SINUMERIK Dynamic Subprogram Call Using String Variable

Overview: Indirect Subprogram Invocation on SINUMERIK 840D sl / 828D

On Siemens SINUMERIK 840D sl, 840D, 828D, and One NCU controllers, the CALL instruction in the NC high-level language accepts a program identifier that may itself be a string variable. This enables dynamic subprogram selection at runtime — for example, letting an HMI operator choose a machining sequence from a list, or letting the NC select a tool-specific dressing program from a recipe without editing hard-coded CALL statements.

The pattern requires three building blocks:

1. A global user data (GUD) string variable large enough to hold the program name (32 characters is the practical default).
2. An ISFILE() existence check against the active subprogram directory (SPF.DIR) so the controller never attempts to CALL a missing program.
3. A plain CALL <string_var> statement — not EXTCALL, which has different semantics for external (CF card / network) storage.

This article documents the full mechanism, including directory semantics, slash handling, error codes, and a reproducible HMI-side commissioning checklist.

Prerequisites

NC File-System Topology: MPF.DIR vs SPF.DIR

SINUMERIK stores part programs in a passive file system rooted at the NC card. The two directories that matter for this pattern are:

The path string handed to ISFILE() must begin with a leading slash and end with the program name. For subprograms the correct form is:

For main programs use /MPF.DIR/.... The dot character . between name and extension is mandatory; the controller does not append it for you.

GUD Variable Definition

Global user data lives in dedicated definition files in the NC active file system. SINUMERIK provides GUD1..GUD9. Files below the leading underscore (_GUD0) are retain — they survive NCK reset. Numbered GUDs are session-only.

Recommended file: GUD4.DEF (or any free number). Place it under NC active / Definitions / GUD4.DEF via HMI menu Start-up > Define.

; GUD4.DEF — global user data
; Cross-channel scope (DEF NCK)

DEF NCK STRING[32] NAME    ; target subprogram name (incl. extension)
DEF NCK BOOL    ISNAME     ; existence flag, 1 = present

Rules of the DEF statement:

• Position: anywhere in the file, separated by semicolons or whitespace.
• Indentation: ignored by the parser.
• Scope keywords: NCK (global), CHAN (per-channel), DOUBLE (per-axis-pair — rarely used for strings).
• Type: STRING[32] reserves 32 characters. STRING[128] is allowed in newer NCK 4.8+ for long paths; default is 32.

The ISFILE() Existence Check

ISFILE(<path>) returns TRUE when the file exists in the addressed directory, FALSE otherwise. The function operates synchronously on the passive file system and is safe to call from MAIN.MPF before a CALL.

The operator << is the NC string concatenation operator (two left-angle brackets with no space). It joins the directory prefix and the variable contents at runtime. The compiled result is evaluated as the argument to ISFILE().

Common error patterns to avoid:

The CALL Instruction: Indirect Program Invocation

CALL in NC syntax expects a program identifier token. Because the NCK parser first performs variable substitution and then looks up the resulting name in the local program memory, you can pass a string variable directly:

The semantics are exactly those of a static CALL MY_SUB.SPF:

1. NCK pushes the current program counter onto the internal subprogram stack.
2. Program counter is reloaded to the first line of the named subprogram.
3. On reaching M17 or RET, control returns to the saved line of the caller.

Up to 8 nested subprogram calls are supported in standard NCK 4.5; NCK 4.7+ extends this to 12 when using 828D sl in compact mode. The depth is consumed by both static CALL and CALL <var> identically.

CALL vs EXTCALL — Do Not Confuse Them

If your subprogram lives in the active SPF.DIR of the NC, use CALL. If the file is on the CF card or a network share, use EXTCALL with the full passive-FS path. The original Siemens wording — “EXTCALL is a separate instruction with a different meaning” — is the rule to remember.

Complete Working Example

The following MAIN.MPF shows the full pattern: read a program name from the operator (or set it elsewhere), validate it, dispatch to it, and loop if the file is missing.

; =============================================
; MAIN.MPF — dynamic subprogram dispatcher
; Requires GUD4.DEF with NAME (STRING[32]), ISNAME (BOOL)
; =============================================

; --- Default value (replace from HMI or PLC as needed) ---
NAME = "DRILL_CYCLE_42.SPF"

BACK: ; branch target for re-entry

ISNAME = ISFILE("/SPF.DIR/" << NAME)

IF ISNAME == FALSE
  ; --- the file is missing ---
  ; Option A: hard fault (raise alarm)
  ; SETAL(65000, "Subprogram not found")
  ; Option B: re-prompt (in production, drive NAME from HMI/PLC)
  GOTO BACK
ENDIF

; --- the file exists, dispatch ---
CALL NAME

; --- back here after M17 / RET ---
G0 X0 Y0 Z100        ; safe position
M30                   ; program end / rewind

Notes on the snippet:

• SETAL(65000, ...) is an OEM alarm; you may replace the number with a free alarm in your machine builder range (typically 65000..99999).
• GOTO BACK inside a MAIN loop is fine; the NCK keeps the subprogram return stack empty in that branch because we never called.
• M30 at the bottom ensures a clean program reset on the next CYCLE START.

String Handling: Length, Trimming, and Special Characters

The default STRING[32] can hold exactly 32 bytes. When a value shorter than 32 characters is assigned, the remainder is padded with BLANK (space, hex 0x20). Two consequences:

1. ISFILE() does not treat trailing blanks as significant — the OS-level lookup ignores them — so an assignment like NAME = "DRILL" followed by manual padding to 32 chars in the HMI still resolves correctly.
2. When you concatenate with << in ISFILE("/SPF.DIR/" << NAME), the trailing blanks are appended to the path. ISFILE() normalizes this; CALL NAME does not, and will fail to find a subprogram whose displayed name has trailing spaces (highly unusual, but possible on older HMIs).

To be defensive, normalize at the entry of the dispatcher:

; Optional safety: left-justify and strip trailing blanks
NAME = NAME                       ; identity assignment forces evaluation
; Some HMI writes append blanks; CALL with blanks is tolerated,
; but EXTCALL would not. Stick to local NC memory and CALL.

Error Codes and Alarms You Will See

Block-Look-Ahead, LookAhead, and the Safe Place to Dispatch

The NCK reads ahead up to ~1000 lines of NC code so that it can pre-compute path velocity. A CALL <var> statement can be processed by the look-ahead, but the contents of the called subprogram can NOT — they are only loaded once the subprogram counter advances. Two practical rules:

1. Place the CALL in the same line where the operator changes a tool or a work offset, so that the look-ahead has no motion to mis-compute across the jump.
2. Do not place a CALL <var> inside a motion block. Use its own line.

Commissioning Procedure (Step by Step)

1. Write the GUD. On the HMI, go to Start-up > NC > GUD and create/append to GUD4.DEF:

   DEF NCK STRING[32] NAME
   DEF NCK BOOL ISNAME

   Activate with Set active and confirm the HMI shows the values in the Parameters soft key.
2. Drop a test subprogram. Upload TEST_DYN.SPF into NC active / Subprograms / SPF.DIR via the HMI file manager. Verify it appears under Program Manager > NC active > Subprograms.
3. Load MAIN.MPF. Use the dispatcher code from the previous section. Replace NAME = "DRILL_CYCLE_42.SPF" with NAME = "TEST_DYN.SPF" for the first test run.
4. Single-block run. In AUTO > SBL ON, hit CYCLE START. The ISFILE check returns TRUE and CALL NAME dispatches. Verify the controller enters the subprogram by looking at the program level display in the HMI status bar.
5. Negative test. Change the assignment to NAME = "DOES_NOT_EXIST.SPF" and reload. ISFILE must return FALSE; the GOTO branch should re-prompt (or SETAL should raise the alarm).
6. HMI integration. Add a string input field on the operator screen and bind it to NAME over the OPC UA / NC variable interface. The string must be a valid .SPF name before the operator can press CYCLE START.
7. End-to-end. Drive 3–5 different subprograms from a recipe and confirm the subprogram stack is clean (no alarm 14000 series when the cycle ends).

Field-Proven Caveats and Notes

• Case sensitivity: the NCK file system is case-sensitive on NCU software ≥ 4.5. drill_cycle_42.spf and DRILL_CYCLE_42.SPF are different files.
• GUD activation: changes to GUD files are applied on the next NCK reset or power-on. A RESET alone is not always sufficient on 828D; perform an NCK reset from Start-up > NCK reset.
• PLC string length: when an S7-1500 / S7-1200 writes into NAME through the NC variable interface, the PLC must always send 32 bytes, even if the textual name is shorter. Trailing blanks are required so the NCK does not keep the previous content.
• Cycle 840D sl Powerline vs Solutionline: syntax is identical. Behaviour around alarm 15370 differs slightly — Powerline raises it on any external medium; Solutionline is more permissive. Either way, prefer CALL for files in local memory.
• Simultaneous channels: DEF NCK shares NAME across channels. If two channels dispatch concurrently, the value seen by the other channel may already have changed. Use DEF CHAN if each channel must hold its own selection.
• Operator security: the dispatcher pattern can call any file in SPF.DIR. Restrict operator write access to the NAME GUD to a curated list — otherwise a malicious or curious operator can CALL tool-change or measurement cycles outside the intended flow.

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