Triggering SINUMERIK 840D Servo Trace via Part Program

Overview

The SINUMERIK 840D and 840D sl controllers expose a high-speed Servo Trace function used to capture NC setpoint/actual-value signals, drive signals, and PLC variables during a defined motion event. The trace is normally armed and started from the HMI (Diagnostics > Trace for HMI Advanced, or Diagnostics > Trace / Servo Trace under SINUMERIK Operate). For automated commissioning, acceptance tests, or repeatable diagnostic routines, the operator typically wants the trace to start sampling exactly when the part program reaches a known motion block — not when the user manually clicks the start button.

Siemens provides two write-only system variables that allow the part program to fire the trace trigger:

• $AA_SCTRACE[Ax] — axis-specific software trigger for traces configured in HMI Advanced (840D / 840D sl classic HMI).
• $AN_SLTRACE — axis-independent global software trigger for traces configured in SINUMERIK Operate on 840D sl.

This reference documents the complete procedure for configuring a servo trace that fires from a part program: variable selection, trigger-condition setup, the part-program syntax, the mandatory manual arming step, and the limitations that apply on both HMI generations. The procedure is the one described in the Siemens support documentation SINUMERIK 840D sl SINUMERIK Operate — Support ID 109769186.

Prerequisites

Before writing the part-program trigger, confirm the following on the controller and the HMI:

1. Controller generation: Identify whether the machine runs the legacy 840D with HMI Advanced, or the 840D sl with SINUMERIK Operate. The trigger variable differs between the two.
2. Operator access level: Access to Diagnostics > Trace requires manufacturer or service password on most installations. Coordinate with the machine OEM if the menu is greyed out.
3. Trace channel licence: At least one trace channel must be available. The default configuration provides two servo-trace channels; additional channels may require licence options depending on the software version.
4. Axis identification: Determine the machine axis name (e.g. X1, Y2, SP1) of every axis you intend to trace. The $AA_SCTRACE trigger is indexed by machine axis, not by channel axis name.
5. NCK access rights: Writing to $AA_SCTRACE / $AN_SLTRACE from the part program requires standard NCK write access. Protected programs (e.g. _N_MPF_DIR/_N_CMA_DIR) may need to be unlocked first.
6. Trace storage path: Confirm that the CF card / NCK file system has space for the trace buffer. Long recordings at high sampling rates (31.25 µs) consume several MB per channel.

Trace System Variables — Reference Table

Step-by-Step: Configure the Trace in the HMI

1. Open the Trace dialog

From the HMI main menu:

• HMI Advanced: Diagnostics > Trace > Servo Trace (or Commissioning > Trace on older 840D software).
• SINUMERIK Operate: Menu Select > Diagnostics > Trace > Servo Trace.

The trace dialog displays the Trace Settings (PLC, NC, Servo) window where start condition, stop condition, recording duration, sampling time, and channel assignments are configured. Reference: SINUMERIK 840D sl SINUMERIK Operate — Support ID 109769186.

2. Add at least one trace variable

Click Add variable and select the signal you want to capture. Typical choices during commissioning include:

• Position setpoint of axis X1
• Position actual value of axis X1
• Velocity setpoint of axis SP1 (spindle)
• Torque-limiting current setpoint of axis Z1

Each additional Add variable click adds another channel. The first variable defines the axis the trigger is bound to (see warning above).

3. Configure the trigger condition

In the Trigger condition section, set:

If you leave the trigger mode on Threshold or Edge, the part-program command has no effect. The trace will still run, but it will start on the signal threshold, not on the NC command.

4. Arm the trace

Press the HMI Start button. The dialog line should change to ...waiting for trigger (HMI Advanced) or Waiting for trigger (SINUMERIK Operate). At this point the trace is armed but idle.

Part-Program Syntax — HMI Advanced

For traces armed in HMI Advanced, the part program must set the axis-specific trigger variable before the motion block you want to record:

; =========================================================
; Sample: trigger servo trace on X1 from HMI Advanced
; =========================================================
G90 G94 G500                       ; absolute, feed/min, modal reset

; --- arm the trace trigger on machine axis X1 ----------
$AA_SCTRACE[X1] = 1                ; (or TRUE)

; --- force the NC to accept the trigger immediately ----
NEWCONF                            ; re-evaluate settable frames/commands

; --- start of the motion to be recorded ----------------
G90 G00 X=STARTPOS                 ; rapid to the start position
G01 X=ENDPOS F1000                 ; linear motion — the trace fires here
...

The write to $AA_SCTRACE[X1] is processed in the IPO cycle. Placing the assignment on its own line above the motion block ensures the trigger is latched before the geometry command starts executing. Without the isolated line, the controller may collapse the assignment with the next motion block and miss the latching window.

Variant: writing TRUE

The numeric value 1 and the boolean literal TRUE are equivalent. Use whichever is consistent with the rest of the part program. Some legacy 840D controllers (NC software earlier than SW 5) only accept the integer form — verify on the target build.

Part-Program Syntax — SINUMERIK Operate

On 840D sl running SINUMERIK Operate, the trace trigger is global. Replace the axis-indexed command with the scalar variable:

; =========================================================
; Sample: trigger servo trace from SINUMERIK Operate
; =========================================================
G90 G94 G500

$AN_SLTRACE = 1                    ; global NC trigger, axis-independent

NEWCONF

G90 G00 X=STARTPOS
G01 X=ENDPOS F1000
...

If the same part program must run on both HMI Advanced and Operate installations (e.g. a portable commissioning routine), keep both lines and comment out the one that does not apply:

$AA_SCTRACE[X1] = 1                ; trigger for trace at HMI Advanced
;$AN_SLTRACE = 1                   ; trigger for trace at SINUMERIK Operate
NEWCONF

Verification Procedure

After running the part program with the trace armed, confirm the recording was captured:

1. The HMI dialog line returns from Waiting for trigger to Trace finished (or Recording while the buffer is being read back).
2. The trace graph populates with at least one full record. If the buffer is empty, the trigger did not fire — jump to the troubleshooting matrix below.
3. Inspect the recorded signals against the known motion. The position setpoint should match the programmed path within the servo lag tolerance; large discrepancies indicate a mechanical, drive, or parameterisation issue.
4. Save the trace (File > Save) with a meaningful name (e.g. X1_rapid_2024-05-22.trc) so the capture can be referenced in the commissioning report.
5. If a second recording is needed, press Start again and re-run the program.

Limitations and Workarounds

Troubleshooting Matrix

Performance and Timing Notes

• IPO latching latency: The trigger is evaluated once per IPO cycle (default 4 ms; can be 2 ms on high-performance machines). Place the assignment at least one IPO cycle before the motion block you want to record.
• Sampling time alignment: Trace sampling times of 0.5 ms, 1 ms, 2 ms, 4 ms, 8 ms, 16 ms, 32 ms, 64 ms, 128 ms, 256 ms, 512 ms, 1024 ms are exposed in the dialog. For a 4 ms IPO the shortest sensible sampling is 1 ms; anything below the IPO clock will produce interpolated data.
• Record length: 4096 samples at 1 ms gives 4.096 s of capture — adequate for a single linear move with settling. Increase to 8192 samples (8.192 s) when capturing spindle acceleration or interpolating-axis settling tails.
• Multiple triggers in one program: If you need to capture two distinct motion segments in a single NC-Start, you must re-arm the trace between segments. The trigger command can be issued twice, but only the first firing will produce a record; subsequent firings are ignored until the trace is restarted from the HMI.

Related System Variables for Diagnostics

These variables can be displayed in the same trace session, allowing the engineer to correlate "what the part program did" with "what the axes actually did" in a single recording.

Integration with Commissioning Workflows

For repeatable acceptance tests, embed the trigger in a dedicated commissioning program that loops over the test points:

; =========================================================
; Commissioning loop: trace each of three test positions
; Requires the operator to press "Start" on the HMI
; before each NC-Start.
; =========================================================
G90 G94 G500

DEF INT _IDX
FOR _IDX = 0 TO 2

  ; --- operator arms the trace here ----------------------
  ; MSG "Arm trace, then press NC-Start"
  ; wait for operator confirmation if your HMI supports it

  $AA_SCTRACE[X1] = 1
  NEWCONF

  G90 G00 X=10 + (_IDX * 50)
  G01 X=20 + (_IDX * 50) F2000
  G04 F0.5

  ; --- second iteration: re-arm required ----------------
  ; MSG "Trace finished. Re-arm and press NC-Start"

ENDFOR
M30

FAQ