Sinamics G120 Switching Speed Setpoint from Profibus to Analog

Problem Statement

A common field requirement on multi-drive Siemens Sinamics G120 installations is mixed control: the PLC keeps Profibus/Profinet control words (STW1 / ZSW1) for run, stop, direction, and fault acknowledgement, while one drive in the group must accept its speed reference from a local analog source (HMI slider, potentiometer, 4–20 mA loop). The naive approach is to change P1070 or P1000 on the Basic Operator Panel (BOP) and rewire. The drive then refuses to write, returns fault F01910, or silently keeps the Profibus word as the active reference. The root cause is parameter protection imposed by P0922 (PROFIdrive PZD telegram selection). This reference explains the parameter model, the lock condition, and every documented workaround.

Sinamics G120 Setpoint Architecture

The G120 evaluates the speed reference through a two-stage selector before it reaches the ramp-function generator (RFG) and the motor-side speed controller. Understanding this chain is required before touching any parameter.

Conceptually, P1000 writes into P1070 at startup. Once the drive is commissioned, an integrator typically overrides P1070 with a free-wired connector (BICO) from the analog input or fieldbus word. The two parameters are not redundant — they sit at different abstraction layers.

P1000 vs P1070 — Functional Difference

Field-proven rule: during live operation edit P1070 directly. P1000 is a starting point, not a runtime switch. Setting P1000 alone without re-coupling P1070 leaves the drive in the previous state until the next power cycle or factory reset.

P0922 — The Lock Mechanism Behind F01910

P0922 selects the active PROFIdrive PZD telegram (1, 2, 3, 4, 20, 350, 352, 353, 354, 999 = free BICO). When a standard telegram is selected, the SINAMICS firmware re-routes the relevant PZD words into fixed BICO sinks. Specifically:

• Telegram 1 / 2 / 3 / 4: PZD word 2 (NSOLL_A) is routed to r0062 and clamps P1070 / P1075 against manual overwrite.
• Telegram 20: full PROFIsafe / process data with speed setpoint in PZD 2.
• Standard telegrams also set P2051 (PZD send) and P2050 (PZD receive) implicitly.

The write-protection on P1070 is enforced by the parameter supervisor when P0922 > 0 (any non-free telegram). Attempting to write the parameter raises F01910 "Fieldbus setpoint timeout" or simply rejects the value with status r3996 = 7 (write protection active). This is the symptom the operator sees at the BOP.

Prerequisites

1. Sinamics G120 with CU240S/D/E-2 control unit (or G120C, G120P with the same firmware behaviour). Verify via r0001 = control unit type and r0964[0..3] firmware version.
2. Firmware ≥ V4.4 recommended for the dual-source pattern below. Earlier versions use the P0922 = 4 default and the BOP write path is identical but parameter access rights differ.
3. Basic Operator Panel (BOP-2) or IOP with write access level ≥ Standard (set in P0003 = 3).
4. Analog input wired to terminals 3/4 (AI1, ±10 V / 0–10 V) or 6/7/8 (AI2, 0/4–20 mA). Confirm scaling in P0756 and P0761.
5. PLC project must continue sending STW1 = 047Eh (UNFREEZE / stop) and 047Fh (RUN) on PZD1; the speed word (PZD2) is ignored once the analog override is in effect.

Analog Input Parameter Stack (Reference)

Step-by-Step BOP Configuration

The procedure keeps Profibus active for control words. It rebinds the speed setpoint path only.

1. Power on the drive, enter Expert access. BOP-2 path: MENU → PARAMETER ACCESS → P0003 = 3.
2. Select the free-BICO telegram so the firmware releases P1070 from the standard PZD2 route:
   P0922 = 999 (free BICO). The drive briefly goes OFF2; the PLC control word is re-read on the next cycle. This is expected.
3. Confirm control-word coupling is intact. The PLC continues to send STW1 on PZD word 1. Verify r2050[0] follows the received PZD1 and that the drive returns to ZSW1 = 0637h (ready, enabled).
4. Wire the analog input as the main setpoint.
   P1070[0] = 755 (CI: r0755[0] — AI0 percentage).
   If a supplementary path is also needed (e.g. for trim), leave P1075[0] = 0; otherwise P1075[0] = 755 for a second analog channel.
5. Disable any pre-set ramp bypasses that may clamp the analog path:
   P1110[0] = 0 (no fixed setpoint override)
   P1113[0] = 0 (no direction reversal from setpoint sign)
   P1140[0] = 1 (RFG active — keep ramp times P1120 / P1121 meaningful).
6. Configure analog scaling for the chosen transducer (see table above). For a 10 kΩ pot on AI1 use P0756[0] = 0, P0759[0] = 10.0, P0760[0] = 100.0.
7. Save parameters to EEPROM: BOP-2 → MENU → SAVE TO MEMORY → OK. Without this step, the change is lost on power-cycle.
8. Cycle the drive (OFF1 → wait 3 s → ON). Watch r0021 (actual frequency) when the potentiometer is rotated. The PLC control word is unaffected.

Workarounds When P1070 Remains Locked

If P0922 = 999 cannot be set (the PLC programmer insists on standard telegram 1, or the HMI is wired against the standard profile), use one of the following:

Workaround A — Free telegram (recommended)

Switch PLC hardware configuration to "Free telegram 999" in TIA Portal / STEP 7. Only the necessary PZD words are mapped. PZD1 = STW1/ZSW1, PZD2 unused. Same drive count, fewer mapping conflicts.

Workaround B — Disable P1070 protection via P0922 = 0

Setting P0922 = 0 disables PROFIdrive PZD setpoint coupling. The drive treats fieldbus as control-word only. This is the original Siemens recommendation for the user's case.

Workaround C — Override the fieldbus word with the analog source

Wire P1070 = 755 first, then issue the write while P0922 = 999. If the application forces P0922 = 1 from the PLC at every startup, place a parameter-init script in the drive that runs P0922 = 999 → P1070 = 755 from a startup macro (P0010 = 0, P0015 macro) so the drive reverts after every PLC handshake.

Workaround D — Drive-based HMI control via BOP / IOP

If the request is "local control via HMI without PLC software changes," fit a Smart Access Module (SAM) or a Siemens Webserver module and run a built-in drive web page. The setpoint is written by the HMI to P1070 directly over the drive's service interface. Profibus control words are untouched.

Workaround E — Motor potentiometer (MOP)

Set P1000 = 7 (analog + MOP) and use P1035 / P1036 for UP/DOWN digital inputs. Field service reports this is the most robust fallback when no analog is available — but the user must wire physical buttons or use the HMI's MOP screen.

Verification Procedure

1. With drive enabled via Profibus (PLC sends STW1 = 047Fh), ramp potentiometer from 0 → 100 %.
2. Monitor r0021 (motor speed Hz) and r0024 (output frequency). The values must track r0755[0].
3. Check r3996 (parameter reject cause) = 0. Any non-zero value means the write was rejected; refer to the table below.
4. Trigger a Profibus stop: PLC sends STW1 = 047Eh. Drive must coast (OFF1 ramp). Speed setpoint from potentiometer is irrelevant during OFF1.
5. Cause a fault: open the AI wire (4–20 mA, P0761 = 2.0 V). The drive should trigger F03505 (AI wire break) within 100 ms, communicate it to the PLC via ZSW1 bit 3 = fault and ZSW1 bit 6 = switch-on inhibit active.
6. Reconnect and acknowledge the fault from the PLC: STW1 bit 7 rising edge. Verify r0949[0] = 3505 and the fault clears.

Troubleshooting Matrix

Parameter Quick-Reference Table

Edge Cases and Field Notes

• Three-drive synchronous runs: the user's three drives share one setpoint. Once drive #2 is rebound to analog, the other two still read the PLC's PZD2. To keep the PLC's process image consistent, write 0 to NSOLL_A for drive #2 from the PLC instead of removing the PZD2 mapping.
• Safety: any PROFIsafe profile (telegrams 30, 31) cannot be broken to free BICO 999 without deactivating safety. For Safety-Integrated drives, use the SAM/Webserver approach only, never P0922 = 999.
• Save and verify: always issue P0971 = 1 or BOP-2 "Save to memory" after the parameter set is final. Cold restart from a power cycle is the only definitive test that the change persisted.
• Acyclic writes from the PLC (DPV1 / acyclic PKW) may overwrite P1070 on every restart. Disable the acyclic write for the setpoint parameter in the PLC's parameter assignment, or the drive will revert on the next PLC cycle.