SINUMERIK 840D sl: Auto-Reset Tool Sequence on Gantry Interrupt

When a cycle is interrupted on a SINUMERIK 840D sl controller mid-sequence (T1→T2→T3→T4) on a gantry-style machine, the default behavior is a REPOS-style resume from the last decoded block. Operators who need to discard the in-process workpiece and restart the sequence from a defined tool index require a deterministic ASUB-driven reset path. This reference covers the NC, PLC, and HMI Operate configuration that implements an auto-reset of the tool sequence and increments a reject counter, with explicit handling of the gantry axis pair so the reset cannot desynchronize the coupled axes.

1. Problem: Interrupt at T3, Resume vs. Reset

The reported scenario: a part-program runs T1 D1 → T2 D1 → T3 D1 → T4 D1 on a gantry machine. An NC STOP / alarm / safety stop fires while T3 is engaged. When the operator presses NC START again, the controller executes the configured REPOS mode (REPOSA, REPOSL, REPOSQ, or REPOSH) and re-runs the T3 work as rework on the same workpiece. The required behavior is to:

1. Treat the interrupted workpiece as scrap.
2. Restart the tool sequence from a defined index (typically T1) on a fresh part.
3. Suppress REPOS for this class of interrupt.

2. Root Cause: Why REPOS Resumes T3

The 840D sl kernel preserves decoded block context across NC STOP and stores the REPOS offset in $AC_REPOS_PATH_MODE and the axis positions in $AA_REPOS_DELAY. On NC START, the interpreter branches to the saved block and re-executes any motion and auxiliary function still pending. Because T"name" or T<n> is decoded as a tool selection request and M6 is the tool change, the interpreter sees T3 as still due; the REPOS path simply completes that block set.

The configured REPOS mode is read from machine data MD20150 $MC_GCODE_RESET_VALUES[2] for G-code group 2 (REPOS) at reset, and the active mode is stored in $AC_REPOS_PATH_MODE. The auto-reset path is implemented by setting MD20108 $MC_PROG_EVENT_MASK to call an ASUB on reset and a separate ASUB on NC START.

3. Auto-Reset Architecture

Three coordinated blocks deliver the behavior:

1. ASUB on RESET — declared via MD20108 bit pattern 'H14' (PROG_EVENT_RESET) and 'H81' (PROG_EVENT_START). The reset ASUB increments the reject counter and writes a GUD flag _GUD_REJECT_ACTIVE.
2. ASUB on START — reads _GUD_REJECT_ACTIVE, forces a tool sequence reset, and either (a) calls the main program from the first T block via INIT(2, "_N_MPF_DIR\\_N_MAIN_MPF") + START(2), or (b) skips the in-progress tool and queues the next tool through the magazine.
3. PLC interface — FC8 / standard tool manager handshake; user DB DB9000 holds REJECT_FLAG, SCRAP_COUNT, and REPOS_DISABLE; FC9 starts the ASUB from the PLC side if needed.

4. NC-Side: Asynchronous Subprogram for Tool Sequence Reset

Create two ASUB files in the part-program directory, e.g. _N_ASUP_RESET_SPF and _N_ASUP_START_SPF. Define the GUD in _N_GUD_DEF:

DEF NCK INT _GUD_REJECT_ACTIVE = 0   ; 1 = scrap current part, restart
DEF NCK INT _GUD_RESET_TOOL_IDX = 1  ; restart from this T index
DEF NCK INT _GUD_SCRAP_COUNT = 0     ; local mirror of PLC reject counter
DEF NCK INT _GUD_LASTMACH = 0        ; last decoded T before interrupt

_N_ASUP_RESET_SPF (called by PROG_EVENT_RESET):

; ASUP_RESET_SPF — called automatically on NC RESET
; Increments scrap count and records the last active tool index

DEF INT _LV_TIDX = 0

IF $AC_ALARM_STAT == 0
  ; operator-initiated reset, do not scrap
  GOTOF _END
ENDIF

; capture the tool that was active at the time of the stop
_LV_TIDX = $AC_TOOL_ACT[0]            ; current tool of channel 1
_GUD_LASTMACH = _LV_TIDX
_GUD_REJECT_ACTIVE = 1

; increment scrap counter (special parts used for reject log)
SETPIECE(3)                            ; $AC_TOTAL_PARTS++
$AC_SPECIAL_PARTS = $AC_SPECIAL_PARTS + 1
_GUD_SCRAP_COUNT = _GUD_SCRAP_COUNT + 1

; signal PLC to disable REPOS for the next START
DO $A_DBW[0] = 1                       ; user HMI signal: REJECT_REQ

_END:
M17

_N_ASUP_START_SPF (called by PROG_EVENT_START):

; ASUP_START_SPF — called automatically on NC START

IF _GUD_REJECT_ACTIVE == 0
  GOTOF _NORM_START
ENDIF

; clear REPOS offset and disable auto REPOS for this cycle
$AC_REPOS_PATH_MODE = 0                ; clear pending REPOS
$AC_REPOS_DELAY = 0                    ; no axis delay
$AA_REPOS_DELAY[X] = 0                 ; clear per-axis
$AA_REPOS_DELAY[Y] = 0

; unload current tool, queue tool index from GUD
T0                                      ; clear spindle tool
M6

_GUD_REJECT_ACTIVE = 0                 ; consume flag

; re-trigger the program from the start with the new tool index
INIT(2, "_N_MPF_DIR\\_N_MAIN_MPF", "/_N_MPF_DIR\\_N_MAIN_MPF")
START(2)

_NORM_START:
M17

Main program pattern that survives the reset:

; MAIN_MPF — tool sequence with reject-safe block structure
N10  G0 G54 X0 Y0 Z100
N20  T1 D1 M6
N30  S3000 M3
N40  WORK_BLOCK(1)                       ; part feature A
N50  T2 D1 M6
N60  WORK_BLOCK(2)                       ; part feature B
N70  T3 D1 M6
N80  WORK_BLOCK(3)                       ; part feature C  <-- interrupt point
N90  T4 D1 M6
N100 WORK_BLOCK(4)                       ; part feature D
N110 M30

5. PLC-Side: FC8 / Standard Tool Manager Interface

The standard tool manager on SINUMERIK 840D sl uses the following exchange blocks. User data for the reject logic is held in DB9000:

SCL example for the reject handshake (FB 9000 "REJECT_HND") stored in DB9000:

FUNCTION_BLOCK FB9000
VAR
    bReject : BOOL;             // DB9000.DBX0.0
    nScrap : DINT;              // DB9000.DBD4
    bRepDis : BOOL;             // DB9000.DBX8.0   "REPOS disable"
    nTime : TIME;
END_VAR

BEGIN
    // rising edge of PLC-side reject request from NCK
    IF "dbNcToPlc".reject_req AND NOT bReject THEN
        bReject := TRUE;
        nScrap := nScrap + 1;
        bRepDis := TRUE;
        // log to operator via HMI message line
        "HMI_MSG_BUFF".msg[0] := 1;
        "HMI_MSG_BUFF".id[0] := 4701;   // 4701 = reject event
        "HMI_MSG_BUFF".text[0] := 'SCRAP PART TOOL=';
        "HMI_MSG_BUFF".text[0] := CONCAT("HMI_MSG_BUFF".text[0],
                                          DWORD_TO_STRING("dbNcToPlc".last_tool_idx));
    END_IF;

    // clear after start
    IF "dbPlcToNck".channel_1_start_ack AND bReject THEN
        bReject := FALSE;
        bRepDis := FALSE;
    END_IF;
END_FUNCTION_BLOCK

The PLC drives the REPOS path via DB32000.DBX0.6 (REPOS offset active) and the ASUB trigger via DB32000.DBB4 (ASUB interface). For alarm-driven ASUBs, set MD11602 $MN_ASUP_START_MASK accordingly and MD20194 $MC_PROG_EVENT_RESTART_BITS to allow PROG_EVENT at NC STOP.

6. Rejecting the Workpiece: Counters and Marking

The Siemens part-counter system uses SETPIECE() and the $AC_*PARTS family. For reject accounting, the canonical approach is:

To mark a workpiece as reject:

; increment the NC-side scrap book-keeping
SETPIECE(3)                            ; $AC_TOTAL_PARTS++
$AC_SPECIAL_PARTS = $AC_SPECIAL_PARTS + 1
$AC_REQUIRED_PARTS = $AC_REQUIRED_PARTS - 0   ; not decremented

; user scrap counter is mirrored in PLC
SBC(64)                                ; set user bit, e.g. for HMI
; or write directly
DO $A_DBD[0] = $A_DBD[0] + 1          ; user-side reject counter

For traceability, write the tool index and timestamp to a user log DB on each reject:

DEF NCK INT _GUD_LAST_TIDX = 0
DEF NCK REAL _GUD_LAST_TIME = 0

_GUD_LAST_TIDX = $AC_TOOL_ACT[0]
_GUD_LAST_TIME = $AC_TIMER[0] / 1000.0   ; seconds
M17

7. Gantry Synchronization Across the Reset

A gantry axis pair (typically two mechanically-coupled drives, e.g. X1 = master, X2 = slave) is defined by:

• MD37100 $MA_GANTRY_AXIS_TYPE[n] — 0 = none, 1 = leading, 2 = following
• MD37110 $MA_GANTRY_POS_TOL_LONG[n] — monitoring tolerance (mm), default 0.5
• MD37120 $MA_GANTRY_POS_TOL_REF[n] — reference tolerance, default 0.05
• MD37130 $MA_GANTRY_BREAK_UP_DELAY[n] — break-up delay in seconds
• MD37140 $MA_GANTRY_BREAK_UP_COND[n] — break-up condition (e.g. follow-up, error)

On NC RESET, alarm 26017 ("Axis %1 gantry not synchronized") can fire if the slave axis position differs from the master beyond GANTRY_POS_TOL_LONG. The ASUB must zero the REPOS offset on the master and slave before any motion, and the gantry must be re-synchronized with GANTRY_ACTIVATE or by leaving reset state if the leading axis is referenced.

; ASUB gantry-safe sequence
IF $AA_LEAD_SP[X] <> X_AXIS_NAME OR $AA_LEAD_SP[X2] <> X_AXIS_NAME THEN
  ; gantry broken up — re-join before continuing
  $AA_LEAD_SP[X2] = X_AXIS_NAME
  $AA_LEAD_OFF[X2] = 0
ENDIF

; clear REPOS on both
$AA_REPOS_DELAY[X] = 0
$AA_REPOS_DELAY[X2] = 0

For machines with active gantry during tool change, leave MD37130 GANTRY_BREAK_UP_DELAY at the default 0.0 s; never re-synchronize the gantry inside the ASUB if the machine builder has not explicitly allowed it, because re-sync on a tilted gantry can crash the mechanical coupling.

8. Machine Data and Setting Data Configuration

For details refer to the Siemens Industry Online Support entry for the SINUMERIK 840D sl list manual, function manual "Tool Management", and the programming manual "Basics". Search for the SINUMERIK 840D sl documentation set on the Siemens support portal.

9. HMI Operate: Operator Interface and Acknowledgment

Display the scrap count and the last reject on the HMI Operate operator screen. Use the user-variable display that calls DB9000.DBD4 for the counter, and the alarm log shows the event with a classification of 4701. In Operate's config (System CFG → operator screen), add the following read-only fields:

• /Plc/DB9000.DBD4 — scrap count (DINT)
• /Plc/DB9000.DBD8 — last tool index rejected (DINT)
• /Plc/DB9000.DBD12 — last reject timestamp in NCK ticks (DINT)

For the operator to manually trigger the auto-reset (without waiting for an alarm), add an MC-Code or HMI softkey that sets DB32000.DBX0.7 (channel-specific "delete distance to go") and DB9000.DBX0.0 (REJECT_FLAG), then a separate "RESET + START" prompt. The MC-Code can be issued as an M-function in the part program:

; Manual reject (operator M-code entry)
M97                                  ; custom M for reject
; — defined via MD10715 $MN_M_NO_FCT_CYCLE[1] = 97
; — calls _CYC97, which sets DB9000.DBX0.0 and triggers PROG_EVENT

Use MD10715 to bind M97 to a custom cycle, e.g. _CYC97_SPF:

; _CYC97_SPF — operator reject
RPF _GUD_REJECT_ACTIVE = 1
SBC(64)
M17

10. Step-by-Step Commissioning Procedure

1. Back up the machine. Archive the current NCK, PLC, and CF card image with HMI Operate → Commissioning → Backup before any change.
2. Create GUD definitions in _N_GUD_DEF and load via HMI Operate → Commissioning → NC → GUD. Re-initialize the NCK to apply.
3. Author the ASUBs _N_ASUP_RESET_SPF and _N_ASUP_START_SPF in the manufacturer area (NCKMADV_DIR or MPF_DIR). Use manufacturer area, not user area, to prevent end-user edits.
4. Configure PROG_EVENT machine data (Section 8). Always set values in NCK reset state; reload with NCK reset.
5. Add DB9000 in the PLC project (TIA Portal or STEP 7 classic) and instantiate FB9000 in OB1 cyclic.
6. Verify FC8 / FC9 in the PLC. Confirm that the tool manager handshake bits in DB4/DB5 cycle through (NCK → PLC → NCK).
7. Compile and load the PLC project. Watch the PLC diagnostic buffer for any cross-reference errors.
8. Test in dry-run: set SD42100 $SC_DRY_RUN_MASK = 1, run a known part-program, and trigger an NC STOP at T3. Confirm ASUB fires, scrap counter increments, REPOS does not resume.
9. Test in a real cut: same sequence, with a test block (no actual cut) on a sacrificial workpiece. Verify the gantry remains synchronized, the tool returns to the magazine correctly, and the next part starts at T1.
10. Alarm-class filter: configure MD11600 $MN_CONTOURHANDLER_CONFIG to only fire the reset ASUB for ALC REWORK alarms. Other alarms (e.g. axis, drive) must not invoke the reset.

11. Verification Tests and Acceptance Criteria

Read the counters from HMI Operate: Diagnostics → NC/PLC Variables → /Plc/DB9000.DBD4 and /NCK/System/_GUD_SCRAP_COUNT — they must match.

12. Troubleshooting Matrix

13. Operational and Safety Notes

For tool life monitoring (TLO), set MD18080 $MN_MM_TOOL_MANAGEMENT_MASK to enable life monitor and use $AC_SPECIAL_PARTS as the increment target when the part consumed a tool. The user scrap counter (DB9000) is the audit-trail for parts that did not consume a tool life unit, only REJECT_HND.

For machines that load a part into the work area only after the tool change, the auto-reset may also need to call a part-load macro. Add a GUD-driven decision:

IF _GUD_REJECT_ACTIVE == 1
  CALL _CYC_PART_LOAD                  ; load a new blank
  _GUD_REJECT_ACTIVE = 0
ENDIF

14. Cross-References and Standards

For the underlying tool-change interface and PROG_EVENT semantics, the relevant Siemens documentation is the SINUMERIK 840D sl list manual (LH1) and the function manual "Tool Management" (FBMA), both distributed through the Siemens Industry Online Support. The IEC 61131-3 standard covers the PLC side of the PLC basic program; ISO 6983 (G-code) covers the NC syntax for M-functions, T-codes, and modal behavior referenced above.

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