Connecting STARTER to SINUMERIK 840D sl NCU 70.2: X127 Ethernet and Profibus Setup

The SINUMERIK 840D sl with NCU 720.2 PN (order code 6FC5 372-0AA00-0AA1, colloquially the NCU 70.2) is one of the most widely deployed CNC platforms in factory automation. Most field engineers need to attach a Siemens STARTER commissioning tool to the integrated SINAMICS S120 drive line for first commissioning, drive replacement, or firmware alignment. The hardware exposes two practical paths: the X127 service Ethernet port (DHCP, 192.168.215.1) and the X126 / X136 PROFIBUS DP sockets. The MPI/DP combined port (X124) is not a STARTER path for the SINAMICS S120; it terminates PROFIBUS I/O segments and MPI participants. This guide identifies every relevant socket on the NCU 70.2, walks through the X127 Ethernet connection, and covers the X136 PROFIBUS option for engineers who must work through the DP bus.

1. System Identification and Required Part Numbers

Before any cable is plugged in, confirm the hardware revision. The order code printed on the front of the NCU is the only reliable identifier, because Siemens has shipped three functionally similar variants under the NCU 70.x label.

The example system in this guide is the 6FC5 372-0AA00-0AA1 - the standard NCU 720.2 PN with PROFINET, two PROFIBUS interfaces, and one service Ethernet port. The SINAMICS S120 is the drive line that ships with the DRIVE-CLiQ backplane inside the NCU box and connects to external Active Line Modules and Motor Modules through DRIVE-CLiQ.

Reference the official structure of the documentation at the SINUMERIK 840D sl / SINAMICS S120 Safety Integrated manual (FBSI, 03/2006 edition) for the legal/functional boundary between NCU firmware and the SINAMICS drive objects. The SINUMERIK 840D sl / SINAMICS S120 Machine Configuration Guidelines (MA1, 10/2009) describes the topology and addressing rules used throughout this article.

2. NCU 70.2 Physical Interface Map

Every NCU 70.2 faceplate labels its sockets identically regardless of the firmware load. Misidentifying the service port is the single most common cause of "STARTER cannot find the device" errors.

3. X127 Service Ethernet Port - The Recommended Path

X127 is a fixed-configuration 10/100 Mbit/s Ethernet port with an embedded DHCP server. It exists exclusively to give a service PG/PC a non-routable access path into the NCU. There is no need to know the IP address in advance: the NCU hands one to your laptop.

3.1 Default Addressing

3.2 PG/PC Network Configuration

1. Connect a Cat 5/6 patch cable from the NCU's X127 (RJ45) directly to the engineering laptop. Do not pass through a managed switch - X127 is a point-to-point segment and any switch in the middle will issue its own DHCP offer and break the lease.
2. On the laptop, open Control Panel > Network and Sharing Center > Adapter Settings (Windows 10) or Settings > Network & Internet > Ethernet (Windows 11).
3. Right-click the wired Ethernet adapter, choose Properties > Internet Protocol Version 4 (TCP/IPv4) > Properties.
4. Select Obtain an IP address automatically and Obtain DNS server address automatically. Confirm with OK.
5. Wait 5-10 seconds, then run ipconfig /all from an elevated command prompt. The Ethernet adapter should show an IPv4 address inside the 192.168.215.0/24 range with the NCU as the DHCP server.
6. Verify the route with ping 192.168.215.1. A reply confirms the NCU's TCP/IP stack is up and the service interface is alive.

3.3 PIN and Service Unlock

The NCU 7x0.2 has a service switch on the front panel - the rotary selector (S1) and the password-protected service login. On firmware from V2.6 SP2 onward, the service interface is locked at the SINUMERIK level (HMI Advanced / Operate) with a four-digit PIN. The factory default is typically 0000 or the customer PIN set at commissioning. STARTER can still browse the drive topology before the HMI unlocks, but writing parameters requires the SINUMERIK access level.

4. STARTER Software Installation and Project Setup

STARTER is the legacy commissioning tool for SINAMICS drives; SINAMICS Startdrive (TIA Portal plug-in) replaces it for projects on the TIA Portal from V15 onward. For an existing NCU 70.2, STARTER V5.4 SP3 or higher remains the safer choice because it has mature DCC (Drive Control Chart) libraries and direct topology import from the NCU backup.

4.1 Install Prerequisites

• STARTER V5.4 SP3 or later (installed on Windows 10 / Server 2016 or newer; 32-bit still supported but 64-bit recommended).
• Microsoft .NET Framework 4.6.2+ (bundled with STARTER installer).
• Siemens SIMATIC Logon (optional, for change-tracking).
• A current STARTER device library (regularly update via >Options>Web>Update).

4.2 Insert the NCU into a New Project

1. Launch STARTER. Choose Project > New > New with wizard (or Project > New > New (offline) for fully offline work).
2. In the Insert SINUMERIK dialog, select 840D sl and the matching NCU designation (e.g. NCU 720.2 PN). STARTER auto-fills the integrated SINAMICS S120 line.
3. Click Finish and go online when the wizard closes, or stay offline until you have built the topology.

5. Going Online via X127

1. Right-click the project node in the navigation tree and choose Target system > Online > Go online (or press F7).
2. In the Accessible nodes window, choose the interface S7ONLINE (STEP7) -> TCP/IP (Auto) -> Intel(R) Ethernet Connection I219-V (your wired adapter).
3. Click Start node search. The NCU 70.2 should appear with its X127 IP 192.168.215.1 and the chassis name (e.g. NCU720.2PN).
4. Select the device and click OK. STARTER negotiates the connection and reports the SINUMERIK access level.

5.1 What STARTER Sees When Online

After the online connection is established, STARTER reads the live drive topology from the NCU. For an NCU 720.2 PN with one Active Line Module and three Motor Modules, the drive objects are typically:

The drive objects are addressed internally through PROFIBUS. From STARTER's perspective, the connection is TCP/IP to the NCU and the NCU proxies PROFIBUS DP telegrams to each drive. This is why STARTER needs no PROFIBUS card on the laptop for X127 access.

6. PROFIBUS Path: X126 / X136 Connection

When the X127 service port is unavailable (locked out by a forgotten password, hardware fault, or commissioning in a Faraday-cage cabinet), the only practical STARTER path is through the NCU's PROFIBUS DP master. The NCU 720.2 PN carries two identical DP interfaces: X126 and X136. Both are 9-pin D-sub female sockets wired to PROFIBUS pinout (pin 3 = data B, pin 8 = data A, pin 6 = VP, pin 5 = DGND). Either socket can be used.

6.1 When to Use PROFIBUS Instead of Ethernet

• The X127 socket is damaged or has no link LED.
• The customer enforces separation between the service network and the office network (X127 cannot be reached, X136 must be used).
• The system is integrated with an upstream SIMATIC S7 PLC that exposes the drive via DP, and you must troubleshoot that path.

6.2 PG/PC Hardware Requirement

PROFIBUS from a laptop requires a CP 5611 / CP 5612 / CP 5711 / CP 5512 card and the Siemens SIMATIC NET PROFIBUS driver. Install the card, then run Set PG/PC Interface in the Windows Control Panel and assign S7ONLINE (STEP7) -> CP5611.PROFIBUS.1 (or the matching CP).

6.3 PROFIBUS Address and Baud Rate

The NCU's DP master on X126 has a default address of 3 and runs at 12 Mbit/s by default. The SINAMICS S120 drive objects on the same internal bus use addresses 2-15 in sequence. Adjust with the rotary switches on the NCU front panel only when commissioning a second NCU on the same segment.

6.4 Going Online via PROFIBUS in STARTER

1. Plug the CP 5611 / CP 5711 into the laptop, install SIMATIC NET, and assign the PG/PC interface to PROFIBUS.
2. Connect a PROFIBUS drop cable from the CP to a free tap on the DP segment (X136 or X126 of the NCU).
3. Ensure the tap's terminator is OFF (mid-segment tap).
4. Launch STARTER, open the project, and press F7.
5. Select S7ONLINE (STEP7) -> CP5611.PROFIBUS.1, then click Start node search. The NCU and all reachable drive objects should appear.

PROFIBUS scans are slower than Ethernet. Allow 10-15 seconds for the segment to enumerate. If no devices appear, check the PG/PC address conflicts: a laptop is usually address 0 on PROFIBUS - confirm with the Set PG/PC Interface dialog.

7. MPI / DP Port (X124) - Why STARTER Does Not Use It

The contributor's follow-up question - "can I connect with an MPI cable?" - has a definitive answer: not for STARTER, and not for the SINAMICS S120. X124 on the NCU 70.2 is the MPI/DP combined port for the legacy HMI-Pro, PLC, and external I/O segment. It is electrically a DP slave on a customer-built bus; STARTER's CP 5611 can theoretically talk to it, but the NCU 70.2 will not proxy the SINAMICS drive objects through X124. The DP proxy for the SINAMICS S120 exists on X126 and X136 only.

If a customer demands a service terminal via MPI, the supported solution is an HMI-Pro panel on X124, not STARTER. Keep the MPI bus separate from the drive PROFIBUS.

8. Parameter Verification After Going Online

Once STARTER is online, verify the following drive-side parameters before any axis is moved. They are the most common sources of "comm loss" faults at first commissioning.

9. Verification Checklist

1. Physical link - both Link LEDs on X127 (green on the NCU, link on the laptop) are lit. A dark Link LED means the cable, port, or NIC is bad.
2. IP lease - ipconfig /all on the laptop shows an IPv4 address in 192.168.215.0/24, default gateway empty.
3. Ping - ping 192.168.215.1 -t returns < 1 ms replies.
4. STARTER online - Project > Online > Go online succeeds; the online bar turns green.
5. Topology import - Drive unit > Topology > Compare with offline shows all drive objects green.
6. Fault buffer - Drive unit > Diagnostics > Fault buffer is empty or contains only historical faults.
7. Access level - Target system > Access level shows Service or Manufacturer.
8. Identical project - in STARTER, Target system > Copy RAM to ROM is greyed out only when the project matches the live configuration. If the button is active, the project diverges from the NCU - load the NCU's actual topology with Load to PG/PC.

10. Troubleshooting Matrix

11. Safety, ESD, and Network Isolation

• Power state. The X127 service port remains live even when the machine's main disconnect is OFF. Treat the NCU as a live device at all times. The integrated SINAMICS S120 may still hold DC link voltage for several minutes after the line contactor opens - wait for the DC-link LED on the Active Line Module to extinguish before touching any drive wiring.
• ESD handling. Wear a wrist strap grounded to the cabinet's PE bar when changing S120 modules. The DRIVE-CLiQ connectors on the front are not ESD-sensitive in normal use, but the SD card slot on the NCU is.
• Network isolation. X127 is a closed, non-routable segment. Never bridge it to the office network. If the customer needs remote commissioning, install a properly configured VPN endpoint at the machine, not at X127.
• Cyber-security. The SINUMERIK 840D sl ships with SINUMERIK Operate. STARTER inherits the SINUMERIK access level, so change the customer password from the factory default before handover. Reference the SINUMERIK 840D sl Safety Integrated manual for the Safety-Integrated-specific access levels.
• Backup. Before any firmware change, take a Commissioning > Series commissioning > Create series commissioning file backup from STARTER. Store it on an off-machine medium and label with the NCU order code and firmware version.

12. Frequently Asked Questions