Overview of the SINAMICS S110 CU305DP Platform
The SINAMICS S110 is a single-axis servo drive in the Siemens SIMOTION/SINAMICS family, designed for positioning and speed-controlled applications up to the kilowatt range typical of laboratory automation, packaging, feed/withdraw axes, and small-format machine building. The drive is paired with a Control Unit CU305DP, which embeds a PROFIBUS DP slave interface (9.6 kbit/s to 12 Mbit/s) and serves as the central communication, control, and diagnostics unit for the power module it supervises (typically a PM340 or a blocksize-format power section).
At commissioning the operator can choose between three control sources for the drive: the front-panel BOP (Basic Operator Panel), the STARTER commissioning tool over PROFIBUS/USB, and a higher-level controller (PLC) over PROFIBUS DP. The decision of which source owns the command word (STW1) and the speed setpoint is encoded in the parameter block P840 to P856. An incorrect assignment in this block is the most common reason the drive rejects the operator's "Run" command and reports the message "the switch-on disable of the drive is active", often accompanied by a red, 2 Hz blinking RDY LED.
The Parameter Block P840-P856: Command and Setpoint Sources
Each input to the drive's internal state machine is wired through a connector input parameter (denoted CI: in the parameter list) that selects the signal source. The numeric value you enter is the source parameter number or a fixed BICO identifier. A wrong assignment leaves the drive waiting for a control word from a device that is not connected, and the drive therefore latches into the OFF1/OFF2/OFF3 disabled state.
| Parameter | Designation | Default | Function in the command chain |
|---|---|---|---|
| P840 | CI: ON/OFF1 command source | r0019.0 (control word BOP) | Edges that switch the drive into "ready to run". |
| P844 | CI: Speed setpoint source | r0019.0 | Main speed/position setpoint input. |
| P845 | BI: Enable / control signal source | 0 | Permits power module gating. |
| P848 | CI: Additional setpoint source | 0 | Auxiliary torque/acceleration feed. |
| P849 | CI: Supplementary setpoint source | 0 | Second setpoint for overlaid control. |
| P850 to P855 | BI: STW1 bit-level sources | 0 / various | Individual routing of control word bits (OFF2, OFF3, enable, jog, etc.). |
| P856 | CI: STW1 (control word 1) source | r0019.0 | Master source for STW1 bits 0-15. |
The numeric values you enter are BICO identifiers. Common identifiers for the SINAMICS S110 are:
| Identifier | Source | Use case |
|---|---|---|
| 0 | Constant zero (signal disabled) | Clearing a default routing. |
| 722.x | STW1 bit from PROFIBUS telegram (PZD word) | PLC-driven control. |
| r0019.0 | Internal control word from BOP/AOP | Local panel start/stop. |
| 840.x | Drive-internal status bits | Advanced interlocking. |
For the local control panel case described in this article, the drive must source STW1 from r0019.0 (BOP control word) and the enable signal must come from a non-zero identifier. If P840, P844, P848, or P856 are left at a value referencing PROFIBUS while no PLC telegram is present, the drive will remain in the OFF state and report F01770.
Fault F01770: Switch-On Disable Active
The diagnostic message "the switch-on disable of the drive is active" maps to fault number F01770 (drive-integrated, control-related). The same root cause can also surface as alarm A17903 or A17904, depending on whether the suppression bit was cleared or whether the OFF2/OFF3 ramp is active.
| Fault/alarm | Meaning | Typical trigger |
|---|---|---|
| F01770 | Switch-on inhibit / OFF1 held | STW1 bit 0 = 0 because the command source is wrong. |
| F17970 | OFF2 active | STW1 bit 1 = 0 due to incorrect enable. |
| F17971 | OFF3 active | STW1 bit 2 = 0; emergency-stop ramp held. |
| A17900 | Drive ready but not enabled | Power-up complete, awaiting enable. |
Each fault number has a default reaction: F01770 forces the drive into the OFF1 ramp-down and latches. To clear the fault, the operator must issue an "Acknowledge" command from the active control source. The BOP key sequence is FN -> ACK; on STARTER the toolbar button "Acknowledge faults" performs the same operation.
RDY LED Diagnostic States on CU305DP
The CU305DP front face has a two-color RDY (Ready) LED and a COM (Communication) LED. The blink patterns are specified by Siemens and should be interpreted with the SINAMICS S110 List Manual (LH1) section "Diagnostics / LED display".
| RDY LED | State | Action |
|---|---|---|
| Off | No 24 V logic supply | Check X124 power, DIP switch S1. |
| Solid green | Drive ready, no fault | Normal. |
| Blinking green 0.5 Hz | Commissioning / download active | Wait for completion. |
| Blinking green 2 Hz | Drive ready, identification routine running | Wait; first run after motor ID. |
| Solid yellow | Warning active | Read alarm buffer. |
| Blinking red 2 Hz | Fault active (e.g. F01770) | Read fault buffer, acknowledge. |
| Solid red | Firmware / hardware fault | Power-cycle, reflash if persistent. |
| Blinking red 0.5 Hz | Bootloader / firmware update | Do not power-cycle. |
The user's report of "RDY LED blinks red" indicates the drive is in the Fault active state. Coupled with the "switch-on disable" message, this points to the command-source wiring rather than a hardware failure.
Root Cause: Mismatch Between P840/P856 and the BOP
When the S110 leaves the factory, the command source is wired to PROFIBUS by default (P856 = 722.0, the first control word PZD from the PROFIBUS telegram). If you try to start the motor from the BOP without first re-routing STW1 to the BOP, the drive will:
- See STW1 bit 0 (ON/OFF1) held at 0 by the PLC that is not yet connected.
- Latch into OFF1 (switch-on inhibit).
- Illuminate the RDY LED in 2 Hz red blink.
- Display the F01770 message in the fault buffer.
The user's original attempt (entering 856, 840, 844, 848) failed because the numeric values were treated as raw BICO identifiers pointing to inactive sources. Re-issuing the same numbers after a factory reset and adding P845 and P849 with correct routing was the path that worked.
Step-by-Step Recovery Procedure
The procedure below returns the drive to a known-good BOP-driven state. Total time: 5-10 minutes including motor ID run.
Prerequisites
- CU305DP powered with 24 V at X124, DIP S1 = ON (or per project).
- BOP20 or AOP30 connected at X100 / X101.
- Motor power cables landed, encoder cable on X202 / X203.
- Motor data sheet (rated current, torque, speed, encoder type).
- STARTER V5.x (optional but recommended for offline parameter inspection).
Procedure
- Factory reset: set P0010 = 30, P0970 = 1, then wait for the unit to reboot (RDY LED cycles). This clears every BICO assignment.
- Confirm motor ID: navigate to P0300 (motor code) or enter nameplate values manually. For Siemens 1FK7 / 1FT6 / 1PH8 servo motors set P0300 to the motor code; for third-party induction motors, enter P0304-P0311 by hand.
- Encoder configuration: set P0400 (encoder type) to match the connected feedback (e.g. 2001 = EnDat 2.1, 2002 = resolver, 2003 = sin/cos incremental, 2050 = DRIVE-CLiQ encoder). The drive will not leave F01770 if the encoder is misconfigured because the position actual value is invalid.
- Control type selection: P1300 = 21 (sensorless vector) or P1300 = 11 (V/f) for induction motors; for servo operation with the S110 use speed control with encoder (default for 1FK7 motors). Verify the choice in the wizard screen.
-
BICO routing for local control (the fix that worked):
- P845 = 1 (enable from BOP / r0019.0).
- P849 = 1 (supplementary setpoint from BOP).
- P856 = r0019.0 or the equivalent BICO identifier 840.0 (STW1 from BOP).
Leave P840 and P844 at the default routing, which after factory reset already points to the BOP. - Save the configuration: set P0971 = 1 to write RAM to the non-volatile CF card.
- Start from BOP: press the green RUN key. RDY LED should transition from blinking red to solid green.
Verification
- RDY LED = solid green.
- STARTER / BOP shows r0002 = "Operation enabled".
- Fault buffer (r0945 / r0947 / r0948 / r0949) is empty.
- Setpoint entry on the BOP drives the motor smoothly with no jerk.
PROFIBUS Integration After Local Start Is Confirmed
Once the motor spins under BOP control, PROFIBUS can be enabled. The standard S110 telegram for an external PLC is the Siemens PZD telegram Standard Telegram 3 (1 control word + 1 setpoint + 1 status + 1 actual value) or Standard Telegram 5 (additional encoder feedback).
- Set P0922 = 3 (or 5) to select the telegram.
- Set P856 to 722.0 (STW1 from word 1 of the PROFIBUS PZD receive).
- Set P844 to 722.1 (main setpoint from word 2).
- Re-route enable signals: P845 = 722.0 with bit-masking via P850-P855, or assign the enable to a separate digital input bit 0 (r0722.x).
- Configure the PROFIBUS node address on DIP switch S1 (1-127) or via P0918.
- In the PLC project, map the drive to the corresponding I/O address and toggle control word bit 0 (ON/OFF1), bit 3 (enable), bit 7 (no OFF2), and bit 8...10 depending on the telegram profile.
Motor and Encoder Configuration Constraints
| Motor family | Encoder | P0400 | Comment |
|---|---|---|---|
| 1FK7 / 1FT7 synchronous servo | EnDat 2.1 / DRIVE-CLiQ | 2001 / 2050 | Motor ID automatic. |
| 1PH8 asynchronous servo | EnDat / sin/cos | 2001 / 2003 | Requires manual P0304-P0311. |
| Third-party induction | Incremental HTL/TTL | 2004 / 2005 | Vector mode requires encoder. |
| No encoder (open-loop) | None | 0 | V/f mode only; no servo operation. |
If the encoder is misconfigured or the cable is reversed, the drive may also report F31117 / F31118 (encoder initialization) in addition to F01770. Always check the alarm buffer before re-trying the start command.
Troubleshooting Matrix
| Symptom | Likely cause | Fix |
|---|---|---|
| RDY blinks red 2 Hz, "switch-on disable" | Command source points to PROFIBUS while no PLC is online | Factory reset (P0970 = 1) or re-route P856 to r0019.0. |
| RDY blinks red, F17970 in buffer | OFF2 input held low (terminal or P840 wrong) | Check P840 = r0019.0 or 722.0. |
| RDY solid green but motor does not move | Setpoint source (P844) wired to a constant 0 | Set P844 to r0019 or 722.1. |
| RDY blinks red after PROFIBUS connect | STW1 bit 0 not toggled by PLC | Verify PLC send word 0 bit 0; check P0922 = 3. |
| RDY blinks red after PLC stop / PROFIBUS drop | Enable bit 0 latched to 0 | Add r2090 handshake or rewire to terminal input. |
| RDY solid red | Hardware / firmware fault | Power-cycle, check CF card, reflash firmware via STARTER. |
| RDY blinks red 0.5 Hz | Firmware update in progress | Wait; do not interrupt. |
Safety and Commissioning Checklist
- Confirm STO (Safe Torque Off) inputs at X126 are wired; otherwise the drive stays in OFF2 by design.
- Verify the DC-link precharge is complete before issuing the run command (P0010 wizard step "Power on" shows DC-link voltage > 540 V DC for 400 V class).
- Set ramp-up (P1120) and ramp-down (P1121) to safe values (default 10 s is too fast for most servo axes; start with 2-5 s).
- Enable the speed setpoint filter (P1500 / P1503) for high-inertia loads to avoid jerk-induced overshoot.
- Run the motor ID at zero load for the first commissioning; subsequent fine-tune with the load applied.
Verification Checklist After Recovery
- RDY LED = solid green, COM LED = solid green (PROFIBUS data exchange active).
- r0002 shows "Operation enabled" or "Operation".
- Fault buffer (r0947[r0950]) is empty.
- Alarm buffer (r2110[r2122]) is empty (warning only when n > 0 has been reported in some installations due to P2150 settings; clear with factory defaults if unwanted).
- BOP can ramp the motor from 0 to rated speed and back to 0 without overshoot or fault.
- PLC can take over the command source by re-routing P856 / P844 to 722.x and toggling STW1 bit 0.
What does F01770 "switch-on disable" mean on a SINAMICS S110?
F01770 means STW1 bit 0 (ON/OFF1) is held at 0 because the command source parameter P856 is wired to a signal that is not active - most often a PROFIBUS word from a PLC that is not in RUN. Route P856 to r0019.0 (BOP) for local start, or 722.0 (PROFIBUS PZD word 1) for PLC control, then acknowledge the fault.
Which parameters must be set to start an S110 from the BOP after a factory reset?
After P0970 = 1 (factory reset) the defaults already wire P840, P844, and P856 to r0019.0 (BOP). Add P845 = 1 and P849 = 1 to enable the BOP enable and supplementary setpoint paths, then save with P0971 = 1.
Why does the RDY LED blink red even after I cycle the 24 V power?
F01770 latches. A power-cycle re-arms the input stage but does not acknowledge the fault. Use the BOP "FN -> ACK" key sequence or the STARTER "Acknowledge faults" button to clear the buffer.
How do I integrate the S110 with a PLC over PROFIBUS DP after the local start is confirmed?
Set P0922 = 3 (Standard Telegram 3), P856 = 722.0 (STW1 source), P844 = 722.1 (main setpoint source), and P0918 to the node address. In the PLC project, map the drive to its configured I/O range and toggle control word bit 0 (ON), bit 1 (no OFF2), bit 2 (no OFF3), and bit 3 (enable).
Can the drive run without an encoder on the SINAMICS S110?
Only in V/f mode (P1300 = 11) for induction motors. Sensorless vector (P1300 = 21) and servo modes require an encoder. Set P0400 = 0 and accept reduced dynamics, or install a matching encoder before commissioning.
What is the difference between F01770 and F17970 on the S110?
F01770 is the OFF1 / switch-on inhibit generated by the command chain. F17970 is the OFF2 inhibit driven by the safety / enable input. F17971 is the OFF3 (emergency-stop) inhibit. All three share the same symptom (RDY red, no start) but require different parameter fixes - P840/P856 for F01770, P845/P849 for F17970, and the STO terminals for F17971.