Siemens Drive P1121 Reset After Stop: RAM-to-ROM Save Procedure

1. Problem Overview

Field engineers commissioning legacy Siemens MICROMASTER 4xx and entry-level SINAMICS drives (V20, G110, G120 with CU230/CU240 control units) routinely encounter a frustrating behavior: ramp-down time parameter P1121 is changed via the Basic Operator Panel (BOP), verified, used during a run cycle, and then found reverted to factory default after the stop command. No power cycle occurred between the change and the apparent reset. The technician reports the change, runs the machine, stops it, reads back P1121, and finds the default value of 10.00 s (or whatever factory default applies for the firmware) sitting where 30.00 s was entered minutes earlier.

This symptom is not a hardware defect, EEPROM write failure, or firmware bug. In virtually every documented case the root cause is one of two conditions:

1. Supervisory controller overwriting the parameter. A PLC, HMI, or SCADA tag is writing P1120/P1121 back to the project default on every scan, every stop event, or every parameter-download cycle. The BOP edit is silently overwritten by the controller.
2. RAM-only change never committed to non-volatile memory. The BOP edit was written to volatile RAM. Without executing P0971 = 1 (RAM-to-ROM transfer), the value is lost on the next power-up or on certain parameter-reset triggers tied to command-source changes.

2. Affected Drive Families and Firmware

The P1120/P1121/P0971 numbering is part of the legacy MICROMASTER parameter schema that Siemens preserved across the entry-level SINAMICS product lines. Parameter availability, however, varies by firmware version and control-unit type.

Refer to the official SINAMICS V20 operating instructions and the MICROMASTER 440 parameter list for parameter map specifics per firmware branch.

3. Root Cause Analysis: Two-Tier Parameter Storage

Siemens entry-level drives expose a two-tier parameter storage model that is identical in principle across MM420/430/440, SINAMICS V20, and the SINAMICS G-series compatibility-mode parameter set:

• Volatile RAM (working memory): All parameter writes from the BOP, AOP, IOP, USS, Modbus, PROFIBUS-DP, or PROFINET land here first. RAM is fast but is cleared on power-down and on certain reset events tied to factory-reset or command-source changeovers.
• Non-volatile EEPROM (back-up memory): Parameters that should persist across power-cycles must be explicitly transferred from RAM to ROM using P0971 = 1. The transfer is a block write of all marked parameters and typically takes 3 to 30 seconds depending on the drive family and EEPROM wear.

The default for P0971 is 0 (idle, no save in progress). When the operator uses the BOP to change P1121, the BOP software automatically initiates a per-parameter write to RAM. Historically, many MM4 BOP interfaces write small parameter changes immediately to EEPROM. With certain SINAMICS V20 firmware revisions and certain IOP/BOP-2 combinations, the behavior switched to RAM-only write unless P0971 = 1 is issued afterward. This change in default behavior is the source of much field confusion.

3.1 Reset Triggers Beyond Power-Cycle

Several events other than power-cycle will silently drop a RAM-only value:

• Factory reset (P0010 = 30, P0970 = 1)
• Parameter-set switchover between parameter sets 0 and 1 (P0820/P0821)
• Restore-factory-defaults command from PLC over fieldbus
• Drive firmware update via STARTER or SINAMICS Startdrive
• Plug-and-play / I-component replacement on SINAMICS G120

Engineers observing P1121 "self-resetting" should record whether any of these triggers preceded the observation.

4. Key Parameter Reference

The following table lists the parameters that govern ramp timing and parameter persistence on the affected drive families. Verify the index and range against the parameter list specific to your firmware branch using Siemens Industry Online Support.

5. Solution A: RAM-to-ROM Transfer via P0971

When the root cause is genuine RAM-only loss, the fix is a one-shot write of the parameter set to EEPROM using P0971. The procedure works on every drive family listed in Section 2.

5.1 Procedure on the BOP / BOP-2

1. Confirm the drive is in ready state (no active run command, no fault active). The BOP displays the drive state in the top-right indicator (MM4) or via status LED (BOP-2/IOP).
2. Navigate to P1121 and verify the desired value. Do not run the motor between edit and save.
3. Navigate to P0971.
4. Press OK / Fn / P to enter edit mode.
5. Set the value to 1 and press OK / Fn / P to commit.
6. Wait until P0971 returns to 0 on its own (3 to 30 s). The BOP typically displays "busy", "save", or a flashing decimal during the transfer.
7. Power-cycle the drive. Verify P1121 retains the edited value.

5.2 Procedure via USS / Modbus RTU

For PLCs controlling the drive over RS485 (USS protocol or Modbus RTU), the same P0971 write can be issued as a parameter write over the bus. The PKW (USS) or register 4031xx (Modbus) address for P0971 must be calculated against the parameter number using the formula:

Modbus register address = 40000 + (parameter_number × 2) + 1

For P0971: register = 40000 + (971 × 2) + 1 = 41943 (read), 41942 (write low word).

Function code 06 (preset single register) or 16 (preset multiple registers) writes the value. Issue write of 1 to register 41942. Poll parameter r3996 (or status word bit) to confirm the save completes. See the SINAMICS V20 Modbus RTU parameter mapping reference for the precise address map on V20 firmware.

5.3 Procedure via PROFIBUS-DP / PROFINET

On PROFINET (SINAMICS G120 with CU240 PN) and PROFIBUS-DP (MM440 with PROFIBUS option), the parameter channel is mapped via PKE/IND/PWE words in the cyclic I/O or via acyclic record (PROFIBUS DPV1 / PROFINET record write). Use STARTER or the PLC's parameter-access FB to write 1 to P0971. Acyclic writes via record 0xB02E (PN) or DPV1 index 0x2F are accepted.

6. Solution B: Identify and Stop the PLC Override

If the PLC is the controlling entity, it is common practice for the HMI or PLC program to write default ramp times from the project database on every start-up, every recipe change, or every stop event. This is normal behavior in well-architected control code, but it makes ad-hoc BOP edits useless.

6.1 Diagnostic Procedure

1. Place the drive in hand / jog mode via P0700 = 1 (BOP) and P1000 = 1 (motor potentiometer / BOP). This isolates the drive from the PLC.
2. Edit P1121 to a non-default value, e.g., 45.00 s.
3. Save via P0971 = 1.
4. Power-cycle the drive.
5. Verify P1121 = 45.00 s after the power-up.
6. If the value survives, the PLC is the source of the overwrites.

6.2 Where to Look in the PLC Project

Search for the following patterns in the PLC code, regardless of manufacturer (Siemens S7-1200, S7-1500, Allen-Bradley CompactLogix, Schneider M340, Beckhoff TwinCAT):

• Function blocks named Drive_Param_Write, VFD_Sync, Parameter_Load, Recipe_Apply
• Logic blocks that execute on First_Scan, Cold_Restart, Drive_Ready_Rising_Edge
• Data blocks or tag databases with strings like ramp_time, decel_time, RAMP_DN, P1121
• Recipe-management blocks that push a complete parameter set on recipe selection

Disable or comment out the offending write block, recompile, and download. Verify that P1121 retains its edited value across a stop event.

7. Verification Checklist

After applying either Solution A or Solution B, run the following verification sequence before returning the machine to production:

1. Confirm drive state = Ready (no active run command).
2. Read P1121 and confirm intended value.
3. Issue a controlled run command from the controlling source (BOP, terminal, or PLC).
4. Run the motor for at least 30 s at a frequency > 10 Hz to verify ramping.
5. Issue stop. Measure ramp-down time with a stopwatch or from r0899 (ramp-function-generator output frequency) trace.
6. Stop and read P1121. Confirm intended value still present.
7. Power-cycle the drive. Read P1121 again.
8. If all three read-backs agree, close the work order.

8. Field-Proven Caveats

• Edit-only-when-stopped: Some BOP-2 firmware revisions reject writes to P1121 while the drive is in Run state. Always edit with the drive stopped. The MM4 manual explicitly documents this requirement.
• Quick-commissioning mode (P0010 ≠ 0): Writes issued while P0010 = 1 (quick commissioning) may not persist the same way as writes in P0010 = 0. Always set P0010 = 0 before issuing the P0971 save.
• Recipe download via STARTER / Startdrive: When the project is downloaded to the drive via STARTER or SINAMICS Startdrive, the tool issues its own RAM-to-ROM transfer at the end of the download sequence. You should still verify with P0971 = 1 if the value reverts.
• Multi-parameter-set drives: MM440 supports parameter set switchover via P0820/P0821. Each set has its own P1121 value. If switchover is active, you are only editing one set.
• Fieldbus fault behavior: On PROFIBUS/PROFINET, a faulted fieldbus or watchdog timeout can revert P0700/P1000 to their fault-handling values, which in some configurations silently restores factory ramp defaults.
• EEPROM wear: Each P0971 = 1 transfer is a full parameter-block write to EEPROM. EEPROM cells on MM4 hardware are rated for ~100,000 write cycles. In test-and-tune environments where engineers issue P0971 = 1 many times per shift, this can become a wear issue. Cache changes in RAM and only issue P0971 = 1 when the parameter set is final.

9. Related Parameter Set to Audit

If P1121 was reverting because of PLC override or generic parameter reset, the same issue applies to every parameter the PLC or operator edits. Audit the following list as a starting point:

10. Troubleshooting Matrix

11. Manual and Documentation References

For deeper parameter context, fault-code listings, and wiring examples, consult the official Siemens documentation:

• SINAMICS V20 Operating Instructions (Issue 2024)
• MICROMASTER 440 Parameter List
• SINAMICS G120 Operating Instructions (CU230P-2 / CU240B-2 / CU240E-2)
• SINAMICS V20 Modbus RTU Communication Manual
• SINAMICS V20 List Manual (Parameter Reference)

Fault-code references for ramp-related faults encountered after a P1121 override attempt:

12. Frequently Asked Questions