Calling Sinumerik 840D sl Subprograms During Axis Motion

1. Problem Overview

On a Sinumerik 840D sl controller running Operate v4.5 SP4, the engineer wants to launch a subprogram the instant a digital input becomes true while an axis is still traversing. The block from the original case looks like this:

WHEN ($A_IN[32]==1) AND ($AA_IM[Y1]>100) DO M80
G0 G90 Y1=400

The synchronous action WHEN ... DO fires while G0 is still moving Y1 toward 400 mm, so the auxiliary function must be interpreted as a subprogram call rather than as a hard-coded M word. A direct swap to CALC_Y1.spf in the action is rejected by the parser because synchronized actions in the NCK only accept a fixed set of keywords (axis names, M/S/T/H functions, GUD writes, and SET/CLR/DEF instructions). The subprogram name is not in that set.

This article documents the only sanctioned path on the 840D sl for that pattern: an Asynchrones Unterprogramm (ASUB / ASUP) bound to an M function via machine data, with the synchronous action calling the M word.

2. ASUB vs ASUP: Nomenclature

Siemens documentation is inconsistent across revisions. Use the mapping below to translate between German and English sources when reading manuals side by side:

For the 840D sl with Operate v4.5 SP4 the active option code in the license file is the English string ASUB, and the licensed option is billed under catalog number 6FC5800-0AS00-0YB0 (always verify against the active license on the CF card under /card/sinumerik/license).

3. Synchronized Action (WHEN…DO) Primer

Synchronized actions are real-time condition/action pairs evaluated every IPO cycle. On the 840D sl the relevant syntax is:

WHEN <condition> DO <action>
WHEN ... DO M<n>     ; M function call (valid)
WHEN ... DO CALC_Y1   ; subprogram call (NOT valid in the action)

The parser accepts only the following tokens in the action of WHEN ... DO on a 840D sl NCU 710/720/730/732:

• Axis names with motion (e.g. Y1=420) — only with implicit/quick couplings
• M, S, T, H, D, F auxiliary functions
• Assignments to GUD, LUD, R parameters and system variables (SET, CLR)
• Pre-defined DEF instructions (DEF INT _n=5)

This is why a direct CALC_Y1 in the action line returns alarm 20150 "Action not permissible in synchronized action" or, depending on firmware, 20160 "Syntax error in synchronized action". The substitution must therefore happen at the M-function layer using machine data.

4. M Function Substitution via MD 10715 and MD 10716

Siemens provides two related machine data items that bind an M number to a cycle/subprogram name:

The original engineer observed that using MD 10715/10716 inside a WHEN ... DO produced an alarm because the default of MD 10717 expects the substituted M to be programmed in a part-program block — the substitution path was not designed for synchronized actions.

5. The Asynchronous Subprogram (ASUB) Solution

The canonical 840D sl pattern for "launch a subprogram mid-motion" is to start an ASUB. There are three trigger sources:

1. SETINT(n) inside the part program — interrupt-driven ASUB bound to a digital/NCK input.
2. PLC request — FB9 / FB10 (PI service _N_ASUP) called from the PLC program.
3. Synchronized action with M function — the M is sent to the PLC, which raises the ASUB via FB9/FB10.

For the case in the source (a synchronized action watching a digital input), the M-trigger path is the cleanest because it keeps the NCK logic identical to a normal M-function call and pushes only the asynchrony into the PLC, where it can be diagnosed with the standard cross-reference.

6. Required Options and Configuration

Confirm the following before commissioning:

If MD 11640 bit 0 is 0, every ASUB start request from FB9/FB10 is rejected with alarm 4250 "ASUB not enabled". Set it to 1 and NCK reset.

7. Complete Working Example

Assume the engineer wants to run a calculation named CALC_Y1.SPF on channel 1 the moment input 32 rises AND Y1 has passed 100 mm. The axis Y1 must not stop.

7.1 Part program _N_MPF_DIR/_N_MAIN_MPF

; --- Main program ---
N10 G0 G90 Y1=0
N20 WHEN $A_IN[32]==1 AND $AA_IM[Y1]>100 DO M80
N30 G0 G90 Y1=400 F5000
N40 M30

7.2 Cycle directory setup

; CALC_Y1.SPF must live in the manufacturer cycle directory
; /card/sinumerik/cma or /card/sinumerik/oem or /card/sinumerik/user
; so that the NCK can resolve it without an explicit path
;
; --- File: _N_CALC_Y1_SPF ---
;$PATH=/_N_CMA_DIR
PROC CALC_Y1 SAVE
  ; axis remains moving; do NOT use G04 or M00
  R10 = $AA_IM[Y1]
  R11 = $AA_VACTM[Y1]
  ; .. user logic here ..
  ; e.g. compute a feed-override adjustment
  IF R11 > 8000
    $AC_OVR = 80      ; soft cap feed override at 80 %
  ENDIF
  RET

7.3 PLC side: trigger ASUB on M80

// SCL fragment - cyclic OB1
// M80 is the in-band marker; the ASUB name is "CALC_Y1"

IF "M_Fct_M80_Ack" AND NOT "M_Fct_M80_Prev" THEN
    "ASUP_DB".INTNO      := 1;            // interrupt slot
    "ASUP_DB".PRIO       := 100;          // higher = more urgent
    "ASUP_DB".LIFTFAST   := FALSE;
    "ASUP_DB".SET_MASK   := 'B0000';
    "ASUP_DB".CLEAR_MASK := 'B0000';
    "ASUP_DB".TYPE       := 'B0010';      // ASUP_START = 1, with PRIO
    "ASUP_DB".PATH       := '/_N_CMA_DIR/_N_CALC_Y1_SPF';
    "ASUP_DB".ADDR       := 'ASUP';
    FB10_DB(REQ := TRUE, INTNO := 1,
            LADDR := "ASUP_DB".LADDR,
            DB_No := "ASUP_DB".DB_NO);
    "ASUP_Triggered" := TRUE;
END_IF;

"M_Fct_M80_Prev" := "M_Fct_M80_Ack";

When M80 is recognized in the synchronous action, the NCK raises the M-output and sets M_Fct_M80_Ack. The PLC opens a window with FB10, which calls the PI service _N_ASUP. The NCK interrupts the current IPO cycle, stores all machine state, and starts CALC_Y1.SPF in a higher priority level. The original G0 Y1=400 continues to interpolate during the ASUB unless the ASUB itself issues a stop. By default the axes do not stop, which is what the engineer needs.

8. PLC-Based Calculation Alternative

If the calculation in CALC_Y1.SPF is a handful of arithmetic operations on real values, Siemens advises moving the math into the PLC. Reasons:

• No ASUB license required.
• No NCK state save/restore (the ASUB mechanism snapshots the interpreter).
• No PLC/NCK round-trip latency for each function call.
• Diagnostic visibility through STEP 7 / TIA Portal cross-reference.

Use the fast NCK interface (F-NCK) for high-rate values, or read $AA_IM, $AA_VACTM from the PLC once per IPO2 cycle (default 4 ms). A typical data path is:

// PLC reads current Y1 actual position
"Y1_ActualPos" := "NCK_Var".read("$AA_IM[Y1]", channel := 1, axis := 5);
IF "DI_32" AND ("Y1_ActualPos" > 100.0) THEN
   "Y1_FeedOverride" := LIMIT("Y1_ActualPos", 0, 120);  // PLC-side logic
END_IF;

9. Second-Channel Alternative

The second recommendation in the field report is to put the calculation in a second NC channel. On 840D sl a single NCU 710/720/730/732 can run up to 10 channels; channel 2 can be configured as a pure computation channel with no axes:

Channel 1 writes the request via $NC variables that channel 2 watches. This isolates the calculation latency from the motion channel completely. It is heavier than a single ASUB but provides the cleanest separation of concerns for repeated, complex evaluations.

10. Alarm Diagnosis and Verification

Common faults and the corrective action when commissioning the pattern above:

Verification procedure after writing the part program and PLC code:

1. Power on the controller, perform NCK reset, and confirm the Channel Status page shows ASUB=READY.
2. Select the part program in Operate > Machining and start it in single-block mode.
3. Raise input 32 by hand, watch the Diagnostics > Alarms page — no alarm should appear.
4. Watch $AC_ASUP in the Service > Display System Data screen. The variable should read the name of the running ASUB while the calculation is active.
5. Confirm that the Y1 actual position continues to advance in the Axis display during the ASUB. If it stalls, the ASUB has issued a stop (likely M0, G04, or an axis-coupled command); remove the offending statement from CALC_Y1.SPF.

11. Field-Proven Caveats

• ASUBs share interpreter state with the calling part program. If CALC_Y1.SPF leaves a modal G code active (for example G91), the calling part program will see it after RET unless the SAVE attribute is used and the modal group mask is handled.
• MD 10717 bit 1 forces a stop before the substituted subprogram. Leave it at 0 to keep the axis in motion.
• Multiple ASUB triggers from the same synchronized action queue up; the NCK executes them in priority order (PRIO in FB10). Do not rely on strict FIFO.
• On a NCU 710 the maximum simultaneous ASUB depth is 2; on NCU 720/730/732 it is 8. Exceed it and alarm 14760 "Channel %1 ASUB queue overflow" is raised.
• The synchronized-action evaluation period is set in MD 10050 $MN_SYSCLOCK_CYCLE_TIME and MD 10060 $MN_POSCTRL_SYSCLOCK_CYCLE_TIME. Polling $A_IN[32] at IPO2 means up to 4 ms reaction latency, which is fine for feed-override adjustments but not for safety stops. For safety, use SETINT instead of WHEN ... DO.

12. Cross-Reference to Other Paths

For the original case (single condition, simple calculation) the PLC-side alternative is usually the right answer; the ASUB is justified only when the calculation is too complex to live in the PLC or when it needs GUDs/LUDs from the NCK context.

13. Summary

A WHEN ... DO synchronized action on Sinumerik 840D sl can only call M, S, T, H, D, F functions or write variables; it cannot call a subprogram by name. The accepted path is to use DO M80, let the M80 be output to the PLC, and have the PLC start an ASUB via FB10. The ASUB requires license 6FC5800-0AS00-0YB0, MD 11640 bit 0 = 1, and the cycle file CALC_Y1.SPF placed in /_N_CMA_DIR or /_N_CST_DIR. With MD 10717 bit 1 = 0 the axis motion continues uninterrupted. For many real-world cases the engineer can drop the ASUB entirely and run the math in the PLC, accepting a small additional latency in exchange for zero new license cost and better diagnostic visibility.