Resolving SINAMICS CU320 Fault F07840 Infeed Operation Missing

Fault F07840 Overview and System Context

SINAMICS fault F07840 ("Drive: Infeed operation missing") is a drive-side reaction to a missing "Infeed in operation" feedback inside an S120 multi-axis topology. The fault is generated on a Motor Module (the inverter controlling the motor) when the inverter has been commanded to run (ON/OFF1 = 1, pulse enable, speed setpoint > 0) but the binary signal that should confirm that the upstream Line Module is ready and powering the DC bus is still 0. In most field cases the fault is not caused by the Line Module itself failing - the IGBTs are intact, the pre-charge has finished, the DC link is at nominal voltage - but by the wiring and parameterization of the feedback path that ties the Line Module "ready" output to the Motor Module "infeed operation" input.

The CU320 / CU320-2 is the central Control Unit of the SINAMICS S120 booksize or chassis format. It hosts the firmware, the closed-loop control, the BICO (Binector/Connector) interconnection matrix, and the digital I/O terminal strips X122 and X132 used for hardwired run/stop, enable, and infeed-OK feedback. Because the same Control Unit is shared by the Line Module and all downstream Motor Modules via DRIVE-CLiQ, the interlock can be propagated in three different ways: as a BICO source from the Line Module, as a digital input on the CU, or as a virtual signal. When the interlock path is misconfigured the result on the Motor Module is a recurring F07840 every time the operator issues a run command and a "no fault / no reaction" the moment the user clears the buffer. The cleared fault returns because the root cause - a missing or inverted binary signal - is still present.

CU320 Hardware Map Relevant to F07840

The CU320-2 (the current production version of the CU320) provides eight bidirectional digital inputs/outputs on terminal strip X122 and eight more on X132. Each terminal can be configured as DI or DO. The status image of these 16 terminals is mapped to parameter r0722; the bit-inverted image is mapped to r0723. These two parameters are commonly confused with the "infeed status", which is wrong: r0722 and r0723 are the digital-input status, not the line module's operational state.

Why the "r722 vs r723" Question Always Comes Up

In the field, a beginner often reads the parameter list in STARTER, sees two parameters that look like an "infeed status", and assumes the system is reporting the line module state on those BICO outputs. The reality is more mundane: r0723 is simply r0722 inverted. If the user wires a physical 24 V signal to DI 0 of the CU and then connects that DI to the Motor Module's "infeed operation" interlock via P0864, they will read r0722.0 = 1 (signal present) and r0723.0 = 0 (inverted, so 0). If the signal is missing the relationship inverts. The fact that one of the two values is "1" tells you nothing about the Line Module; it only confirms that the DI is wired in a defined state. The classic field mistake is to read r0723.0 = 1, interpret it as "infeed in operation", and wire the design around the wrong input.

Fault F07840 Technical Definition

The fault value of F07840 is defined in the SINAMICS S120 List Manual (LH1) as follows: the drive is in state "Ready to switch on" or "Switched on" (control word 1, bit 0 or bit 3 set, depending on interpretation) and the signal "Infeed operation" (r0863.0) is 0 at the moment ON is commanded, or the signal goes from 1 to 0 while the drive is running. The reaction by default is OFF2 (immediate pulse cancellation, coast to stop). The acknowledge model is POWER ON; the user must either power cycle the CU, issue a safe Acknowledgement via STARTER/Startdrive, or re-trigger the run sequence with the permissive restored.

Root Cause Tree for F07840

Although the fault has a single text, the root cause is a five-bucket problem. Working the tree in this order avoids the trap of replacing a wiring or parameter issue with a Line Module swap.

1. Source signal is not present. The Line Module is in Operation state (r863 of the line object, bit 0 set) but the binary signal has not been routed to the Motor Module. Check p0864 on the Motor Module and confirm a valid BICO source (e.g. p0864 = r0863.0 from the line object, or p0864 = 722.0 for a wired DI).
2. Source signal is present but inverted. The Line Module reports a "ready" contact that is N.C. (normally closed) rather than N.O. (normally open). When the infeed is healthy the contact opens (signal = 0) and the Motor Module therefore never sees r0863.0 = 1. Reconfigure the DI as inverted (p0722 bit settings) or invert via BICO (b0000 with negation).
3. Source signal is a CU digital input that is wired wrong. The DI is open, floating, or wired to the wrong terminal. Read r0722 and r0723 on the live drive. The two values must be complementary; if r0722.0 = r0723.0 the input is faulted (typically a short to 24 V or a wiring break that the input stage reads as undefined).
4. Source signal is a parameter that the user has overwritten. P0864 has been left at the default 0 (interlock always satisfied) on a system that depends on the interlock. After a firmware upgrade or a project upload, the interlock disappears and the fault becomes latent. Reconnect the interlock to its physical source.
5. Line Module is genuinely not in operation. The Line Module itself has a fault (e.g. F07801, F07802, F30002), the pre-charge is still active, or a 24 V supply to the SLM control card is missing. Check the line object's r863 and the drive object's r070 first; if the Line Module is not in Operation, fix that before chasing the Motor Module interlock.

Parameter Map for the Infeed-Ready Feedback Path

The feedback path between the Line Module and each Motor Module runs through the BICO matrix of the CU320. The default configuration on a single-axis booksize S120 is the interlock satisfied by default (P0864 = 0). The default on a multi-axis chassis system depends on the topology and on the topology wizard that STARTER/Startdrive generates. The following parameter map lists every object that influences F07840.

Wiring Verification Procedure

Use the following procedure to localize a recurring F07840 on a CU320 system. The procedure assumes STARTER (V5.x) or Startdrive (V15 and later) is online with the drive.

1. Open the project, go online with the CU320, and navigate to the drive object that raised F07840 (typically the Motor Module identified in the fault buffer).
2. Read r0863.0. If the bit is 0 while the Line Module is in Operation, the interlock source on the Motor Module is not connected. The fault is then a parameter/wiring problem on the Motor Module, not a Line Module problem.
3. Inspect P0864. If it is 0, the drive is configured to ignore the interlock and F07840 cannot be generated by hardware - any fault in the buffer is residual from earlier state. Re-acknowledge the buffer with a power cycle and re-run the application; if F07840 returns, the interlock is being set by a non-default parameter that you have not yet located. Re-export the project and diff against the live online image.
4. If P0864 points to a CU digital input (a value in the form 722.x or 723.x), navigate to the CU's r0722 and r0723 online. Verify the bit in question is showing the expected level. The two values must be complementary (r0722.0 + r0723.0 = 1). If they are equal, the input is either shorted, open, or wired to a reference that is invalid for the 24 V input stage.
5. Pull the wire off the terminal and measure the voltage to ground with a multimeter. The CU320-2 digital inputs read high when the voltage is between 11 V and 30 V with respect to the reference ground of X122/X132. Below 5 V the input reads low; above 30 V the input protection clamps and the read is undefined.
6. Trace the wire back to the Line Module. On a Smart Line Module (SLM) the contact is typically on the X9 terminal block of the SLM Control Interface Module (CIM), wired through the user's auxiliary contactor or through the integrated "line contactor control" output of the SLM.
7. Reconnect the wire and re-arm the drive. Monitor r0722 on the CU and r863.0 on the Line Module in a long trace. The bit must be 1 within the pre-charge window (typically 1 s to 4 s on a booksize SLM, longer on a chassis SLM). If the bit is 1 and F07840 still triggers, replace the wire from the line contactor with a hard strap to confirm that the source itself is not bouncing.

Configuring P0864 in STARTER and Startdrive

The BICO source for "Infeed operation" is parameter P0864 inside the drive object. To set it from the parameter view in STARTER:

1. Open the drive object (double-click in the project tree), then double-click Commissioning > Parameter list.
2. Filter on P0864.
3. Click the field for P0864; the BICO editor opens.
4. From the source list, pick either a binector output from the Line Module (typically r863.0 of the line object) or a CU digital input. For a CU DI, pick r0722.x or r0723.x as appropriate.
5. Click OK, then Copy RAM to ROM so the configuration survives a power cycle.

In Startdrive (TIA Portal), the same parameter is under Drive > Parameters > Commissioning > Function diagrams > 2510 (status word, sequence control). Right-click on the input of the "Infeed operation" BICO block, choose Interconnection to... and pick the source.

Verifying the Line Module is in Operation

Before changing the Motor Module's interlock, confirm the upstream state. The Line Module's own status word (r863.0) is the authoritative answer.

If the Line Module is in Ready for switching on but not in Operation because the user's external 24 V enable is missing, the Motor Module will not see r0863.0 = 1 and F07840 will trigger every run. Fix the Line Module enable (typically p0840 ON/OFF1 and p0844 OFF2 and p0848 OFF3 on the line object) and the F07840 will clear as a side effect.

Step-by-Step Resolution Checklist

1. Capture the fault buffer (STARTER > Drive > Diagnostics > Faults/Alarms) and note the timestamp and the drive object.
2. Read r0863.0 of the affected drive. If 0, the interlock source is the problem.
3. Read P0864. If 0, the drive is not configured to receive the interlock; check whether a previous project upload overwrote the configuration.
4. Read r0722.0...r0722.15 and r0723.0...r0723.15 of the CU. Identify which bit is intended to carry the interlock.
5. Verify the bit is complementary to its inverse. r0722.x XOR r0723.x = 1 is mandatory.
6. With a multimeter, measure the voltage at the terminal block. The level must be 11 V ... 30 V for a high read, 0 V ... 5 V for a low read.
7. Trace the wire to its source. Confirm the source is the Line Module "ready" or "in operation" contact, not a generic 24 V supply.
8. If the source is correct and the wire is correct, the BICO interconnection is wrong. Reconnect P0864 to the correct binector source.
9. Copy RAM to ROM, power cycle the CU, re-run the application. The fault must not return.
10. If the fault still returns, capture the BICO trace during the run command. The bit feeding P0864 must be 1 within the configured monitoring window. If the bit flips from 1 to 0 mid-run, the Line Module is dropping out and the upstream problem must be solved first.

Verification and Commissioning

The validation of a fix to F07840 should be performed under three operating conditions: idle, commanded run, and emergency stop. Use the STARTER trace or the TIA Portal trace viewer to record r0070 (DC link), r0722 (CU DI), r0863.0 (drive coupling), and r863.0 (line module status) on four channels at 1 kHz sample rate for 10 s.

• Idle verification: With the drive not commanded, r0070 should be 0, r0863.0 should be 0, and the line module r863.0 should be 0. These are the resting states.
• Commanded run verification: Issue ON/OFF1, then within 1 s ... 4 s r0070 should rise to 100 %, r0863.0 should rise to 1, and the line r863.0 should be 1. F07840 must not be present in the buffer after the trace.
• Stop verification: Issue OFF1. The pulses cancel, the motor decelerates along the ramp, r0070 stays at 100 % (the Line Module remains in Operation), and r0863.0 falls to 0 only after the line contactor is told to open. F07840 must not be present in the buffer after the trace.

Common Pitfalls and Field-Proven Caveats

Five recurring mistakes show up on every service call where F07840 has been chased for hours.

1. Confusing r0722 / r0723 with infeed status. They are CU DI status, not Line Module status. If the user wires the interlock to r723.0 instead of r722.0, the interlock is permanently satisfied (r723.0 stays 1 only when r0722.0 is 0, which is the idle state) and F07840 will not trigger, but the drive will also start without a real permissive. This is a hidden safety problem, not just a wiring problem.
2. Default 0 on P0864. Inexperienced commissioning engineers leave P0864 = 0 because the drive then runs without complaint. The interlock is then missing entirely, and the first time the Line Module is offline (24 V removed, line contactor not pulled in) the drive will start anyway. P0864 must be wired to a real source on any multi-axis system where the Line Module can be disabled independently of the Motor Modules.
3. Reading the wrong drive object. F07840 raises on the Motor Module, but the BICO source of the interlock can live on the CU or on the Line Module. Inspect the entire BICO chain by following the green connectors in the STARTER function diagram, not just the drive object that owns the fault.
4. Confusion between 24 V reference grounds. The CU320-2 X122/X132 digital inputs have their own reference terminal. If the user's infeed-OK contact is referenced to a different ground (e.g. the field ground of the cabinet, not the CU ground), the input will float and read undefined. Bridge the reference terminal of X122/X132 to the reference terminal of the source.
5. EMI-induced glitching. Inverter outputs on the same cabinet generate dV/dt that couples into 24 V control wiring. If r0722 is bouncing for a few milliseconds during a Line Module pre-charge, the drive will register the loss of permissive and trigger F07840. Use shielded cable, tie the shield to cabinet ground at one end, and add a small RC debounce if the source is a mechanical contact.

Related Faults and Where They Fit

F07840 sits inside a small family of "infeed coupling" faults. The relationships are useful for diagnostics because they tell you whether the problem is upstream (Line Module) or downstream (Motor Module).

Safety Integration Considerations

When the SINAMICS S120 is used in a safety-relevant application (SS1, STO, SLS, SDI), the F07840 path must be kept separate from the safety path. The safe stop functions (via PROFIsafe on PROFINET or via the TM54F terminal module) bypass r0863.0 and use the safe control word 1 (S_STW1) to disable the pulses. A persistent F07840 in a safety-integrated system is therefore almost always a permissive interlock problem, not a safety problem, but the safety wiring must be inspected in parallel to be sure the safe channel is not the source of the missing enable. Confirm S_STW1 bit 0 and bit 1 are correctly driven by the F-host, and confirm the safe stop time in p9556 / p9856 is longer than the Line Module pre-charge so that the safety path does not cancel the pulses before the infeed has reached the Operation state.

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