Adding a New Axis to Siemens Sinumerik 840D NCU 572.5 CNC

1. Overview: Why a New Axis Is Not a Plug-and-Play Operation

The Siemens SINUMERIK 840D sl with NCU 572.5 is a modular CNC platform where adding a spindle, a linear axis, a rotary table, or a tool changer axis means modifying four tightly coupled domains simultaneously:

1. Hardware – drive bus topology, SINAMICS power modules, motor modules, motors, encoders, NX expansion module (if the basic line module is at capacity), and the 24 V / 600 V DC link distribution.
2. Drive configuration – STARTER / SIMOTION SCOUT or the integrated Setup & Commissioning inside HMI, with PROFIdrive telegrams and PROFIsafe slot allocation.
3. NCK machine data (MD) – channel/axis MDs (MD20050, MD20060, MD20070, MD30130, MD30350, MD31020, MD32000…MD32990, MD38000…MD38600) plus the drives-DB generated from the drive configuration.
4. PLC program – PLC 317-2DP that lives inside the NCU, with axis interface DB31/DB32 (and DBnn for further axes), FC18, FC19, FC24, FC25, FC26 axis-handling blocks, plus the safety logic (SPL) and HMI softkey / MCP wiring.

Skipping any one of these layers results in alarms such as 26017 (axis type not configured), 26001 (no hardware), 201410 (drive error), 201414 (telegram failure), or the famous 8080/8081 PLC stop. The remainder of this document walks through the procedure in the order a commissioning engineer should perform it on the shop floor.

2. Pre-Engineering Checks Before You Touch the Cabinet

Before any machine data is touched, collect the following information and place it in a function specification (Siemens calls this the “FBS” / Anlagen-Funktionsbeschreibung). Anything missing here will surface as a costly rework later.

Verify the NCU 572.5 firmware / NCK software release. The 840D sl line moved from NCK System 2.6 through 4.7 to 4.95 SP3, and the HMI from SW 4.5 / 4.7 / 4.8 / 4.91 / 4.95. Telegram and FB syntax differ; in particular, FB40001+ libraries (Easy Archive, Easy Extend) are only available on System 4.5 SP2 and above. Capture the existing cardano.cfg, active.cfg, and the full *.arc series-archive from CF card before any change — if you break the configuration you can always return to the as-delivered state.

3. Hardware Architecture of the NCU 572.5 Drive Line

The NCU 572.5 (6FC5371-0AA30-0AA0 or its variants) is delivered with a base power line and either an internal I/O (NCU on-board) or external I/O via NX10.3 / NX15.3. The drive bus is a differential PROFIBUS DP (MPI/DP-X122) running PROFIdrive V3 profile, clock 12 Mbaud typical, isolation 1.5 kV. Modern replacements use PROFINET IRT on X150 P1/P2 with CBE30-2 / CBE40-2 communication boards, switching telegram type 6 / 116 / 125 / 136 to PROFINET equivalents.

3.1 SINAMICS S120 line-up for the new axis

3.2 Wiring topology (logical)

For each new axis, the cabling is essentially:

1. Three-phase 400 V (or 480 V UL) feed into the Line Module.
2. DC link bus between Line Module and Motor Module(s) (24 V ribbon cable on top of the modules, internal).
3. Motor power cable (MOTION-CONNECT) from the Motor Module to the motor.
4. DRIVE-CLiQ chain: NCU → Line Module → Motor Module → Sensor Module (if used) → motor encoder.
5. PROFIBUS DP / PROFINET from the NCU X122 / X150 to each Motor Module's CU320-2 control unit (every second module shares a CU when double modules are used).
6. PROFIsafe channel to the F-DI / F-DO that releases the Safe Stop.

Torque and sizing formulas (single-phase and three-phase interpretations are shown because machine tool datasheets are sometimes ambiguous):

• Single-phase apparent power: kVA = V · I / 1000 (example: 230 V, 16 A → 3.68 kVA)
• Three-phase apparent power: kVA = √3 · V_LL · I_line / 1000 (example: 400 V, 30 A → 20.78 kVA)
• DC link capacitor pre-charge energy: E = 0.5 · C · U_dc² (verify against the line module datasheet for the kJ limit)

If the existing line module is at 80 % nominal current, an additional axis must trigger a line module upgrade. A 22 kW spindle typically needs a 36 kW ALM (rule of thumb: sum of all motor module rated currents × 1.0, then derate per site altitude / ambient).

4. Drive Bus Configuration (PROFIdrive Telegrams)

Two parallel addressing schemes exist on the 840D sl:

• Logical drive numbers (1…31) used in NCK MD (MD30110, MD30120, MD30130, MD30220). These are the axis-to-drive mapping.
• PROFIBUS slave addresses / PROFINET device names (set on the CU320-2 DIP switches or via STARTER). These are the physical addresses.

The mapping is stored in the SDB container generated by SIMATIC Manager / TIA Portal when you insert the drives from the “SINAMICS S120 CU320-2 DP/PN” GSD file. Always use the most current GSD: GSDML-V2.31-Siemens-Sinamics-S120-20191025.xml or newer. Outdated GSD files are the number one source of 201414 (telegram configuration error) on retrofits.

4.1 Telegram choice for the new axis

For a standard feed axis on the 840D sl, leave the SINAMICS S120 standard telegram 6 assigned. Spindles use telegram 136 or 116 with the second encoder (spindle encoder) bit set in MD30220.

5. NCK Machine Data Configuration

All NCK MD changes are performed through the HMI → Startup → Machine Data or by loading a .ini from the CF card. Once you save with NCK reset, the new values are persistent. Below is a representative configuration for a 7th axis (call it AX7) on a 6-axis lathe or mill.

5.1 Global channel/axis options

; Global: declare the new axis as machine axis number 7
$MN_AXCONF_TOTAL = 7          ; was 6
$MN_CHAN_AXCONF = [ ... , 7 ] ; add 7 to the channel

5.2 Axis channel assignment

; Channel 1 gets the new axis
$MC_AXCONF_CHANAX_NAME_TAB[6] = "AX7"
$MC_AXCONF_CHANAX_DEFAULT_NAME[6] = "AX7"
$MC_GEOAX_ASSIGN_TAB = ( 1, 2, 3 )   ; unchanged, AX7 is auxiliary
$MC_AX_USAGE_ATTRIB[6] = 0x0001      ; 0x0001 = auxiliary axis

5.3 Axis machine data (machine axis AX7 = index 6)

; ----- Identification -----
$MA_AXCONF_NAME_TAB[6]           = "AX7"
$MA_AXCONF_LOGIC_NR[6]           = 7

; ----- Axis type -----
$MA_AXIS_TYPE[6]                 = 1     ; 1 = linear, 2 = rotary, 3 = spindle

; ----- Drive assignment -----
$MA_DRIVE_AX_RATIO_NUMERA[6]     = 1
$MA_DRIVE_AX_RATIO_DENOM[6]      = 1
$MA_DRIVE_AX_RATIO[6]            = 1.0

$MN_DRIVE_AX_USED[6]             = 7     ; logical drive number
$MN_DRIVE_LOGIC_NR[6]            = 7
$MA_CTRLOUT_MODULE_NR[6]         = 1     ; setpoint slot
$MA_CTRLOUT_TYPE[6]              = 1     ; PROFIdrive standard telegram
$MA_CTRLOUT_LOG_ADDR[6]          = 4100  ; depends on PLC; offset base + (drive-1)*16

$MA_ENC1_MODULE_NR[6]            = 2     ; actual value slot
$MA_ENC1_TYPE[6]                  = 1     ; incremental sin/cos
$MA_ENC1_LOG_ADDR[6]              = 4108

; ----- Mechanical data -----
$MA_LEADSCREW_PITCH[6]           = 10.0  ; mm/rev
$MA_AXIS_DENOM[6]                = 10.0
$MA_AXIS_NUMER[6]                = 1.0

; ----- Encoder scaling -----
$MA_ENC_SEGMENT_NR[6]            = 1
$MA_ENC_RESOL[6]                  = 2048  ; lines per rev on motor
$MA_ENC_ABS_TURN_ENABLE[6]        = 0

; ----- Speed / acceleration limits -----
$MA_MAX_AX_VELO[6]               = 5000.0  ; mm/min
$MA_MAX_AX_ACCEL[6]               = 1.0     ; m/s²
$MA_MAX_AX_JERK[6]                = 10.0    ; m/s³

; ----- Software limits -----
$MA_POS_LIMIT_MINUS[6]            = -500.0
$MA_POS_LIMIT_PLUS[6]             = 1000.0

; ----- Reference / homing -----
$MA_REFP_CAM_DIR_IS_MINUS[6]      = 1
$MA_REFP_VELO_POS[6]              = 500.0
$MA_REFP_VELO_SEARCH_CAM[6]       = 2000.0
$MA_REFP_MOVE_DIST[6]             = 2.0
$MA_REFP_SEQ_ENA[6]               = 1

; ----- Monitoring -----
$MA_POSCTRL_GAIN[6]               = 16.667  ; Kv in (m/min)/mm
$MA_STANDSTILL_POS_TOL[6]         = 0.2     ; mm
$MA_STANDSTILL_VELO_TOL[6]        = 1.0     ; mm/min
$MA_ABSOFF_ENABLE[6]              = 1       ; absolute encoder handling

; ----- Compensation (if needed) -----
$MA_SSC_IS_ENABLE[6]              = 0       ; friction comp, axis-specific
$MA_CEC_ENABLE[6]                 = 0       ; sag/angularity comp
$MA_QEC_ENABLE[6]                 = 0       ; quadrant error comp

5.4 Drive-DB (DO) regeneration

Every PROFIdrive axis slot requires a DO in the drive-DB. The SDB is regenerated automatically when the GSD is re-loaded into STEP 7 / TIA Portal. The DO entries are referenced by:

• MD30110 $MA_CTRLOUT_MODULE_NR
• MD30120 $MA_CTRLOUT_TYPE
• MD30130 $MA_CTRLOUT_LOG_ADDR
• MD30220 $MA_ENC_MODULE_NR / $MA_ENC_TYPE / $MA_ENC_LOG_ADDR

If the DO does not match, the controller raises alarm 201414 <axis> drive 007 “telegram mismatch” or 26017 "axis 7 has no drive assignment" on the next NCK reset.

6. PLC Program Modifications (PLC 317-2DP Inside the NCU)

Adding an axis in the NCK alone is insufficient. The integrated PLC must know the axis exists, must release the controller enable, must process the NCK->PLC interface, and must signal the safety chain. The standard 840D toolbox library is included with the “Toolbox 840D sl” archive from Siemens and provides the following FBs / DBs.

6.1 Axis interface DBs (DB31, DB32, …)

Each axis is exposed to the PLC in a fixed DB number:

Adding axis 7 means creating an instance in DB37 (axis 1=DB31, axis 2=DB32, … axis 7=DB37). The instance data block is automatically generated by the toolbox FB15/FB16/FC18 chain. After the symbol table is updated, the “NCK Interface” view in the toolbox editor fills in the offsets.

6.2 Calling the axis handler FCs

CALL  FC  18   // Axis enabling / reset / cancel
      Axis      := AX7_IDB                    // DB37
      Enable    := I_StartEnable_AX7
      Reset     := I_Reset_AX7
      Error     := O_FAULT_AX7
      Ready     := O_READY_AX7

CALL  FC  19   // Brake / clamp control
      Axis      := AX7_IDB
      BrakeOpen := O_BrakeOpen_AX7

CALL  FC  24    // Spindle / axis "ready" check
      Axis      := AX7_IDB

6.3 Softkey wiring to the HMI

If the new axis needs operator control (e.g. a manual jog menu), the MCP buttons must be re-mapped in FC 19/FC 25 of the toolbox. Add the new axis identifier in the HMI startup screen “Axis selection” configuration. The keys are mapped to DB10.DBB36..….

6.4 Safety logic (SPL)

Inside the safety program (SPL = Safe Programmable Logic, generated with the S7 Distributed Safety editor on TIA Portal or classic STEP 7 + F-library), add the new axis to the safety logic. Minimum requirement: a separate PROFIsafe slot for the new drive’s STO. Typical wiring:

// SPL block for AX7 STO
F-Channel  := SLP_Input.ProfisafeCh_AX7_StO;
F-Output   := SLP_Output.DriveStop_AX7;
F-Ton      := 200ms;     // STO delay (SS1 time)
F-StopCat  := 1;         // SS1, controlled stop -> STO

After editing, regenerate the safety signature and log the new CRC in the safety acceptance test report per EN ISO 13849-1 / EN 62061.

7. Mechanical Integration, Encoder & Brake Wiring

1. Encoder: Mount according to the Festo / Heidenhain installation drawing. Use a torque arm or coupling; a 0.05 mm offset on a sin/cos encoder can introduce 50 µm periodic error.
2. Brake: Connect the 24 V holding brake through the Motor Module's X5 brake connector. Verify the polarity (brown = +, blue = −) and add a flyback diode (1N4007) externally if the motor does not integrate one.
3. Limit switches: Hard limits in series into the F-DI module. Software limits in MD36100 / MD36110 as a second barrier.
4. Drive end stop force / torque clamp: Do not rely on MD for the mechanical safety; always include a hard stop at ±10 mm beyond the software limit.

8. HMI & Operator Panel Configuration

• Add the new axis to the Axis Monitor and Service Axes screens in easyscreen or in the SINUMERIK Operate configuration menu (Setup → Axis List).
• Update the Tool List / magazine if the new axis is a tool-changing axis.
• Re-export the alarm and message texts if the new axis has any axis-specific alarms (the standard 840D sl already has them at 300000+ range).

9. Commissioning Procedure (Step-by-Step)

1. Back up the CF card image and the PLC project (full archive via the HMI → Commissioning → Series start-up). Save the binary separately.
2. Mechanical fit & electrical wiring with the cabinet off; verify the PE/ground and the 24 V aux supplies.
3. Power up Line Module only (no motor module). Check 600 V DC link voltage within 60 s; an SLM/BLM typically needs 5…20 s for pre-charge.
4. Connect the DRIVE-CLiQ chain and run Commissioning → Drive system → Automatic configuration in STARTER. Confirm the topology.
5. Configure telegram 6 in STARTER for the new drive; download and save the project in the drive.
6. NCK MD changes as per Section 5. Save with NCK Reset.
7. PLC program changes as per Section 6. Compile, download, perform a warm restart (OB100) to load the axis DB instances.
8. Run the axis without drive enable by setting DB31.DBX4.7 = 0 (pulse enable) and DB31.DBX4.6 = 0 (controller enable). Verify that NCK reports the axis in the “available axes” list.
9. Enable the controller and jog the axis in JOG / INC mode with low override (5 %).
10. Run the reference-point homing sequence; verify DB31.DBX1.4 = 1 (referenced) afterwards.
11. Run a G-code test program (e.g. G01 AX7=100 F500) under feed hold and single block. Cross-check position, following error and Kv.
12. Sign off the safety acceptance test per the safety matrix.

10. Verification Checklist

11. Common Faults & Diagnostic Map

12. Documentation & Re-Audit Trail

• Save the final series start-up archive on a network share. The .arc file contains NCK, PLC, drive and HMI project.
• Print the safety acceptance test report and the modified machine data list and sign them off.
• Update the CE technical file: Schmersal / SICK / Pilz safety architect, the new axis block diagram and the operating manual.
• Train operators on the new axis functionality, including new error messages and softkeys.

FAQ