How to Disable an Axis in Siemens SINUMERIK 840D: MD and PLC Methods

Disable a rotary or linear axis on a Siemens SINUMERIK 840D / 840D sl controller without mechanically removing the motor. This technical reference covers the machine-data (MD) approach, the PLC encoder-park interface, the reference-point cycle suppression, and the required drive-pulse safety steps that prevent motion when the axis is logically removed from the part program and from the referencing sequence. The most common case is a C axis (or any 360° rotary table) that is taken out of service temporarily while the rest of the machine is still in production.

Problem Definition

On a SINUMERIK 840D, every axis that the NCK knows about must report a referenced position before the part program is allowed to start. When an operator needs to skip the C axis to run an X/Y/Z-only job, the controller still raises alarms because the missing axis is in the active channel configuration. Typical alarm lines seen at NC start are:

• 27000 "Axis %1 not referenced"
• 27001 "Axis %1 no measurement system"
• 21612 "Channel %1 axis %2 traverse enable withdrawn"
• 26100 "Channel %1 axis %2 not available"
• 8080 "Channel %1: program processing resumed after block search"

The root cause is not a hardware fault — it is the controller's expectation that the axis is still mechanically present. The correct fix is to tell the NCK that the axis is no longer part of the kinematic chain, while the drive is independently commanded to remove its torque-producing capability. The controller does not disconnect the motor for you; that is the PLC's job.

Prerequisites

Before touching machine data, confirm the following on a 840D sl installation:

• Operator access to the SINUMERIK Operate HMI with the manufacturer or service password (password level 1 for machine data read, level 2 for write).
• PLC access to the STEP 7 / TIA Portal project that owns the axis interface DBs (DB31 through DB3n, one per axis).
• Knowledge of the axis number n for the C axis on this channel (typically 4 on a 3+1 configuration, but it varies with the project).
• Drive bus topology confirmed: a SINAMICS S120 line module + motor module combination is assumed; the procedure also works with 611U on older 840D (Powerline) hardware.
• The axis is in a safe state: no part loaded, no coolant pressure, no chips in the work envelope, and the spindle stopped.
• A backup of the project machine data and PLC program, with date and author logged, so the change can be reverted cleanly.

Axis Configuration Model in SINUMERIK 840D

The 840D stores axis-level behavior in machine data arrays indexed by axis number. The disable task touches three families of MDs and one PLC interface family:

Reference: SINUMERIK 840D sl List Manual (Parameter Manual) for the complete MD hierarchy.

Method 1 — Machine Data Permanent Disable

This is the cleanest approach when the C axis is removed for an extended period and an NCK power-on is acceptable. The procedure zeroes both the setpoint and encoder types of the axis and forces the NCK to skip referencing for that axis. Once the values are loaded and the NCK is restarted, the axis is invisible to the part program and to the referencing sequence.

MD 30130 — Setpoint output type

$MA_CTRLOUT_TYPE[n, 0] = 0
$MA_CTRLOUT_TYPE[n, 1] = 0

MD 30130 CTRLOUT_TYPE[n] defines how the speed setpoint is output to the drive. The valid values on 840D sl are:

Setting both channel-specific entries to 0 tells the NCK there is no drive to command. There are two indices because some installations mirror the setpoint over a second channel for redundancy; on a single-channel C axis the second index can usually be left at 0 if the original value was non-zero.

MD 30240 — Encoder type

$MA_ENC_TYPE[n, 0] = 0
$MA_ENC_TYPE[n, 1] = 0

MD 30240 ENC_TYPE[n] defines the encoder configuration. The same 0 = no encoder convention applies. With both entries zeroed, the NCK stops evaluating position feedback, which is what removes the "axis not referenced" condition downstream.

MD 34110 — Reference-point cycle

$MA_REFP_CYC_NR[n] = -1

MD 34110 REFP_CYC_NR selects which reference-point cycle runs when the axis is asked to reference. The value -1 means "no reference-point cycle" — the axis is treated as permanently referenced. This single MD is what closes the loop; without it, even the setpoint/encoder zeroing above will still leave the axis in an unreferenced state on a fresh NCK reset.

MD 14512 — Project user-data flag (optional)

MD 14512 [n] = 0x8 (changed from 0x0)

MD 14512 is an axis-specific user data field (USER_DATA_HEX). The bit value 0x8 is sometimes used as a project-side marker to remember that the axis is logically parked. This is not a Siemens-defined disable function; treat it as a project flag, document it in the machine data book, and do not rely on it for any safety or control behavior.

Loading the MDs

Changes to MD 30130, 30240, and 34110 require a full NCK power-on to take effect. On Operate, navigate to Commissioning → Machine Data → Axis MD, edit the three values, then trigger an NCK power-on. On Powerline hardware with HMI Advanced, the equivalent path is Start-up → Machine Data. After the power-on, the controller comes up with the C axis absent.

Method 2 — PLC Interface Encoder Parking

Use this method when the axis must be enabled and disabled dynamically by the PLC — for example, on a machine where the C axis is shared with a manual tailstock or a swiveling operator platform, or when the toggle frequency is high (multiple times per shift). The PLC controls a single bit in the axis interface; when set, the NCK "sleeps" the axis and reports it as unavailable until the bit is cleared.

Required PLC interface bits

where x is the axis index. Axis 1 lives in DB31, axis 2 in DB32, and so on. Reference: SINUMERIK 840D sl PLC Interface Description (NC Var Selector).

PLC logic (STEP 7 ladder)

// Title: Park encoder for C axis (axis 4, DB34)
// Author: Service engineering
// Date:   YYYY-MM-DD

NETWORK 1    // User request: park C axis
A     M      100.0     // Operator HMI button "Park C axis"
S     DB34.DBX1.3      // Set encoder parking

NETWORK 2    // Confirmation
A     DB34.DBX2.1      // Encoder parking active
S     M      100.1     // Parked flag latched

NETWORK 3    // Pulse enable chain (safety)
A     M      100.1     // Parked flag
AN    DB34.DBX21.7     // Drop pulse enable while parked
=     DB34.DBX21.7     // Pulse enable OFF → drive pulses removed

NETWORK 4    // Position controller enable
A     M      100.1
=     DB34.DBX21.0     // OFF2: position controller disabled

What happens inside the NCK

When DB3x.DBX1.3 is set, the NCK treats the encoder as logically disconnected: no position control, no speed control, no encoder evaluation. Any attempt by a part program to traverse the parked axis raises alarm 21612 ("traverse enable withdrawn") because the axis is sleeping, not because the drive is faulted. Clearing the bit brings the axis back into normal mode, and the operator must run a referencing pass before the axis can move under program control.

Method 3 — Reference-Point Cycle Suppression Only

If the only symptom is the "axis not referenced" alarm at program start and the part program does not call the C axis, the minimum-impact fix is to set MD 34110 to -1 and leave the setpoint and encoder types intact:

$MA_REFP_CYC_NR[n] = -1     ; No reference cycle — axis is permanently referenced

This change is reversible in seconds and, in most installations, does not require an NCK power-on — a normal NCK reset is sufficient. It is the correct tool when the axis is logically present but no motion is required in the current job. If the part program does call the axis (a C block, a SETMS spindle swap, or a CYCLE800 invocation), alarms return and you must escalate to Method 1 or Method 2.

Drive Safety: Pulse Disable

None of the three NC-side methods removes electrical energy from the motor. The drive power unit MUST be commanded to remove pulses so that no torque-producing current can flow. On a SINUMERIK 840D with a SINAMICS S120 drive line-up, the pulse-enable chain has three layers:

Reference: SINAMICS S120 Function Manual (FH1).

For an encoder-parked axis, the standard wiring rule is:

1. PLC sets DB3x.DBX1.3 = 1 to park the axis.
2. PLC waits for feedback DB3x.DBX2.1 = 1 (encoder parked confirmed).
3. PLC clears DB3x.DBX21.7 to remove the drive pulse enable for that Motor Module.
4. PLC drops DB3x.DBX21.0 for an additional position-controller disable.
5. The axis is now mechanically and electrically inactive, with the drive still in standby for re-activation.

For a method-1 MD-disabled axis, the PLC still holds responsibility for the drive enable chain, even though the NCK no longer commands motion. Treat the drive as live and supervise the pulse enable just as you would for an active axis.

Programmatic Detection — $MA_CTRLOUT_TYPE

Before a part program commands a parked or disabled axis, it should read the runtime machine data to decide whether the axis is alive. The standard NC variable is $MA_CTRLOUT_TYPE[axis, index]. A return value of 0 indicates no setpoint output. The two ways to use it are:

From an NCK variable read in the part program

; SINUMERIK 840D sl — part program guard
R10 = $MA_CTRLOUT_TYPE[C, 0]
IF R10 == 0 GOTOF NO_C_AXIS
G1 C=45.0 F500
NO_C_AXIS:
; continue without the C move

From the PLC as a readiness flag

; PLC function — expose parked state to HMI
L     DB34.DBD0        ; first 4 bytes of axis status word
L     W#16#0004        ; bit pattern corresponding to encoder parking
AW
T     MW 110           ; HMI tag "C_Axis_Parked"

Reference: SINUMERIK 840D sl Programming Manual — System Variables and SINUMERIK 840D sl Cycles Programming Manual for the higher-level use inside CYCLE800.

Commissioning Procedure (Method 1)

1. Verify the C axis is in a safe state (no part, no clamps engaged, no chips in the work envelope, spindle stopped).
2. On the HMI, log in with service-level password and navigate to Commissioning → Machine Data → Axis MD.
3. Set $MA_CTRLOUT_TYPE[AX_C, 0] = 0 and $MA_CTRLOUT_TYPE[AX_C, 1] = 0.
4. Set $MA_ENC_TYPE[AX_C, 0] = 0 and $MA_ENC_TYPE[AX_C, 1] = 0.
5. Set $MA_REFP_CYC_NR[AX_C] = -1.
6. (Optional) Set the project user-data flag MD 14512 [AX_C] = 0x8 and document it in the machine data book.
7. Save the machine data to the CFast / SSD card and the NCK backup file.
8. Trigger an NCK power-on through HMI Commissioning → NCK Power On, or by toggling the NCK power button on the NCU module.
9. In the PLC, add or enable the rung that drives DB3x.DBX1.3 = 1 for the parked axis and clears DB3x.DBX21.7.
10. Run a dry-run part program with no C-axis motion (block search to a known position, single block, M00 for a stop) to confirm the controller accepts the start without alarm 27000.
11. Log the change in the project machine data book: date, author, reason, MD values before and after, and the PLC commit revision.

Verification Checklist

Restoring the Axis

When the C axis is needed again, the procedure mirrors the disable:

1. Confirm the work envelope is clear and the axis is mechanically free (no collision).
2. Restore the original non-zero values for $MA_CTRLOUT_TYPE[AX_C, 0/1] and $MA_ENC_TYPE[AX_C, 0/1].
3. Restore $MA_REFP_CYC_NR[AX_C] to the project default (typically 0 or the dedicated cycle number for the rotary encoder).
4. Save the MDs and trigger an NCK power-on.
5. Clear the PLC park bit DB3x.DBX1.3 = 0 and re-establish the pulse enable DB3x.DBX21.7 = 1.
6. Run the standard referencing pass for the C axis from the HMI: Reference → C axis → Reference point 1.
7. Run a low-speed test move to confirm position control and encoder feedback.
8. Update the machine data book with the restore date and author.

Troubleshooting Matrix

Cycle 800 and CAM-Side Handling

If the parked C axis is referenced by the swivel cycle CYCLE800 — common on 5-axis machines where the post processor invokes CYCLE800 automatically whenever a 5-axis tool vector is needed — disabling the axis in the NCK is not enough. The CAM post processor must also stop emitting CYCLE800 calls, or the cycle's internal checks will raise a different alarm even when the MDs are correctly zeroed.

The standard Siemens fix on the controller side is the cycle user-data flag useMultiAxisFeatures (or its machine-data equivalent in the CYCLE800 user area). Setting it to FALSE causes CYCLE800 to skip the C-axis block when no 5-axis transformation is active. The CAM-side fix is to update the post processor to suppress the cycle for this specific machine configuration, or to add a guard in the post that reads the axis configuration and decides whether to emit CYCLE800 at all.

Reference: SINUMERIK 840D sl Cycles Programming Manual (CYCLE800 chapter).

Field-Proven Caveats

• Method 1 is invisible to the part program. If the post processor emits a C axis move, the alarm returned is "C axis does not exist" rather than "C axis is parked." This is the desired behavior because it surfaces the issue to the CAM operator at code-generation time.
• Method 2 keeps the axis visible in the channel but unreachable. Use it when the part program is trusted and you need fast toggling without an NCK power-on, for example on a single-shift work cell that swaps between C-axis and chuck-mode jobs.
• Method 3 is the surgical fix. It does not change any other behavior. The axis is referenced immediately at NCK reset, so no referencing alarm appears at program start. If the part program does call the axis, you will see alarm 21612 and you must escalate.
• All three methods require the drive pulse enable to be handled in the PLC; otherwise the parked axis can still receive torque from the drive on a software fault, which is a safety hazard that is invisible to the NCK alarms.
• On coupled-axis systems (e.g. TC, CAX), decouple the C axis from its master before parking. The NCK will not let a parked axis be a coupling follower; alarm 20085 is the visible symptom.
• On machines with a 611U drive line-up (older 840D Powerline), the PLC interface DB numbers are the same, but the drive parameters differ (p0845 becomes the pulse-enable parameter, not the S120 equivalent). Verify the drive manual before adapting the procedure.

Related Standards and Safety Architecture

The disable behavior interacts with functional safety (drive-integrated STO/SS1, SS2, SLS, SOS) governed by IEC 61800-5-2 (Adjustable speed electrical power drive systems — Safety requirements for drives). Disabling a non-safety axis through the NCK interface described above does not, on its own, satisfy the requirements of a safety-rated stop function. A STO input at the SINAMICS terminal strip is required for PL d / SIL 2 or higher, and the wiring must be validated against the safety category of the machine.

The general machinery safety context is the harmonized standard EN ISO 13849-1:2023 (Safety of machinery — Safety-related parts of control systems). Verify the safety architecture and the stop category (0, 1, or 2) against this standard before declaring the axis out of service for any safety-relevant application. Disabling an axis on the NCK side is an operational change, not a safety function; the two must not be conflated in the machine's safety case.