Outputting Sinumerik 840D Axis Torque via Profibus Analog Output

Overview

On a SINUMERIK 840D control, axis-internal drive quantities such as torque ($AA_TORQUE), current ($AA_CURR), power ($AA_POWER) and load ($AA_LOAD) can be made available to external hardware through a Profibus-mounted analog output module. Two engineering paths exist, both supported from the NCK side and both delivering a continuous, cycle-accurate signal during the entire machining cycle:

1. FAST DMP analog output with a synchronized action writing to the system variable $A_OUTA[n]. The signal path is NCK → DMP module → analog voltage, bypassing the PLC cycle.
2. FB2 PLC function from the SINUMERIK 840D PLC Toolbox basic program, which transfers the NC variable into the PLC via NCVar selector, after which the value is written to the analog output image.

Both methods require the load/torque option bit (6FC5251-0AB17-0AA0) to be enabled and axis machine data MD 36730 to be configured. This article documents the configuration of each path, the option set-up, the NCVar selector wiring, and field-proven troubleshooting steps for FB2 when the read data appears as zero or uninitialized on the PLC side.

Prerequisites

• SINUMERIK 840D (or 840D sl) with NCK software version that supports $AA_TORQUE / $AA_LOAD / $AA_CURR / $AA_POWER.
• Active software option 6FC5251-0AB17-0AA0 (axis load/torque reading option).
• Configured analog output hardware on Profibus (DP slave) or FAST DMP module wired to NCK.
• Configured Profibus network, GSD file imported into SINUMERIK hardware configuration.
• PLC Toolbox basic program loaded into the PLC (S7-300/400) – required for FB2 path.
• Axis machine data MD 36730 configured (see dedicated section below).

Method 1 — FAST DMP Analog Output with Synchronized Actions

The FAST DMP analog output module is mapped to the NCK-side system variable $A_OUTA[n]. Each index corresponds to a physical analog channel on the DMP hardware. The signal is driven directly by the NCK, so the latency between the underlying drive quantity and the analog voltage is one IPO cycle, irrespective of the PLC cycle.

Hardware Topology

Synchronized Action Syntax

Define a synchronized action in the part program or in a static synchronized-action block (e.g., DEF INT ... with SYNC) that writes the axis torque to the analog output every IPO cycle:

This keeps the analog channel 1 permanently bound to the X-axis torque throughout the machining cycle. To condition the output (e.g., scale the value, blank it during rapid, or invert it for a positive-going voltage):

For a windowing mask that disables the output when the X axis is not moving:

WHEN $AA_IM[X] <> 0 DO $A_OUTA[1] = $AA_TORQUE[X]
WHEN $AA_IM[X] == 0 DO $A_OUTA[1] = 0

Use $A_OUTA[1]..$A_OUTA[8] (typical) depending on the DMP module slot count. The index n in $A_OUTA[n] corresponds to the physical channel ordering on the DMP hardware as configured in the SINUMERIK hardware configuration.

Method 2 — FB2 PLC Function from PLC Toolbox

The PLC Toolbox basic program provides a set of function blocks (FB2/FB3/FB4/FB5) for bidirectional NC-variable access. FB2 reads a single NC variable from the NCK into the PLC using a pre-configured NCVar selector (typically DB120). The data lands in a PLC-side word/double-word where it can be scaled, monitored, and written to a Profibus analog output via standard S7 I/O image or an FB to a Profibus DP slave.

NCVar Selector DB Configuration

The NCVar selector DB (commonly DB120, but the index is configurable via PLC machine data) describes which NCK variable to read, its area code, axis index, and other parameters. Typical fields include:

FB2 Call Example

The call is typically issued from OB1 (or OB35 cyclic interrupt for a deterministic refresh):

CALL FB2, DB_NCVar (    // instance DB for FB2
    REQ     := TRUE,           // continuous read
    NR      := 1,              // refers to NCVar entry 1
    LADDR   := W#16#0,         // local NCK
    UNIT    := 1,              // X axis
    COLUMN  := 1,
    LINE    := 0,
    VALID   := #validX,        // output: data valid
    BUSY    := #busyX,         // output: read in progress
    ERROR   := #errX,          // output: error flag
    ERR_CODE:= #errCodeX,      // output: error code
    VALUE   := #torqueX        // output: real value
);

Once VALUE contains the torque, scale and write to the analog output via the Profibus DP slave's output words. Typical scaling: drive torque is reported in Nm referenced to the motor; convert to a ±10 V range using the maximum expected torque:

Send Vout_scaled to the analog output module's process data word (e.g., QW 256 on the PLC image, mapped to the analog Profibus channel).

Required Software Option: 6FC5251-0AB17-0AA0

The four axis-internal drive quantities are bundled into a single Siemens option:

Without this option, accessing $AA_TORQUE[X] returns zero or a constant placeholder, regardless of the synchronized action or FB2 configuration. Verify the option bit is set via the HMI under:

• Commissioning → NCK → Options (SINUMERIK Operate)
• Or in the option file (typically under /oem/sinumerik/ or via the HMI's installed options view).

Axis Machine Data MD 36730

The source documentation explicitly calls out that after enabling the option bit, axis machine data MD 36730 must also be configured. The exact semantic role of MD 36730 in the load/torque option enablement path must be verified against the SINUMERIK 840D machine-data manual for the specific NCK software version, because the meaning of MD numbers in the 36xxx range varies between NCK versions.

Verification steps:

1. In HMI Operate, navigate to Commissioning → Machine Data → Axis MD.
2. Locate MD 36730 and confirm the expected value (refer to the NCK-specific machine data list / DoConWeb / Siemens support portal for the build).
3. Save with the proper startup file (NCK reset / warm restart).

For definitive MD 36730 values, consult the SINUMERIK 840D machine data manual for the installed NCK version, available through the Siemens Industry Online Support portal.

Available Axis Variables

All four are accessed with the same option and the same calling convention. The choice between them is a measurement question — for a load-cell surrogate on a servo, torque is the most direct equivalent; for thermal stress, current is more meaningful.

Method Comparison: FAST DMP vs. FB2

Configuring the NCVar Selector for $AA_TORQUE

The NCVar selector DB must be edited before FB2 can be called. The procedure below assumes the PLC Toolbox basic program is loaded and the NCVar area is initialized in OB100 (cold restart):

1. Open the PLC project in STEP 7 (or TIA Portal if migrated).
2. Locate the NCVar selector DB — by convention DB120 — and open it.
3. Fill row 1 with the parameters for $AA_TORQUE[X] as documented in the section above (NR=1, LADDR=W#16#0, Area=B#16#20, Unit=1, Column=1, Line=0, VarName='AA_TORQUE[X]').
4. Compile and download the PLC program.
5. On NCK side, verify option 6FC5251-0AB17-0AA0 is active and MD 36730 is configured.
6. Trigger a PLC cold restart (OB100) to initialize the NCVar selector.
7. From a programmer-side watch table, force DB120.DBW0 = 1 (NR) to match the row index used by the FB2 call.
8. Monitor FB2 instance VALUE in the watch table — it should update to a non-zero value as soon as the axis moves.

Troubleshooting FB2 Returning No Useful Data

The original question reports that FB2 is implemented but no useful data appears on the PLC side. The following matrix covers the recurring failure modes seen in the field:

Diagnostic Procedure for an FB2 Stuck-at-Zero

1. Confirm the option: in HMI Operate, go to Commissioning → NCK → Options; verify 6FC5251-0AB17-0AA0 is active.
2. Open the PLC online watch table; force the NCVar selector row 1 (NR=1, LADDR=W#16#0, Area=B#16#20, Unit=1).
3. In the same watch table, force the FB2 inputs to match. Monitor VALID, BUSY, ERROR, ERR_CODE.
4. Manually jog the X axis and observe VALUE. If it stays 0 with VALID never asserting, the selector row index or area code is wrong.
5. Cross-reference the NC variable name in the SINUMERIK list of system variables — the spelling AA_TORQUE (capitalization irrelevant) must be the one supported by the installed NCK build.
6. For the PLC Toolbox basic program reference (FB1, FB2/3/4/5 and the NCVar selector), refer to the SINUMERIK 840D sl PLC Basic Program programming manual available on the Siemens Industry Online Support portal.

Verification Procedure

1. Option verification: HMI → Commissioning → NCK → Options → confirm 6FC5251-0AB17-0AA0 active.
2. MD verification: HMI → Commissioning → Machine Data → Axis MD → MD 36730 → confirm value matches NCK manual.
3. Synchronized action check: In the synchronized-action monitor, verify $A_OUTA[1] = $AA_TORQUE[X] is firing every IPO cycle.
4. FB2 check: Watch FB2.VALUE — must reflect instantaneous torque while X axis is in motion.
5. Analog output check: With a multimeter or scope on the Profibus analog-output terminals, confirm voltage scales with torque per the configured scaling.
6. Continuity check: Through a full machining cycle (rapid + feed + dwell + rapid), confirm the analog voltage follows the X-axis torque profile without dropouts or frozen values.

FAQ