SINAMICS S120 CU320-2 DP Resolving F07840 and PG/PC Connection

Problem Overview

The SINAMICS S120 CU320-2 DP Control Unit is the central drive controller that manages up to six Motor Modules and one active Line Module over the internal DRIVE-CLiQ serial bus. Two separate but frequently co-occurring failure modes appear in field service:

1. Loss of PG/PC online access – STARTER, Startdrive (TIA Portal), or SIMOTION Scout cannot establish a connection over PROFIBUS DP, with the diagnostic hint "Set PG/PC interface or check the network".
2. Fault F07840 – Drive: Infeed operation missing – reported on one or more drive objects, even though the central Line Module (Smart Line Module / Basic Line Module / Active Line Module) is energized and shows no fault of its own.

In a multi-axis lineup with three parallel CU320-2 DP Control Units tied to a single central infeed through DRIVE-CLiQ, a single bad download or a topology corruption on one Control Unit can cascade into all CUs reporting inconsistent online state. This reference describes the diagnostic procedure, the X127 service-port recovery path, and the parameter-level checks that clear F07840.

Affected Hardware, Firmware, and Topology

The topology assumed here is the canonical central-infeed pattern: a single Line Module supplies a common DC bus shared by multiple Motor Modules. Each Motor Module is wired through DRIVE-CLiQ (RJ45, shielded, yellow) to port X200 / X201 of the CU320-2 DP. In this topology the Line Module also has a DRIVE-CLiQ connection to port X200 of the master CU, but the drive objects on every CU in the project expect that infeed to be in operation.

Root Cause Analysis

F07840 – Drive: Infeed operation missing

F07840 is raised by a drive object (Servo, Vector, V/f) when the controller attempts to release the drive (ON/OFF1 via p840[0]) but the assigned Line Module is not in Operation state. The fault source is the binary interlock signal that the drive object reads as infeed ready. Three physical causes dominate:

1. Infeed enable chain broken. Parameter p0864 (BI: Infeed operation) on the affected drive object does not see r0863.0 of the Line Module. After a full project download STARTER sometimes rewires BICO interconnections if the user did not include the infeed in the "Download target device(s)" selection.
2. DRIVE-CLiQ link to infeed defective. Cable damage, port mismatch (X201 instead of X200 on CU), or a missing terminator on the Line Module breaks the cyclic status exchange. The drive object times out waiting for Status word 1 (r0899) bit 0 from the infeed and raises F07840.
3. Wrong offline project loaded. The drive objects may have been configured for a non-existent infeed because the manufacturer-supplied project contained a different topology than the physical cabinet. STARTER and Startdrive do not always detect this mismatch at download time.

PG/PC interface fault ("Set PG/PC interface")

STARTER and Startdrive route every online action through the PG/PC interface configured in the Windows Control Panel (Set PG/PC Interface) or in TIA Portal (Online Access). For PROFIBUS DP the active interface must be CP5611 / CP5612 / CP5621 / CP5711 / PC-Adapter USB A2. For Ethernet the active interface must be the TCP/IP binding pointing to the same subnet as the CU320-2 DP.

The reported symptom – one of three CUs downloads successfully while the other two fail – almost always means that the PROFIBUS bus connector on the failing CUs is either:

• Terminated to a non-existent bus address (address set differently than the DIL switch on the unit),
• Connected to a stub that is not powered (24 V to PROFIBUS connector missing),
• Short-circuited by a damaged cable,
• Or, more commonly, the PC's CP card sees only the address of the third CU because the other two nodes are not broadcasting their token because of the missing terminator or 24 V supply.

The clean recovery path is to bypass PROFIBUS and use the X127 Ethernet service port, which is a fixed 169.254.x.x link-local interface independent of the fieldbus address.

Diagnostic Sequence

1. Verify power and DC link. Check LED states on the CU320-2 DP: RDY (green steady = run, green flashing = commissioning, red = fault). All CU RDY LEDs should be green or green-flashing before proceeding.
2. Confirm PROFIBUS address. Read the DIL switches on the back of each CU and compare with the project offline configuration (Drive device → Communication → PROFIBUS address).
3. Try X127 service port. Plug a standard patch cable into the X127 socket on the front of the CU320-2 DP (not the PROFINET ports X150 P1/P2 – those are not present on a CU320-2 DP). Set the notebook Ethernet adapter to 169.254.11.21 / 255.255.0.0 and ping 169.254.11.22.
4. Open STARTER / Startdrive and run Accessible Nodes. If the device responds, the Control Unit firmware is healthy and PROFIBUS is the only broken link.
5. Read fault buffer. From online access, navigate to Drive device → Diagnostics → Fault buffer on each drive object. Record F07840, any A07841, and any DRIVE-CLiQ diagnostic alarms (A5000–A5007).
6. Inspect DRIVE-CLiQ topology. Open Configuration → DRIVE-CLiQ topology in STARTER. Verify that the Line Module port assignment matches the physical wiring.
7. Compare offline/online BICO for p0864. Online Expert list → p0864 on the faulted drive object must show a BICO source pointing to the correct infeed's r0863.0.

Solution A – Restoring PG/PC Online Access via X127

The X127 service interface is an Ethernet port on every SINAMICS S120 Control Unit and on SIMOTION D4x5 controllers. It uses a link-local IP scheme that does not require a DHCP server and is reachable without configuring the fieldbus address. The port supports STARTER, Startdrive, SIMOTION Scout, and SIMATIC S7 TCP/IP routing.

Step-by-step – STARTER

1. In Windows Control Panel open Set PG/PC Interface. Select S7ONLINE (TCP/IP) -> [Intel/Realtek NIC used for X127] and click OK.
2. Open STARTER. Click the Accessible nodes button (or press F5). The accessible-node dialog starts a broadcast scan.
3. When the CU320-2 DP appears with IP 169.254.11.22, double-click it to enter online mode. STARTER will load the actual configuration from the device.
4. If STARTER reports an inconsistency, accept "Load to PG" only when you are sure the offline project is older than the device; otherwise choose "Load to target" only after confirming with the machine manufacturer.
5. Repeat on the other two CU320-2 DPs by changing the notebook IP to 169.254.12.21 / 255.255.0.0 and 169.254.13.21 / 255.255.0.0 only if the CUs have been reconfigured with secondary IPs (default is 169.254.11.22 on every CU; you must therefore bring them online one at a time).

Step-by-step – Startdrive in TIA Portal V20

In TIA Portal the CU320-2 PN is added from the hardware catalog; the DP variant is configured as a PROFIBUS DP device on an S7 CPU's PROFIBUS subnet. When commissioning offline-only, online access via X127 follows the standard TIA path:

1. Project tree → right-click the drive device → Go online.
2. Select PN/IE as the type and the appropriate NIC.
3. Enter the X127 IP 169.254.11.22 in the Accessible nodes dialog.

For TIA Portal projects containing a SIMOTION D controller together with a SINAMICS S120 CU320-2 PN, Siemens documents the insertion sequence – drag the SIMOTION D455-2 DP/PN (S120) from Controller → SIMOTION into the network view first, then drop the CU320-2 PN as a PROFINET IO device. See the official TIA Portal V20 documentation for the full sequence and IP/profinet-name assignment rules: Inserting SIMOTION D455-2 DP/PN and SINAMICS S120 CU320-2 PN.

Why X127 works when PROFIBUS does not

X127 is wired to the Control Unit's internal ARM/A40 processor and operates independently of the PROFIBUS DP ASIC. The X127 TCP/IP stack uses link-local addressing and does not require the user to download a hardware configuration before going online. This makes X127 the canonical fallback channel whenever the fieldbus cable plant is suspect.

Solution B – Clearing F07840 (Drive: Infeed operation missing)

Verify BICO interconnection p0864

On every drive object that reports F07840, open the Expert list in STARTER and inspect:

The default source for a Servo DO fed from an Active Line Module is the r0863.0 bit of the Line Module DO. After a partial project download – for example only modified FB/DB blocks and HMI screens – the BICO connection is not reissued if the infeed DO was excluded from the download set. Result: the drive sees a "1" at p0864 because the offline value differs from the online value, but the live signal from the infeed is missing on next power-up.

Force a consistent download of the entire Drive device

1. Go online with the CU320-2 DP via X127 (as described in Solution A).
2. Right-click Drive device → Download to drive device. Do not deselect any DO; the Line Module must be included.
3. Confirm the dialog "Download target device(s) – all selected".
4. Perform a POWER ON (24 V cycle on CU320-2 DP). F07840 should not reappear.

If F07840 persists after a full download

1. Read r0863.0 on the Line Module DO online. If it never goes to "1" even though the Line Module shows Operation (r0899.0 = 1), the BICO source is wrong – rewire p0864 manually.
2. Open Configuration → DRIVE-CLiQ topology in STARTER and click Compare with offline. Mismatched port numbers or missing components indicate a corrupted topology – recreate it via Configuration → Automatic configuration after physically checking each DRIVE-CLiQ LED.
3. Replace suspect DRIVE-CLiQ cables one at a time. Each cable must be ≤ 70 m, shielded Cat5e or better, and use the Siemens green or yellow DRIVE-CLiQ connectors (6FX2002-1DC00-series).

DRIVE-CLiQ Topology Verification

DRIVE-CLiQ is a deterministic serial ring with a maximum of 64 nodes per CU320-2 DP. The physical port numbering on a CU320-2 DP is:

The DRIVE-CLiQ LED on each port is solid green when the link is up and flashing green during data exchange. A red LED signals a topology mismatch or a baud-rate failure; an off LED means no link at all. STARTER reports the topology in two views:

• Online → Topology shows the actual physical connections read from the device.
• Configuration → DRIVE-CLiQ topology shows the configured offline view.

The two views must match port-for-port. A common source of F07840 in field service is a Motor Module being assigned to port X202 in the offline project but physically wired to X204 because of a previous cabinet rework. STARTER will not flag this at download time; only F07840 on the corresponding drive object plus an A5001 "DRIVE-CLiQ topology error" warning will appear.

Handling Online/Offline Inconsistencies

After every successful online connection STARTER compares the offline project with the device image. The dialog can show three choices:

If inconsistencies recur every time the project goes offline, the cause is usually one of the following:

• A parameter is write-protected by password (p7760) and was changed online but not committed to the offline project.
• Free function blocks (FFB) or DCC charts were modified online without being saved to the CF card.
• Two engineering workstations are loading different offline versions to the same device – enforce a single source via the project server.

Verification and Commissioning

After the connection is restored and F07840 is cleared, perform the following verification sequence before returning the machine to production:

1. LED check. RDY green on every CU, DP green (PROFIBUS cyclic), DRIVE-CLiQ green on every used port.
2. Fault buffer empty. Online → Drive device → Diagnostics → Fault buffer: confirm no F07840 and no A5000–A5007 DRIVE-CLiQ alarms.
3. Jog test each axis. Use STARTER control panel with enable signals; verify torque and speed reference follow without F07840.
4. DC-link voltage. Online read r0070[0] on the Line Module; must be within ±5% of nominal (e.g., 600 V on a 400 V line, 720 V on a 480 V line).
5. Power off / power on cycle. 24 V off, wait until CU RDY goes dark, re-energize. F07840 must not return.
6. Document the configuration. Save the project to the project server, archive the parameter set via Drive device → Save parameter set to CF card, and export the drive trace buffer.

Preventive Measures and Best Practices

FAQ