Resolving FbDrive Bad Output and ErrorNum 1 with SINAMICS G120C in PCS 7

When interfacing a SINAMICS G120C variable frequency drive to an APL MotSpdCL block via driver block FbDrive (FB1797) in SIMATIC PCS 7, output flags BAD = 1 and ErrorNum = 1 (invalid process value / invalid operating mode) occur when parameter Mode is unassigned or set incorrectly. In standard PCS 7 engineering, the 'Generate Module Drivers' compiler function automatically links Mode to the drive's diagnostic address (e.g., 16#8000FFFF). If the drive was inserted into HW Config using a generic third-party GSD file rather than the official Siemens Drive ES or PCS 7 Hardware Support Package (HSP), driver generation fails to recognize the module. Resolve this by installing the Drive ES / PCS 7 G120 profile or manually parameterizing Mode.

The Role of FbDrive and Module Driver Generation

In SIMATIC PCS 7, the Advanced Process Library (APL) separates technological drive control from fieldbus communication. The technological block MotSpdCL (Motor with Speed Control) handles operator commands, interlocks, and speed ramping. It relies on the lower-level driver block FbDrive (FB1797) to exchange cyclic process data (PZD words) with the physical frequency converter.

FbDrive communicates using standard PROFIdrive telegram profiles (typically Telegram 1: 2 words PZD In / 2 words PZD Out, or Telegram 20). During CFC compilation, the PCS 7 driver generator (Generate Module Drivers) automatically queries HW Config, identifies the drive module, creates diagnostic blocks (MOD_DRV), and connects the hardware diagnostic address and mode structure to FbDrive.Mode.

Meaning of BAD = 1 and ErrorNum = 1 in FbDrive

When FbDrive executes in the AS controller, it continuously validates the integrity of the interconnected hardware:

BAD = 1: Signals that the process value is invalid or communication to the drive has failed. When BAD = 1, the downstream MotSpdCL block enters fault state, drops the run command, and displays a communication error on the WinCC faceplate.
ErrorNum = 1: Directly decodes to Invalid Mode Setting or Invalid Process Value. The block checks input parameter Mode. If Mode remains at its default unconfigured state (DW#16#00000000) or contains an unsupported telegram code, the block refuses to process cyclic I/O data and asserts BAD = 1.

Why Generic GSD Files Cause Driver Generation Failures

The most common root cause of an unlinked Mode parameter is improper catalog selection in STEP 7 HW Config:

Generic GSD / GSDML Files: If an engineer downloads a generic fieldbus GSD file from the internet and inserts the G120C under Additional Field Devices, STEP 7 treats the drive as a black-box third-party slave. The PCS 7 driver generator does not recognize the internal module family ID and silently skips driver generation.
Certified Drive ES / HSP Objects: When the drive is inserted using the official Siemens Hardware Support Package (HSP) or Drive ES PCS 7 catalog (PROFINET IO -> Drives -> SINAMICS -> G120C), the driver generator identifies the module family and binds Mode, LADDR, and diagnostic OB82/OB86 routines automatically.

Comparison of SINAMICS G120C Integration Methods

The table below contrasts different methods of integrating SINAMICS G120C drives into SIMATIC PCS 7.

Engineering MethodHardware Catalog PathModule Driver GenerationMode Parameter AssignmentFull APL FaceplatesDrive ES PCS 7 LibraryDrives -> SINAMICS -> G120C (HSP)Fully AutomatedAutomatically connected to MOD_DRVNative APL Faceplates & DiagnosticsStandard PROFIdrive GSDPROFINET IO -> Standard PROFIdriveSemi-Automated (requires standard telegram)Auto-connected if telegram matchesBasic MotSpdCL controlGeneric GSDML DeviceAdditional Field Devices -> GSDMLFails (Not recognized)Unconnected; requires manual hex entryRequires custom script faceplates

Step-by-Step Troubleshooting and Parameterization Procedure

Follow this procedure to eliminate ErrorNum = 1 and establish communication between FbDrive and the G120C:

Verify Drive Hardware Catalog Integration:
- In HW Config, check where the G120C was inserted. If inserted via a generic GSD, delete it.
- Install the official Siemens HSP for SINAMICS G120C (or install Drive ES PCS 7).
- Insert the drive from PROFINET IO -> Drives -> SINAMICS -> G120C.
- Assign standard Telegram 1 (or Telegram 20) in both HW Config and the physical G120C drive parameters (p0922 = 1).
Check Interconnections in CFC:
- Open the CFC chart: interconnect FbDrive.PZD_In1 to the drive hardware input address (e.g., IW 512).
- Interconnect FbDrive.PZD_Out1 to the drive hardware output address (e.g., QW 512).
- Set parameter PZD_Type = 1 (for Telegram 1).
Execute CFC Compilation with Driver Generation:
- In SIMATIC Manager, initiate CFC compilation.
- Ensure the checkbox Generate module drivers is checked.
- Inspect the compiled chart: verify that parameter Mode is now interconnected to output Mode of a generated MOD_DRV block.
Manual Mode Parameter Workaround (If Using Generic GSD):
- If forced to use generic GSD, set Mode manually in CFC object properties to DW#16#8000FFFF (or DW#16#80000201 for Telegram 1 with active monitoring) to clear the invalid mode fault.

Verification

Verify drive operation through the following steps:
1. In CFC online test mode, inspect FbDrive: verify that BAD = 0 and ErrorNum = 0.
2. Confirm that FbDrive.ActSpd displays the actual motor speed in RPM or percentage matching the physical drive display.
3. On the WinCC OS client, open the MotSpdCL faceplate: confirm that the drive state shows 'Ready to start' with no communication flags.
4. Issue a forward start command at 50% speed: verify that the G120C accelerates smoothly to the commanded setpoint.

Limitations and Operational Precautions

Never mismatch PROFIdrive telegram types between HW Config, drive parameter p0922, and the FbDrive.PZD_Type parameter. Configuring Telegram 1 in HW Config while parameterizing the drive for Telegram 352 causes word alignment shifts, which can lead to uncontrollable motor movement or catastrophic overspeed trips. Always confirm telegram parity before applying main 480V power.

Engineer FAQ

What does Mode = 16#8000FFFF mean on PCS 7 driver blocks?

It represents the standardized Siemens driver quality code indicating that the channel is active, healthy, and operating under automatic module driver supervision.

Can I use FbDrive with third-party variable frequency drives?

Yes, provided the third-party drive complies strictly with the standard PROFIdrive Telegram 1 or Telegram 20 specification. However, module drivers must be configured manually.

What is the difference between MotSpdCL and FbDrive?

MotSpdCL is the high-level technological APL block providing operator control, interlocks, and alarming. FbDrive is the low-level hardware channel driver that communicates directly with the physical fieldbus telegram.