Sinumerik 840D POSMO SI Servo Replacement: Engineering Reference

1. Problem Statement and Component Status

A 2006-era turning lathe based on a Sinumerik 840D NCU-Box with MLFB string 6FC5247-DAA00-DAA3 is fitted with a POSMO SI AH83 servo axis: an integrated servomotor/drive combination built around a 1FT6 stator stack and a SIMODRIVE-class power section, communicating with the NCU over PROFIBUS DP. With the drive power section and motor both failed, the field engineer faces a forced obsolescence situation:

• Siemens formally discontinued the POSMO SI product line in 2009. Repair/exchange is routed through regional support and is not always commercially attractive on a dedicated single-purpose lathe.
• Replacement as a like-for-like integrated axis is no longer supported by normal spare-parts logistics.
• Mechanical envelope (flange, shaft, key, feedback orientation) constrains physical replacements.
• PLC project archive cannot be opened because the engineering workstation lacks the SINUMERIK 810D/840D STEP 7 add-on package.

This reference consolidates the engineering options that realistically remain in 2024–2026 for a working shop that needs to restore production within days, not months. It treats the third-party drive as the principal retrofit path, the SIMODRIVE/SINAMICS drop-in as the preferred native path, and the missing PLC add-on package as a recovery task.

2. Hardware Identification: NCU, POSMO SI, and the PROFIBUS Layer

Before any drive swap, confirm the exact hardware on site. The string on the nameplate tells you what drive objects (DOs) the NCU expects to bring up at boot.

2.1 NCU identification

The 6FC5247 prefix denotes an NCU 561.x class controller. The NCU hosts the PROFIBUS DP master port through which the POSMO SI axis spoke as a slave. The active machine data set (in MD30300 / MD30310 / MD30320 and the per-axis MD3xxxx family) defines which PROFIBUS slot the axis occupies. Without those numbers, a replacement drive cannot be addressed correctly even if it is hardware-identical.

2.2 POSMO SI as a Profibus node

POSMO SI variants (AH = high inertia, BH = compact) integrated the motor, resolver/encoder evaluation, infeed, and IGBT bridge in a single IP65 housing. To the NCU the unit presented itself as a PROFIdrive Class 1 slave on slot 1 (drive object 1), with optional slot 2 for the internal feedforward/limit-word handling. The GSD file for POSMO SI is the historical SIEM811F.GSD (or vendor-specific equivalents). Without that GSD loaded on the NCU's PROFIBUS master configuration, no third-party or Siemens-native replacement will be recognised.

3. Replacement Path Decision Matrix

Three engineering paths are realistically available on a 2006-vintage 840D with discontinued POSMO SI hardware. The matrix below ranks them by risk, downtime, and disruption to the existing PLC/HMI project.

For a dedicated 3- or 4-product lathe that must resume production in days, Path A or Path B are the only realistic answers. Path D is correct for new machine investment but out of scope for emergency restoration.

4. Third-Party Drive Integration: Protocol and Authorisation Requirements

A non-Siemens servo amplifier can be commanded by a Sinumerik 840D NCU only if it satisfies three conditions simultaneously:

1. PROFIBUS DP slave with a GSD file compatible with the Siemens master, and an explicit PROFIdrive profile implementation (Class 1 minimum, Class 3 if you need DSC - dynamic servo control).
2. Drive state machine matching PROFIdrive Figure 14 (S1..S12): OFF, Power-on, Run-up, Encoder config, Parameterisation, Ready, Switch-on enabled, Operation enabled, Quick Stop, Fault Reaction, Fault. The NCU's startup behaviour (MD20150 GCODE defaults) assumes exactly this state machine.
3. Setpoint interface on PZD words: NSOLL_A/B (MDI setpoints), NIST_A/B (actual values), STW1/ZSW1 (control/status word 1) and STW2/ZSW2 if the drive supports extended functions. These are standardised in PROFIdrive profile v4.2; any third-party drive whose GSD advertises PROFIdrive Class 1 must respect them.

4.1 Drive authorisation on the NCU

Sinumerik 840D implements a per-drive authorisation check. Each drive telegram is bound to a drive object number (DO), and the NCU reads that DO number from PROFIBUS slot 1. When swapping a Siemens POSMO SI for a third-party drive, the slot and DO number must be preserved or the NCU will raise alarm 25201 "Drive %1 hardware fault" or 300700 "Drive fault, axis %1" at power-up.

4.2 Slot mapping rule of thumb

For each axis that is being replaced, do not change the PROFIBUS slave address or the slot assignment. The PLC program, HMI commissioning screens, and tool-management screens all reference the logical axis number. Only the GSD file in the SDB container changes.

5. Native Siemens Drive Replacement Options

Even with POSMO SI discontinued, Siemens still offers two physically compatible drop-in options for older 840D installations.

5.1 SIMODRIVE 611U on the same drive bus

The SIMODRIVE 611 U family accepts a 1FT6 or 1FK7 motor on the same resolver/encoder and supplies a PROFIBUS DP module (6SN1118-0DJ21-0AA1 or equivalent) that speaks PROFIdrive Class 1. Configuration differences from POSMO SI are limited to:

• Power module size (the 611 U may need a different infeed module on the drive bus).
• Motor code number (1FT6xxx-xxx) entered into MD1102 / MD1103 via HMI commissioning.
• Encoder alignment: POSMO SI used absolute encoders with EnDat; 1FT6/1FK7 motors are typically configured with the same protocol but the P-0-0001 parameter for encoder type must be checked.

5.2 SINAMICS S120 with a CU320-2 DP control unit

For a longer-life retrofit, the SINAMICS S120 line with a CU320-2 DP control unit gives a modern PROFIdrive Class 4 interface, integrated safety (SINAMICS Safety Integrated F-DI), and a much broader motor catalogue. A 1FT6 / 1FK7 replacement motor preserves the mechanical envelope; the STARTER commissioning tool (Siemens STARTER drive commissioning tool) walks you through the topology configuration in offline mode and produces the firmware load image for the CU.

6. NCU Drive Object and Machine Data Configuration

Once the replacement drive is on the desk, three groups of machine data need verification on the NCU before the axis will come up.

6.1 PROFIBUS topology data

6.2 Axis mechanical and encoder data

The mechanical constants of the lathe (lead screw pitch, gear ratio, position-loop Kv factor) must remain unchanged; only the motor parameters change. The relevant MDs are MD30300, MD30310, MD30320 for the encoder pitch, and MD31020, MD31030 for encoder resolution. The motor code (e.g. 1FT6108-8AF71) is entered in MD1102 / MD1103 via the HMI commissioning menu; the NCU then auto-loads the matching motor data set from the internal table.

6.3 Drive telegram mapping

For a PROFIdrive Class 1 telegram 2 (4 PZD words in / 4 PZD words out), the slot layout is:

• Word 1 (in): STW1 — control word 1
• Word 2 (in): NSOLL_B — speed setpoint
• Word 3 (out): ZSW1 — status word 1
• Word 4 (out): NIST_B — actual speed value

If the third-party drive uses a different default telegram (e.g. Siemens Standard Telegram 3, 5, or 102), MD13060 must be set accordingly and the SDB rebuilt with HW Config.

7. PLC Program Package Recovery: STEP 7 with the 810D/840D Add-On

The PLC archive saved from the original machine uses STEP 7 libraries that are not part of the base STEP 7 V5.x install. They are delivered as an add-on package on the Sinumerik commissioning DVD or via the regional Siemens support. Without them, the archive opens as a S7-300 program but the FM/CP blocks and the SINUMERIK FB references are missing or shown as unknown.

7.1 Recovery steps

1. Contact the local Siemens representative (Automation & Drives sales channel) and request the Toolbox SINUMERIK 810D/840D add-on for the relevant STEP 7 version (typically V5.5 SP4 or V5.7).
2. Match the add-on to the NCU firmware version on the machine: NCU 561.x typically pairs with SINUMERIK Toolbox V08.x or higher.
3. Install the add-on into the existing STEP 7 install; restart the SIMATIC Manager.
4. Re-open the project archive. The SINUMERIK FB/DB blocks now resolve against the installed library.

8. Commissioning Procedure

The end-to-end commissioning sequence for a single-axis swap on an 840D NCU is intentionally conservative; never skip the bus diagnostics step.

1. Power down and lockout the drive bus (24 V DC logic and three-phase 400 V AC) per local safety rules. Tag-out the main contactor and verify zero energy.
2. Remove the failed POSMO SI unit. Note the bus address set on the address switch (typically a DIP block on the back of the unit).
3. Mount the replacement drive/motor. Wire the resolver/encoder cable, power cables, and PROFIBUS drop connector. Terminate the bus segment at the new drive if it is the end node.
4. Set the new drive's PROFIBUS address to match the value read from the old unit.
5. Apply logic power only. On the HMI, go to Commissioning → Bus Diagnostics and confirm the slave appears at the expected address and slot.
6. Import the new drive's GSD into STEP 7 HW Config, rebuild the SDB, and download it to the NCU.
7. Re-enter the motor code in MD1102/MD1103 via Commissioning → Axis. Save the NCK data.
8. Run the drive's first commissioning via STARTER (Siemens-native) or the third-party vendor's tool. Pre-charge, ramp-up, encoder calibration, current-loop tuning, speed-loop tuning, position-loop tuning.
9. Perform a controlled warm restart of the NCU. Verify that the axis comes up in Follow-up mode with no alarm.
10. Run the part-program test cycle on the lathe with reduced feed override (25%). Verify following-error envelope and velocity profile against the machine's acceptance record.

9. Safety, Functional, and EMC Considerations

Sinumerik 840D originally integrates STO/SS1 stop functions on the SINAMICS Safety Integrated bus. When swapping to a non-Siemens drive, the Safety Integrated channel is broken; the replacement must either provide its own STO input wired to the NCU's safety relay, or the system must be re-evaluated as a category 3 / PL d chain. The Siemens support note 36308201 on POSMO SI discontinuation explicitly warns about safety-related spare parts.

EMC: a third-party PROFIBUS cable must be shielded with 360-degree termination on both ends. The motor cable shield should be bonded to the cabinet ground bar at the drive end only when the motor housing provides a high-frequency path; otherwise use a gland plate with 360-degree clamp at the cabinet entry.

10. Field Verification Checklist