OSSD + EDM: DQC × Siemens S7-1200 Wiring (I/O Map, Reset & Commissioning)

1) OSSD / EDM principles & selection (PNP/NPN)

OSSD is a diagnostic safety output, typically two independent channels (OSSD1/OSSD2). Any channel fault, desynchronization, or beam interruption triggers a safe stop.

EDM (External Device Monitoring) supervises the external contactors/relays (K1/K2) to prevent reset if their contacts are welded or failed.

2) Wiring Option A: PNP (sourcing, recommended)

Use when S7-1200 digital inputs are sourcing or the common is tied to L− (0 V). Wire the DQC Tx/Rx per the factory terminal map, route OSSD1/2 to S7-1200 inputs, and return EDM via the series NC contacts of K1/K2.

3) Wiring Option B: NPN (sinking)

Use when the S7-1200 input common COM is tied to L+ (24 V). Logic is the same as PNP but polarity is inverted; mixing input types causes constant ON or false alarms.

4) I/O map & naming convention

Adopt a consistent naming scheme so technicians can audit quickly and reuse across projects. The table below can be copied as your I/O list (export to CSV if needed).

SCL example (portable to TIA Portal)

// Condition: both channels valid + EDM OK
IF I0.0 AND I0.1 AND I0.2 THEN
    M100 := TRUE;      // LC_STATUS_OK
ELSE
    M100 := FALSE;
END_IF;
// Manual reset (rising edge) while status is OK
IF R_TRIG(I0.3) AND M100 THEN
    M101 := TRUE;      // LC_INTERLOCK
END_IF;
// Auto-cancel interlock when status error occurs
IF NOT M100 THEN
    M101 := FALSE;
END_IF;

Note: R_TRIG() is a rising-edge FB; for a two-stage reset (double-action), add a time window and debouncing.

5) Reset strategies (manual / two-stage / rising edge)

• Mandatory manual reset: no auto-restart after the beam is cleared.
• Two-stage action: press → release → press again within 0.5–3 s to reduce accidental resets.
• Rising-edge detection: hold-to-reset is prohibited; edge-trigger avoids continuous reset.
• Ergonomics: the operator must clearly view the hazard zone before resetting.

6) Commissioning & self-test (scope/meter checkpoints)

1. Polarity & earth: verify L+/L− and PE; ensure PNP/NPN matches DI polarity.
2. Channel coherence: block/release beams and confirm OSSD1/OSSD2 change simultaneously; desync should alarm.
3. EDM logic: mechanically hold a contactor—EDM must prevent reset.
4. Reset validation: long-press ineffective; only rising edge or two-stage is accepted.
5. Safety distance: calculate with ISO 13855 and file it.
6. Capture points: record levels/waveforms at OSSD1, OSSD2, EDM, Reset.

Self-test record template (copy to CSV)

Item,Test,Expected,Actual,Result,Notes
1,OSSD1/OSSD2 beam block response,Both channels drop/alarm simultaneously,,,,
2,Desync alarm,Any channel delay → alarm,,,,
3,EDM welded-contact blocking,Hold K1/K2 → reset disabled,,,,
4,Reset strategy,Only rising edge / two-stage valid,,,,
5,Safety distance,Calculated & recorded by ISO 13855,,,,
6,Waveform/level captures,Saved to as-built file,,,,

7) Cabling & EMC (shielding / grounding / separation)

• Separate OSSD/EDM from power cables by ≥ 200 mm; avoid long parallel runs; cross at 90°.
• Terminate shields at the cabinet end with single-point grounding to avoid loops.
• Use a dedicated breaker for the 24 V sensor supply; keep it apart from drives.
• Use terminal numbers and wire markers consistent with drawings; place an I/O cheat-sheet on the cabinet door.

8) Alignment tips & interference troubleshooting

1. Three-step alignment: maximize horizontally → maximize vertically → fine-tune/lock while watching input stability.
2. Strong light/welding arc: add shrouds or filters; change incident angle; shorten range if necessary.
3. Reflective parts: keep ≥ 300 mm from mirror-like surfaces; consider blanking bars.
4. Mist/dust: choose IP65/67/69K units and set a cleaning schedule.

9) Downloads & tools

• OSSD/EDM wiring pack (PNP/NPN, S7-1200/FX5U, relay matrices)
• ISO 13855 safety-distance calculator
• DQC quick selector (resolution / height / range)

10) FAQ