Resolving Missing Free Function Blocks on SINAMICS G120 CU250S-2

Resolving Missing Free Function Blocks on SINAMICS G120 CU250S-2

This field reference walks through the exact procedure to surface the Free Function Blocks (FFB) library inside a SINAMICS G120 CU250S-2 project when STARTER commissioning software does not show the FFB node in the drive navigator. The same root cause and fix apply to any SINAMICS G120 family Control Unit (CU230P-2, CU240B-2, CU240E-2, CU250S-2) shipped from the factory, but the parameter range and instance counts shown here are tailored to the CU250S-2 with firmware V4.7.11 and STARTER V5.1.1.2 plus the matching support package.

1. Problem Description

After creating or opening a STARTER project that targets a SINAMICS G120 CU250S-2 Control Unit (PROFINET or PROFIBUS variant), engineers frequently cannot locate the Free Function Blocks. Typical symptoms include:

• The entry Free function blocks is absent from the project navigator under Control Unit > Commissioning > Function blocks.
• Inserting an FFB instance (AND, OR, RSR, TIMER, CNT) via the parameter tree is impossible because the parameter group p20000-p20999 is hidden.
• The online trace does not expose any FFB inputs or outputs because the runtime group is disabled.
• Parameter list filter "free function blocks" returns zero hits even after a full offline-to-online upload.

The drive itself functions correctly: motor control, ramp-function generator, ON/OFF1, and PROFIdrive telegram handling are all healthy. The FFB visibility issue is a configuration oversight in STARTER, not a hardware fault.

2. Root Cause

FFB are gated behind a feature bit that STARTER only exposes when the user explicitly activates the function module during the Control Unit configuration wizard. Factory default projects ship with the FFB module deactivated, which causes:

• The FFB node is filtered out of the navigator.
• Parameters in the p20000-p20999 range remain hidden in the parameter list.
• The runtime group sampling time (p20000[0]) is set to 0 ms, so no execution occurs even if instances are inserted via TIA Portal/Startdrive later.

This is documented in chapter 6.14 "Free function blocks" of the SINAMICS G120 CU250S-2 operating instructions. See the official manual at SINAMICS G120 CU250S-2 Operating Instructions.

3. Prerequisites

Verify the firmware version before commissioning. From STARTER online, navigate to Drive > Diagnostics > Version and confirm r0018 reports 041011 (V4.7.11). The support package must match the firmware branch; mismatched combinations cause STARTER to refuse to load the project or to mark FFB parameters as "unknown".

4. Step-by-Step: Enabling FFBs in STARTER

The procedure below assumes an existing offline project with the CU250S-2 inserted. If a fresh project is required, create one with Project > New > Drive > SINAMICS G120 and select CU250S-2 explicitly.

1. Open the project in STARTER and connect to the target device via Online > Accessible nodes.
2. In the project navigator, right-click the CU250S-2 drive icon and select Configuration (or double-click Control Unit > Configuration). The configuration dialog opens with a list of wizard pages on the left.
3. Navigate to the wizard page labeled Function modules (sometimes titled Select function modules or Activate free function blocks).
4. Tick the checkbox Activate free function blocks.
5. Click Next through the remaining wizard pages. Accept defaults if other modules (e.g., technology controller, extended setpoint channel) are not required.
6. Click Finish. STARTER will prompt to save and recompile the project.
7. After recompilation, right-click the drive > Properties > Free function blocks. The FFB entry should now appear in the navigator.
8. Right-click Free function blocks > Insert new block. Choose from AND, AND4, OR, OR4, XOR, XOR4, NOT, RSR, RSE, DFF, TIMER, MFP, PCL, PDE, PDF, PST, CNT, ADD, SUB, MUL, DIV, AVA, NCM, PLI, MUX, BSW, or LVM.
9. Configure the instance: name, input connections (binector/connector numbers), and behavior (e.g., RSR reset-dominant flag).
10. Compile the project (Project > Compile) and download to the drive (Online > Download to target device).

5. Available Free Function Blocks on the CU250S-2

The CU250S-2 with firmware V4.7 ships a fixed pool of FFB instances. The maximum instance count per block type is firmware-dependent; the table below reflects V4.7.11.

Each FFB instance consumes one slot in the runtime group. The runtime group sampling time is set by p20000[0] and must be non-zero (typical: 4 ms or 8 ms) for any FFB to execute. See chapter 6.14.3 of the CU250S-2 operating instructions for parameter layout.

6. RSR Configuration Example: Latched Start/Stop Pushbutton

The classic use case for an RS flip-flop on a G120 drive is creating a maintained start/stop logic from two momentary pushbuttons wired to digital inputs, then feeding the latched output into the ON/OFF1 command source (parameter p0840).

6.1 Required connections

• DI0 (terminal 5) = START pushbutton (NO contact to +24 V)
• DI1 (terminal 6) = STOP pushbutton (NC contact to +24 V, with a parallel resistor for source-type wiring) or wired as NC with sinking mode
• FFB:RSR = RS flip-flop, set = DI0, reset = DI1, output Q = binector r2090.0 (or directly to p0840)

6.2 Parameter wiring

6.3 Configuration procedure

1. In STARTER, after enabling FFB, insert an RSR instance named e.g., "RSR_StartStop".
2. Open the RSR parameter dialog. Set p20150[0] = r0722.0 (DI0 state) for the set input.
3. Set p20151[0] = r0722.1 (DI1 state) for the reset input.
4. Set p20152[0] = 0 (dominance = reset, standard RSR behavior).
5. Note the RSR output bit address (e.g., r20251.0) shown in the navigator.
6. Open parameter p0840[0] and set it to the RSR output: p0840[0] = r20251.0 (or use a CDS binector connector such as r2090.0 if the wizard places the RSR output there).
7. Save, compile, and download.

7. Verification

After downloading, perform the following checks to confirm FFB are active and the start/stop logic works:

1. Runtime group active: Online > Drive > Function blocks > Runtime group. Confirm p20000[0] = 4 (ms) and the group status displays green.
2. RSR output toggles: Online > Watch table. Force r0722.0 = 1 (simulate START). Observe r20251.0 = 1 within one cycle. Force r0722.1 = 1. Observe r20251.0 = 0.
3. ON/OFF1 propagation: Observe r0898.0 (control word bit 0) follows r20251.0. When r0898.0 = 1, the drive enters "Ready to run" (r0002 = 0x...C). When r0898.0 = 0, drive commands OFF1 and ramps down per p1121.
4. Trace confirmation: Use STARTER trace with a 4 ms sampling interval to capture DI0, DI1, RSR.Q, and r0898.0 over a 5-second window. Trigger on START edge.
5. Save RAM to ROM: After successful test, perform Copy RAM to ROM (or set p0971 = 1) to persist the configuration across power cycles.

8. Firmware and Software Compatibility Matrix

STARTER V5.1.1.2 is the last widely-deployed version for V4.7 firmware. For V5.x firmware, Siemens requires Startdrive integrated in TIA Portal. Cross-version projects require a STARTER -> TIA Portal migration or a firmware upgrade before commissioning.

9. Migration to Startdrive in TIA Portal

For new installations or firmware upgrades beyond V4.7, the FFB library is configured in TIA Portal using Startdrive. The equivalent procedure:

1. Open the TIA Portal project containing the G120 with Startdrive V15.1 or later.
2. In the device view, double-click the CU250S-2. Open Properties > Function modules.
3. Enable Free function blocks under the Function modules tab.
4. Drag FFB from the catalog into the FFB editor.
5. Use the interconnection editor to wire binectors and connectors.
6. Compile and download to the drive.

Parameter numbering shifts slightly between STARTER and Startdrive; the binector/connector addresses remain consistent (e.g., r0722.0 for DI0), but the FFB instance numbers may differ. Existing STARTER projects with V4.7 firmware can be migrated to TIA Portal via Project > Migrate; verify FFB interconnections post-migration before relying on them.

10. Safety Considerations

The CU250S-2 supports Safety Integrated functions when ordered with the appropriate variant (-1FA0 for STO/SS1/SBC). FFB operate in the standard (non-safety) control path. Critical considerations:

• STO (Safe Torque Off) must use the dedicated safety terminals (e.g., F-DI on CU250S-2) and cannot be latched via FFB.
• SS1 (Safe Stop 1) requires parameterized deceleration and STO activation; the FFB-based start/stop logic must be subordinate to SS1 (i.e., SS1 overrides ON/OFF1).
• Emergency stop per ISO 13850 and IEC 60204-1 must be wired to the safety circuit, not into a non-safety FFB path.
• The FFB runtime group is single-threaded with the standard control loop; a slow scan (p20000[0] > 16 ms) can delay responses to operator commands.
• Do not implement SIL-rated functions using FFB logic. For SIL 1/2/3, use the integrated Safety functions or a separate safety relay.

11. Troubleshooting Matrix

12. Diagnostic Flowchart

13. Frequently Asked Questions