Resolving STARTER Drive Unit Discovery on SINUMERIK NCU 710

The Siemens SINUMERIK 840D sl line uses an integrated NCU (Numerical Control Unit) that contains a PLC-style control CPU plus an internal SINAMICS S120 drive cluster connected via PROFIBUS or PROFINET. Commissioning engineers familiar with stand-alone SINAMICS S120 cabinets expect to open STARTER and let the "Find Drive Units" wizard populate the project tree automatically. On a SINUMERIK 840D sl with NCU 710.3B PN (article number 6FC5371-0AA30-0AB0) and a NX10.3 module (6SL3040-1NC00-0AA0), that wizard often returns an empty result even when the service laptop can ping the NCU and reach it over SSH/VNC. This article documents the underlying topology that causes the failure, the version-compatibility trap that hides it, and the field-proven procedure for getting STARTER online with the embedded drive cluster.

1. System Configuration in This Scenario

The reported machine uses the following hardware and catalog numbers:

The SINAMICS catalog string 6SL3120-1TE21-8AA4 decodes as a Booksize Single Motor Module (booksize Compact / booksize format, 50 mm width) with 18 A rated output current, 600 V DC link, internal air cooling, and no integrated line filter. Five of these modules are daisy-chained inside the NCU/NX10 cabinet and reach the NCU control CPU over the internal PROFIBUS or PROFINET link that SINUMERIK provides between the integrated CU and the physical drive units.

From STARTER's point of view there are two distinct SINAMICS domains in the cabinet:

1. The integrated Control Unit (CU) that lives inside the NCU 710. This is a virtual CU that the NCK firmware instantiates over its internal PROFIBUS link. STARTER cannot discover it as an Ethernet participant; it must reach it through the NCU gateway.
2. The external SINAMICS S120 drive units (the five 6SL3120-1TE21-8AA4 Motor Modules). These are daisy-chained to the integrated CU.

Both domains are reachable only through the NCU's two Ethernet ports. Understanding the addressing on those ports is the first step in fixing the discovery failure.

2. Network Topology and Port Addressing

The NCU 710.3B PN exposes two Ethernet interfaces that engineers routinely confuse. Their default behaviour is fixed and cannot be changed by STARTER:

On this machine the laptop was placed on X127 first and received 192.168.215.2 from the NCU's built-in DHCP server. The laptop could ping 192.168.215.1, open an SSH session, and open VNC. So the Ethernet link to the NCU control CPU was healthy. The problem was not the physical layer.

When the laptop was moved to X120, it received 192.168.214.241 and the NCU appeared at 192.168.214.1. STARTER was able to populate an entry for the NCU in its "Accessible Nodes" view, but it could still not expand the integrated CU and the SINAMICS drive cluster under it.

3. Root Cause of the Discovery Failure

Three independent failure modes combine to produce the "STARTER cannot find any device" symptom. All three must be addressed before the drive cluster becomes visible.

3.1 Wizard vs. Accessible Nodes

The "Find Drive Units" wizard in STARTER sends a PROFINET DCP multicast identify-all request on the local subnet. The NCU control CPU does not implement the DCP identify reply for the integrated CU, and the SINAMICS Motor Modules are not addressed on the wire at all. Therefore the wizard will time out and report an empty list. The user sees the "discovery takes a long time (few minutes) but no units are finally found" symptom.

STARTER's separate Accessible Nodes view, in contrast, lists any Ethernet device that answers to a single IP probe. Once the NCU is listed there, you can manually type its address in the IP Address of the sought node field and let STARTER do a unicast probe. That probe successfully reaches the integrated CU through the NCU's internal routing logic, and the drive tree expands.

3.2 Routing Between X127 and X120

Accessible Nodes does not perform routing. If the laptop is on 192.168.215.x (X127) and the SINUMERIK reports the integrated CU on 192.168.214.x (the X120 plant network), STARTER will not bridge the two subnets. The fix is one of the following:

• Connect the laptop to X120 instead of X127, so the laptop and the SINUMERIK share the same 192.168.214.x subnet.
• Add a static route on the laptop so that the 192.168.214.0/24 subnet is reachable through the 192.168.215.1 gateway (X127). On Windows: route ADD 192.168.214.0 MASK 255.255.255.0 192.168.215.1. This works only if the NCU control CPU is configured to route between its two interfaces, which is the default on current NCU firmware.

In the field case described in the source, option 1 was used and STARTER could see the NCU at 192.168.214.1 when the laptop was on 192.168.214.241.

3.3 STARTER Version Incompatibility

The most subtle cause is the STARTER build version. The user originally installed STARTER 4.4 HF3, which is the wrong hotfix branch for SINUMERIK 840D sl. Siemens publishes version compatibility notes on the SINAMICS STARTER entry (ID 26233208) on Industry Online Support. HF3 contains changes that are required for stand-alone SINAMICS S120 commissioning but break the SINUMERIK-side connection. The supported build for SINUMERIK 840D sl with current NCU firmware is STARTER V4.4 SP1 (or the SP/HF chain explicitly listed in the compatibility matrix for the installed NCU version). After upgrading, both the wizard timeout and the empty Accessible Nodes expansion disappeared.

4. Prerequisites for a Successful Connection

Before STARTER can be brought online, verify the following:

1. STARTER V4.4 SP1 or later installed on a Windows 7 SP1 / Windows 10 engineering laptop. The release notes for the SP branch list which NCU firmware revisions are supported. Avoid V4.4 HF3 and any later HF that is not paired with a SINUMERIK SSP.
2. SINAMICS Support Package (SSP) installed for the SINAMICS firmware version on the integrated CU and the drive units. The SSP is delivered as a separate setup (for example SSP SINAMICS S120 V5.x). Without the matching SSP, STARTER can read but not write parameters, and the project tree will not list all objects.
3. TCP/IP routes configured so that the laptop can reach both the X127 and X120 subnets, or alternatively the laptop is physically on the same subnet as the NCU port that STARTER will use as entry point.
4. SINUMERIK Operate access to confirm the SINAMICS objects are alive. Open Commissioning > Drive System > Drive Objects on the HMI and verify the number of DO matches the physical count (5 Motor Modules in this scenario).
5. Read/write authorisation. STARTER's default access level for SINUMERIK-hosted drives is read-only. The user must log on to the NCU with the OEM or customer password before write access is granted, or parameter writes will be rejected with F01910 / F08501 type codes.

5. Step-by-Step Connection Procedure

The procedure below is the field-proven sequence. It assumes the hardware is already powered and the NCU is in the cyclic state (no NCK stop, no PROFIBUS/PROFINET alarms). If you are in the cold-start phase, finish the HMI commissioning first, including the drive-object list, before opening STARTER.

1. Open STARTER in the Siemens project view, not the older S7 project view.
2. Select the project tree, then choose Project > Accessible Nodes (or the magnifier icon). Do not use the Find Drive Units wizard. The wizard will not work on a SINUMERIK-hosted CU.
3. Once Accessible Nodes lists the NCU (for example at 192.168.214.1), click on the field IP Address of the sought node and type the NCU's IP address exactly as shown in the list.
4. Click Update. STARTER performs a unicast probe. Within a few seconds the SINAMICS device icon (a +) appears next to the NCU entry.
5. Click the + to expand the SINUMERIK entry. The integrated CU appears, and below it the five drive objects (DOs) corresponding to the 6SL3120-1TE21-8AA4 modules.
6. Right-click the integrated CU > Connect to target system. STARTER asks for the access password if write access is required. Type the appropriate password (manufacturer, service, or customer level).
7. Once online, the parameters in the expert list can be edited. The Drive Object list should match the five physical Motor Modules; if it does not, the DO configuration in SINUMERIK Operate must be re-checked before continuing.
8. Save the project offline as a backup before any parameter download. STARTER Download to target device writes to the CU's RAM and (if "Copy RAM to ROM" is selected) to the NV media on the Motor Modules.

6. Verification Checklist

After STARTER reports an online connection, run through the following checks before tuning any current or speed controller:

1. Online state: every DO in the STARTER project tree shows a green icon and the status "Online". A red icon means STARTER could not reach that DO over the internal PROFIBUS; check the SINUMERIK alarms first.
2. DO count: the number of DOs in STARTER equals the number of physical SINAMICS axes (5 in this scenario). A mismatch means the SINUMERIK topology configuration is out of sync with the cabinet.
3. Firmware version: the firmware version of the integrated CU and each DO matches the version listed in the SINAMICS STARTER compatibility table (ID 26233208). A version mismatch triggers F01000/F01001 family faults during parameter download.
4. Access level: the active access level in STARTER is at least Expert. If it shows Standard, STARTER will refuse to write current-controller parameters.
5. Save project: the offline project is saved to disk with a descriptive name including the NCU serial number. This is the only record of any parameter changes made through STARTER; the NCK does not store the STARTER project itself.

7. Common Faults and Their Meaning

The following alarm/fault codes are routinely encountered during this procedure. They are not exclusive to the wizard failure path; they may appear after the connection is established, during parameter download, or during the first controller optimisation pass.

8. Tuning the Current and Speed Controllers via STARTER

Once STARTER is online, the typical fine-tuning sequence for the SINAMICS S120 drive objects is:

1. Open the Drive Object (for example DO2 = spindle) and select Commissioning > Motor > Motor data identification. Run the standstill identification. Wait for the value to switch from "Running" to "Finished".
2. Switch to Commissioning > Speed controller > Automatic controller setting. The controller computes a starting point for the Kp/Tn of the speed controller from the identified mechanical data.
3. Move to Commissioning > Current controller > Manual optimisation. Adjust p1715 (current controller P gain) and p1717 (current controller integral time) only if the current loop response measured with the trace function is not within the SINAMICS specification (typical 1.0 to 1.5 kHz bandwidth for a 6SL3120-1TE21-8AA4 Motor Module).
4. Use the Trace function to record the speed actual value and the torque setpoint. Trigger on a step in the speed setpoint; the resulting step response should reach 90% of the setpoint in less than 20 ms with no overshoot above 5%.
5. Save parameters to ROM (Copy RAM to ROM) from STARTER or via p0977 = 1 in the expert list. Restart the NCK if prompted.

For the 6SL3120-1TE21-8AA4, the SINAMICS S120 parameter manual lists the rated current at p0305 = 18 A and the maximum current at p0640 = 150 % of rated, both of which are useful sanity checks before downloading a controller preset.

9. Alternate Connection Path: Using Startdrive in TIA Portal

STARTER remains the canonical tool for SINAMICS commissioning on legacy and mid-life SINUMERIK 840D sl lines. For newer machines, Siemens has migrated drive commissioning into Startdrive, which is integrated into the TIA Portal project tree. Startdrive understands the same drive objects and parameters as STARTER, and it can also reach the integrated CU through the NCU. If STARTER is being phased out of the customer's tool stack, the equivalent procedure is:

1. Open the TIA Portal project that contains the SINUMERIK PLC program.
2. Open the Devices & networks view, right-click the SINAMICS integrated CU, and select Online & Diagnostics.
3. Use the same direct IP address entry that STARTER would use, and the integrated CU will appear under the TIA Portal project tree.

Startdrive is not a replacement for the SINUMERIK HMI commissioning; it is a parallel tool. STARTER can still be installed side-by-side and used to read out parameters from older cabinets where TIA Portal does not understand the SINAMICS firmware version.

10. Troubleshooting Matrix

11. Field Notes and Safety

• Do not change the PROFINET device name or the IP address of the integrated CU from STARTER. The NCU owns these settings. Use SINUMERIK Operate Commissioning > Network instead.
• The integrated CU on a SINUMERIK 840D sl uses the same firmware stream as a stand-alone CU320-2 PN. The STARTER SSP that supports the matching firmware version is the one to install; do not install both the "S120" and "S150" SSPs in the same STARTER installation unless the support tool explicitly allows it.
• Always save the project before downloading. The project file is the only place the parameter set is recorded; the NCU does not keep a STARTER-format project.
• If the cabinet has Safety Integrated (SINAMICS S120 Safety) functions, the safety parameters are write-protected and require the safety password as well as the regular access level. STARTER's Safety Integrated view prompts for both.
• For multi-axis spindles and torque motors, run the motor data identification at standstill and at rotating speed (p1900 = 12) before any controller tuning. The standstill identification alone is not sufficient for high-bandwidth spindles.