Overview
The SINAMICS G120 modular drive system with CU240S DP Control Unit communicates on PROFIBUS DP as a slave at selectable node addresses (default 3, configurable via P918 or DIP switches on the CU240S DP variant). When a commissioning engineer needs to perform a parameter upload/download (parameter backup or setpoint cloning) on an installed drive that is already networked with a SIMATIC S7-300 master, several PROFIBUS topology and STARTER software constraints affect whether the drive is visible to the engineering tool. This reference documents the field-proven diagnostic path, the correct hardware adapter choice (CP5711 vs. PC connection kit with USB-RS232), and the difference between STEP 7 HW Config Online and STARTER Accessible Nodes scans.
System Topology and Components
A typical service scenario involves the following hardware:
- SIMATIC S7-300 CPU (e.g., CPU 314) acting as the PROFIBUS DP Class 1 master on the first DB9 (MPI/DP) port.
- SINAMICS G120 with CU240S DP Control Unit, set as a DP slave at addresses 15-18 in the field case described.
- PROFIBUS DP cable (violet, 6XV1830-0EH10 or equivalent) terminated with 220 Ω at both physical ends.
- Engineering PC running STARTER (or Drive ES) for parameter upload/download.
PROFIBUS Addressing on CU240S DP
The PROFIBUS node address of the CU240S DP is defined in parameter r0022 / P0918 (PROFIBUS address of the drive object). Two methods are available to set or read it:
- DIP switch on the CU240S DP front – Setting all switches to OFF enables software addressing (P0918). Any switch in the ON position overrides P0918 with the binary-coded value (valid range 1-126, address 0 reserved for master).
- Parameter P0918 via BOP/IOP/STARTER – Set parameter P0918 to the required address; the drive accepts the new value at next power-on or after Commissioning > Copy RAM to ROM.
Confirm the address by reading P0918 from the Basic Operator Panel (BOP) and verifying it matches what is configured in STEP 7 HW Config on the slave station properties.
STARTER Communication Channels and Adapter Selection
STARTER supports multiple paths to reach a SINAMICS G120:
| Path | Hardware | Typical Use | Limitations | ||||
|---|---|---|---|---|---|---|---|
| PROFIBUS direct (online via DP master) | CP5711, CP5611, CP5613 A2, CP5512 | Live system access while PLC is master | Requires correct bus termination, master/slave configuration | RS232 (PC connection kit 6SL3255-0AA00-2AA10) | Point-to-point to CU240S service port | Drive must be standalone (no other master) | USB-to-RS232 converters often fail; genuine Siemens cable recommended |
| Ethernet (via CU240S PN or external module) | Cross-over cable, STARTER > Accessible nodes | Modern installations | Not available on CU240S DP variant without option module |
PC Connection Kit 6SL3255-0AA00-2AA10
This kit provides an RS232-to-TTL cable for direct connection to the service port under the front cover of the CU240S. The USB-to-RS232 converter requirement is critical: many inexpensive converters (especially those using Prolific PL2303HX clones or FTDI clones with non-genuine VID/PID) do not assert the required DSR/DTR handshake timing that the CU240S service port expects. Field experience shows intermittent communication or complete failure to enumerate the drive. If the converter is required, use a model with a real FTDI or Silicon Labs CP2102N chip and confirm the driver reports USB Serial Port (COMx) at 115,200 bps, 8E1.
CP5711 (Recommended for PROFIBUS Service)
The CP5711 (Siemens 6GK1571-1AA00) is a USB 2.0 PROFIBUS master/slave adapter with integrated Softnet PB. It can act as a Class 2 master, allowing STARTER to perform Accessible Nodes scans on a live PROFIBUS network even when the S7-300 is the Class 1 master. In the field case described, switching from a non-original S7-LAN converter to the CP5711 immediately resolved the connectivity issue and allowed a full parameter backup to be performed.
Accessible Nodes vs. HW Config Online – Why the Drive Disappears
STEP 7 and STARTER implement two distinct PROFIBUS discovery mechanisms, and the behavior differs based on the active master class:
| Function | Discovery Method | Sees DP Class 1 Slaves | Sees DP Class 2 / Standalone Slaves | Notes |
|---|---|---|---|---|
| HW Config Online (Monitor/Modify) | Active polling of configured slave slots from project | Yes – shows diagnostic symbols (green/grey/red) | Yes – but only if the project includes the slave | Works through routed interfaces; shows cyclic I/O health |
| Accessible Nodes (STARTER / STEP 7) | DP-V1 read of all reachable stations from local adapter | Yes – if no master is active or master permits class 2 | Yes | Will not list slaves of a different DP master class without only master on the bus set on the adapter |
When a SINAMICS G120 is connected to a live S7-300 PROFIBUS network, two DP masters can be physically present on the bus at the same time:
- The S7-300 CPU port 1 (Class 1 master) – handling cyclic I/O to drives 15-18.
- The CP5711 on the engineering PC (Class 2 master) – trying to scan for accessible nodes.
The CU240S DP will respond to the Class 1 master requests and to DPV1 reads from the Class 2 master, but STARTER's Accessible Nodes view will only populate the PG/PC interface with stations that answer its own broadcast. If the bus is improperly terminated, the CP5711 may not see responses from stations at the far end of the segment, even if HW Config Online on the same physical port reports them as healthy. The discrepancy – drives visible in HW Config but missing in Accessible Nodes – is almost always one of the following root causes.
Root Cause Matrix for Missing Nodes in STARTER Accessible Nodes
| # | Symptom | Root Cause | Resolution |
|---|---|---|---|
| 1 | Accessible Nodes shows only PLC (3) and stops at 7 | Bus termination missing or only on one end; reflections corrupt DP-V1 response frames | Verify both physical ends of the violet PROFIBUS cable have termination ON; intermediate connectors must be OFF. Measure 220 Ω between pin 3 and pin 8 at each end with PLC and drives powered off |
| 2 | Accessible Nodes empty; HW Config Online shows all drives green | CP adapter not configured as a master or the S7-300 is rejecting the second master class | In STARTER/PC Interface, set CP5711 to PG/PC is the only master on the bus or temporarily disconnect the S7-300 from DP for the backup procedure |
| 3 | Accessible Nodes shows wrong set of addresses (0,1,2,3,5,6,7 but not 15-18) | PC connected to wrong PROFIBUS segment (e.g., S7-300 second interface or MPI segment with converter in MPI mode) | Check the active interface in Set PG/PC Interface > PC Adapter / CP5711 > PROFIBUS; confirm MPI/DP switch on the PC Adapter cable is set to DP, not MPI |
| 4 | Accessible Nodes times out; HW Config freezes for 30 s | Baud rate mismatch (1.5 Mb/s vs. 187.5 kb/s) or P0918 not equal to HW Config | Force 1.5 Mb/s in the PC interface, read P0918 from drive BOP, verify in HW Config slave object properties |
| 5 | Drive visible briefly, then disappears | Non-original or clone PROFIBUS adapter; timing violations in FDL/DP-V1 | Replace with genuine CP5711, CP5611, or CP5512 with Softnet PB license |
| 6 | STARTER finds drive but upload fails with F01910 fieldbus timeout | PG/PC adapter dropped off bus during bulk parameter transfer | Disable power management on USB port; use a powered hub; shorten parameter list filter |
Step-by-Step Parameter Backup Procedure (PROFIBUS via CP5711)
- Install STARTER (V5.x or higher) and the CP5711 driver package. Confirm Set PG/PC Interface &em>CP5711(PROFIBUS) – Softnet PB is selected in the Windows Control Panel.
- Connect the CP5711 to a powered PROFIBUS drop connector on the running network (a 9-pin D-sub drop connector tapped into the violet trunk, not at the very end of the segment). The CP5711 LED must show RUN solid green and TOKEN blinking.
- Set baud rate on the CP5711 to match the live bus (1.5 Mb/s in the field case). The CP5711 auto-detects but a manual override reduces acquisition time.
- Launch STARTER, open the project, right-click the drive in the navigator, and choose Online > Accessible Nodes. The CU240S DP at P0918 = 15, 16, 17, 18 must appear with the green drive icon.
- Go online by right-clicking the drive and selecting Target Device > Online > Connect to target system. The status bar shows Online (green) when the connection is active.
- Load the CPU / drive to PG: Project navigator > drive > right-click > Target Device > Load to PG…. Choose Load all parameters from drive to PG/PC and confirm. The status bar displays progress; a typical G120 with CU240S DP at 1.5 Mb/s takes 25-45 s for a full parameter upload (4,096 parameters).
- Save the project with Project > Save As… to a network share. The .dproj/.lap files are the offline backup; an additional Export to Data Set on the drive (function r9781) creates a portable parameter file on a MMC/CF card if available.
Step-by-Step Backup via RS232 (PC Connection Kit 6SL3255-0AA00-2AA10)
- Power off the G120. Open the front cover of the CU240S to access the service X501 (4-pin RJ11) port.
- Connect the PC connection kit cable: USB-to-RS232 adapter (genuine FTDI/CP2102N) on the PC, RS232-DB9 to the kit's serial end, RJ11 plug into X501.
- In STARTER, set Set PG/PC Interface > COMx > USS/PPI/MODBUS at 115,200 bps, 8E1.
- Set PG/PC is the only master on the bus in the interface properties; the CU240S will only answer USS on the service port.
- Choose Accessible Nodes; the drive appears with address 0 (USS) even if P0918 is set. Go online and perform Load to PG as in the PROFIBUS procedure.
Verification Checks
After the parameter backup completes, confirm the integrity of the captured set:
- In STARTER, navigate to Drive > Parameters > Parameter list; the filter Compare expert list with offline values must return no red rows.
- Open the Commissioning overview in the offline project and verify the motor data (P0304-P0311), encoder data (if present), and PROFIBUS telegram configuration (P0922) match the connected hardware.
- Check the parameter checksum: read r9781 in the live drive, save as a binary file on a card, and compare with the file produced by the Export to Data Set function in STARTER. Identical checksums indicate a clean backup.
LED Status on CU240S DP During Service
The status LED on the front of the CU240S DP indicates communication health. Reference the CU240S Control Units operating instructions for the complete table; the most relevant states are:
| LED State | Meaning | Action |
|---|---|---|
| Off | No control voltage or drive offline | Check 24 V supply at terminals 1-2; check XD24 DIP for analog/digital I/O mode |
| Green slow flash (2 Hz) | Drive ready, cyclic PROFIBUS active | None – normal operation |
| Green solid | PROFIBUS configured but no cyclic I/O from master | Verify master configuration; check P0922 (telegram selection) matches HW Config |
| Red solid | Firmware error or PROFIBUS bus fault | Read r0947 fault buffer; check termination |
| Red fast flash (2 Hz) | F01910 / F08501 fieldbus timeout | Check PROFIBUS cable, master state, and P2042 (DP-V1 watchdog) |
Common Fault Codes During PROFIBUS Parameter Service
| Fault Code | Meaning | Likely Cause in Service Scenario | Remedy |
|---|---|---|---|
| F01910 | Fieldbus setpoint timeout | Master stopped cyclic I/O during upload | Pause S7 program or put CPU in STOP; do not interrupt DP during bulk transfer |
| F08501 | PROFIBUS configuration error | P0922 telegram mismatch with HW Config | Align HW Config slot assignment with drive parameter; power cycle drive |
| F08502 | PROFIBUS address invalid | P0918 = 0 or > 126 | Set valid address 1-126; verify with BOP |
| A08502 | PROFIBUS baud rate fault | Adapter and drive negotiated different rates | Force 1.5 Mb/s on CP5711 to match the live bus |
| F01900 | Internal PROFIBUS error | Excessive CRC errors on cable | Inspect connectors, replace damaged segments, verify shield bonding |
Best Practices for Field Service
- Always carry a genuine CP5711 in the field service kit. It is the most reliable way to access a live PROFIBUS network with a SIMATIC master active.
- Document P0918 (PROFIBUS address), P0922 (telegram selection), and the active baud rate on the cabinet door before any service call. This avoids 30 minutes of bus scanning on arrival.
- Use the Compare expert list with offline values filter in STARTER to validate the upload, not just a green online status.
- For long-term parameter archival, use the Export to Data Set function with a CF card in the CU240S. The result is firmware-version independent and survives STARTER version upgrades.
- When the PC connection kit is the only option, prefer the genuine Siemens RS232 cable and a known-good USB-to-RS232 adapter. Reject any third-party RJ11 breakout – the TTL levels on the service port will not survive a wrong cable.
- During bulk parameter transfers, put the S7-300 in STOP or freeze the cyclic I/O via P2034 = 0 (fieldbus interface mode) to avoid F01910 timeouts.
Notes on Firmware and Software Compatibility
The CU240S DP shipped with several firmware branches (FW 3.x, 4.x). Parameter sets are forward-compatible within a major branch; a backup from FW 3.2 can be downloaded into a drive running FW 3.2 SP1 or later in the same branch without issue. Crossing a major branch (e.g., 3.x to 4.x) requires a STARTER-driven Online > Load firmware first, then the parameter download, to avoid configuration conflicts in BICO interconnections. Refer to the CU240S Control Units manual for the supported firmware upgrade path and the minimum STARTER version required for each G120 firmware branch.
FAQ
Why can I see the SINAMICS G120 in STEP 7 HW Config Online but not in STARTER Accessible Nodes?
HW Config Online polls only the configured slave slots from the project; STARTER Accessible Nodes performs an independent DP-V1 broadcast from the local PC adapter. If the CP adapter is not configured as a master, or the bus is not properly terminated, the drive may respond to the S7-300 master but not to the broadcast. Set PG/PC is the only master on the bus on the CP5711 or temporarily disconnect the S7-300 from DP to perform the upload.
Can I use a cheap USB-to-RS232 converter with PC connection kit 6SL3255-0AA00-2AA10?
It may work with a converter based on a genuine FTDI FT232 or Silicon Labs CP2102N chip, but many low-cost adapters use clones that violate the DSR/DTR handshake timing required by the CU240S service port. Field experience shows frequent failures; a genuine Siemens USB-to-Serial adapter (e.g., 6ES7972-0CB20-0XA0) is the recommended solution.
Which PROFIBUS adapter should I use for parameter backup on a live S7-300 + G120 network?
The CP5711 (6GK1571-1AA00) is the most reliable choice. CP5611 (PCI) and CP5512 (PCMCIA) are also supported with Softnet PB. The CP5711 is USB 2.0 and can coexist with the S7-300 Class 1 master when the only master option is set, allowing STARTER to perform Accessible Nodes scans without disrupting the live cyclic I/O.
How do I read the PROFIBUS address of a CU240S DP without connecting via STARTER?
Use the Basic Operator Panel (BOP) attached to the CU240S. Navigate to parameter r0022 (display of the active address) or P0918 (PROFIBUS address). The address must be in the range 1-126; address 0 is reserved for the master. Address 15-18 was the field case described.
What is the maximum number of parameters transferred in a typical G120 backup, and how long does it take?
A fully configured SINAMICS G120 with CU240S DP has approximately 4,000-4,500 user-visible parameters. At 1.5 Mb/s PROFIBUS, a complete upload to STARTER takes 25-45 seconds. At 187.5 kb/s the same transfer can take 3-5 minutes and is more likely to time out; force the highest supported baud rate on the CP adapter when possible.