To Siemens S7-1200: sample program & common input wiring
Engineer-ready. Map a safety light curtain’s OSSD1/OSSD2, EDM, and RESET to Siemens S7-1200 digital inputs (PNP/NPN), with copy-ready I/O list, TIA Portal example logic (standard CPU monitoring and S7-1200F safety option), plus commissioning records.
1) Common input wiring (PNP / NPN)
PNP (sourcing) — recommended
Light Curtain
OSSD1 ───────► I0.0 (DI)
OSSD2 ───────► I0.1 (DI)
EDM FB (K1 NC ─┐
├─► I0.2 (DI)
K2 NC ─┘
RESET PB ─────► I0.3 (DI, rising-edge)
PLC DI COMMON: M (0V)
Sensor outputs source +24V into inputs.S7-1200 DI referenced to M (0 V); OSSD outputs source +24 V.
NPN (sinking)
Light Curtain
OSSD1 ───────► I0.0 (pulled to 0V)
OSSD2 ───────► I0.1
EDM FB ──────► I0.2
RESET PB ────► I0.3
PLC DI COMMON: L+ (+24V)
Sensor outputs sink to 0V.Do not mix PNP and NPN on the same DI common block.
2) Copy-ready I/O map & naming
| Variable | Address (typ.) | Source | Purpose | Remarks |
|---|---|---|---|---|
LC_OSSD_CH_A | I0.0 | OSSD1 | Safety channel A | Separate harness/fuse |
LC_OSSD_CH_B | I0.1 | OSSD2 | Safety channel B | Desync ⇒ fault |
LC_EDM_FB | I0.2 | K1/K2 NC | EDM feedback | Blocks reset if open |
LC_RST_BTN | I0.3 | Pushbutton | Manual reset | Rising edge only |
LC_STATUS_OK | M100.0 | Logic | LC healthy | Internal |
LC_INTERLOCK | M100.1 | Logic | Enable-to-start | Clears on error |
3) Example program — standard S7-1200 (monitoring)
This example is for a non-safety CPU. It provides status & interlock only. The actual safety stop is executed by the external safety relay/controller.
LADDER (conceptual)
// Rung 1: LC_STATUS_OK when OSSD1 & OSSD2 & EDM are TRUE
| I0.0 I0.1 I0.2 |----( M100.0 )
// Rung 2: Rising-edge of RESET enables interlock if status OK
| I0.3 one-shot M100.0 |----( M100.1 ) // LC_INTERLOCK
// Rung 3: Auto-cancel interlock on status loss
| NOT M100.0 |----( R ) M100.1STRUCTURED TEXT (TIA Portal SCL)
// One-shot rising edge for I0.3
IF (I0_3 = TRUE) AND (RSTprev = FALSE) AND (M100_0 = TRUE) THEN
M100_1 := TRUE; // LC_INTERLOCK
END_IF;
RSTprev := I0_3;
// Status OK
M100_0 := (I0_0 AND I0_1 AND I0_2);
// Cancel interlock if status lost
IF NOT M100_0 THEN M100_1 := FALSE; END_IF;4) Example concept — S7-1200F (safety CPU)
With an S7-1200F CPU and certified F-DI/F-DO, implement the safety function in F-blocks and drive K1/K2. Below is a conceptual mapping; use certified F-library blocks and follow your company validation plan.
// F-Logic concept (pseudo)
Inputs: FDI_OSSD1, FDI_OSSD2, FDI_EDM, FDI_RESET
Block: F_LightCurtain (dual OSSD + EDM + reset)
Outputs: FDO_SAFE_OUT_A -> K1 coil
FDO_SAFE_OUT_B -> K2 coil
// Rules:
- Dual-channel equivalency with discrepancy time.
- Monitored manual reset (edge or two-stage).
- EDM feedback required; welded contactors block restart.
- Safe outputs drive force-guided contactors (two channels).5) Commissioning & periodic verification
- Polarity: confirm DI common (PNP→M=0V, NPN→L+). Verify 24 V, 0 V, PE.
- Channel coherence: block/release beams; I0.0/I0.1 change together; mismatch faults.
- EDM logic: hold a contactor; reset must be inhibited.
- Reset behavior: edge or two-stage only; no hold-to-reset.
- Safety distance: compute per ISO 13855 and file it with the I/O list and photos.
Self-test record (CSV copy)
Item,Test,Expected,Actual,Result,Notes
1,OSSD channels simultaneous,Both inputs change together,,,
2,Desync detection,Fault on mismatch,,,
3,EDM welded-contact block,Reset inhibited when K1/K2 held,,,
4,Reset function,Only rising-edge/two-stage accepted,,,
5,Interlock cancel,LC_INTERLOCK clears on status loss,,,
6,Records,I/O list + ISO 13855 + photos archived,,,
6) Common mistakes & fixes
| Mistake | Symptom | Fix |
|---|---|---|
| Mixing PNP/NPN on same DI group | Inputs stuck ON/OFF | Unify polarity per DI common; segregate wiring |
| No EDM feedback | Restart despite welded contactor | Series K1/K2 NC back to EDM input |
| Shared protection for OSSD1/2 | Single-point failure hidden | Separate harnesses/fuses |
| Reset held continuously | Unintended restart risk | Edge or two-stage; audit with counter |
| Assuming standard CPU is “safe” | Compliance failure | Use safety relay/controller or S7-1200F with F-I/O |
7) FAQ
Can I connect OSSD directly to S7-1200 and perform the stop in PLC?
With a standard CPU: No. PLC only monitors/interlocks. Use a safety relay/controller to cut power. With an S7-1200F and certified F-I/O, implement the safety logic in F-blocks and still drive K1/K2 contactors.
Which polarity should I pick, PNP or NPN?
Prefer PNP for interoperability and clarity. Match the DI common reference and keep all channels in the same polarity domain.
How do I log reset attempts for audits?
One-shot the RESET input and increment a counter with timestamps in a ring buffer. Export during periodic inspections.