.

Root Cause: X24 Electronics Supply Not Connected

Every booksize Line Module, Motor Module and Control Unit in the S120 family requires 24 V DC on its X24 electronics power supply connector. On booksize units this connector sits behind/under the protective cover on the top of the module and is bridged from unit to unit using the supplied 24 V terminal adapter (also referred to as the X24 jumper/adapter), which snaps across adjacent modules to distribute 24 V and its return along the whole drive line-up.

Two misunderstandings commonly cause this symptom:

The 24 V terminal adapter ships in the SLM packaging. If it was lost or never delivered, order it from Siemens as a spare part; alternatively the same 24 V DC and M potential can be brought in with the appropriate booksize 24 V connector hardware specified in the equipment manual.

Solution: Bring Up the 24 V Electronics Domain

1. De-energize completely. Open the line supply, lock out, and wait for the DC link to discharge. Verify 0 V between DCP and DCN with a meter rated for the voltage class before touching busbars or covers — 570 V DC is present and stored energy persists after the AC is removed.
2. Remove the top protective cover of the Smart Line Module to expose the X24 electronics supply contacts.
3. Fit the 24 V terminal adapter across X24 of the SLM and the adjacent module(s) — typically SLM → Motor Module(s), and SLM → Control Unit position — so all booksize units share one 24 V rail and one common M.
4. Feed the rail from an external 24 V DC power supply. Use a supply sized for the sum of the electronics current demand of every module on the rail plus any encoders, brakes and digital I/O loads; take the per-module electronics current from the device data in the equipment manual and add margin. Observe polarity: +24 V and M must not be reversed.
5. Bond the 24 V reference (M) to the cabinet PE/reference point as described in the installation instructions to keep the signal ground and EMC reference stable.
6. Check the X21/X22 wiring for the EP (Enable Pulses) terminals. The safe-torque/pulse-enable input must see +24 V for the module to leave the inhibited state. Pull the exact pin assignment for 6SL3130-6AE15-0AB0 from its equipment manual before relying on a TB30 output — do not assume it matches a Motor Module.
7. Re-apply 24 V first, then 3-phase AC. With 24 V present the SLM front LED must illuminate immediately, before the line contactor closes.

Verification

Why STARTER Does Not Diagnose This

The commissioning tool attaches to the Control Unit over its PROFIBUS/PROFINET/Ethernet interface. The small booksize Smart Line Module has no DRIVE-CLiQ interface, so it does not appear as an intelligent node in the DRIVE-CLiQ topology and reports nothing to the CU by itself. Its status is communicated only through:

• the front-panel LED,
• the hardware ready/fault contact and EP input on X21/X22, wired to CU or TB30 terminals.

That is why a dark SLM cannot be diagnosed from the software side. If you intend to monitor the line module state in the program, the ready contact must be wired into a digital input and evaluated in the drive or PLC logic. Confirm the terminal designations and the LED state table against the equipment manual for your exact order number before commissioning, and keep the 24 V and line-side power-up sequence in the cabinet's control logic rather than relying on operator procedure.

Commissioning Checklist for Booksize Line Modules

1. 24 V DC supply sized for all electronics loads on the rail, with the M reference bonded per the installation instructions.
2. X24 terminal adapter fitted across every adjacent booksize module; protective covers refitted afterwards.
3. EP terminals supplied with 24 V (or driven by the safety circuit) so pulses can be enabled.
4. DC link busbar links torqued and continuous across all modules.
5. PE and shield connections per the EMC installation guidelines.
6. Line-side fuses/contactor and any line reactor or filter matched to the module rating stated on the nameplate.
7. Power-up order: 24 V electronics → verify LEDs → close line contactor → verify ready state.