OSSD Meaning & Wiring (NPN/PNP with EDM)

1) OSSD & EDM in practice

OSSD channels must be separately evaluated by compatible safety inputs. The exact device and controller determine test pulses, discrepancy timing and fault responses; do not impose a generic “few milliseconds” rule.

EDM monitors the required external switching-device feedback. Suitable mechanically linked NC or mirror contacts provide evidence of the specified main-contact state. It does not detect every possible relay, valve, brake or mechanical failure by itself.

2) Topologies: PNP vs NPN (with EDM)

// Functional roles only — not a terminal drawing
Approved PNP OSSD1 ──► Safety input A ┐
Approved PNP OSSD2 ──► Safety input B ├─► validated safety logic
Specified reference ─► per manuals  ┘       │
Suitable K1/K2 linked NC feedback ─► monitored feedback input
Approved reset control ───────────► restart-interlock input
Validated safety logic ──► suitable final elements ──► required safe state

// Conditional interface check — not a generic conversion
Exact device has approved NPN safety outputs?
  NO / UNKNOWN → do not wire as an OSSD safety function
  YES → exact safety input explicitly supports that output/pulse behavior?
    NO / UNKNOWN → select compatible equipment; do not invert in a standard PLC
    YES → follow the manufacturer's approved separate-channel drawing
COM = +24 V alone does not prove safety-input compatibility.

Wiring checklist for the approved drawing

• Record exact transmitter, receiver, safety input and final-device order codes.
• Check separate OSSD channels, permitted cable and reference; no raw-output joins.
• Where required, linked NC feedback normally closes on de-energization after the main contacts open; verify both transitions and timing.
• Apply model-specific cable separation, shielding and bonding. A generic 200 mm distance or single-ended shield rule is not universal.

3) Functional I/O roles & naming

Structured Text — separate HMI diagnostics only

// Ordinary PLC / HMI diagnostics only.
// Inputs below are separate auxiliary/status outputs from validated safety logic.
// No raw OSSD tap, no safety reset, no interlock or actuator-enable output.
HMI_GUARD_STATUS := AUX_GUARD_CLEAR;
HMI_SAFETY_FAULT := AUX_SAFETY_FAULT;
HMI_RESET_REQUIRED := AUX_RESET_REQUIRED;
// Loss or stale communication must be displayed as UNKNOWN, not SAFE.

These status assignments are not a safety function and do not process the OSSD pair or command restart. No generic press/release timing window is prescribed.

4) Reset strategies

• Determine restart prevention from access and remaining-presence risks; clearing beams must not cause an unexpected hazardous start.
• Use the exact approved reset press/release sequence and timing. Rising-edge or two-stage behavior is not universal.
• Locate reset outside the hazard area with the necessary view; add measures where the full zone cannot be checked.
• Reset acknowledges a safe condition; it must not itself initiate hazardous motion.

5) Commissioning routine (5-step)

1. Document review: confirm exact interface, configuration and approved machine drawing before power is applied.
2. Detection: with hazardous motion disabled, use the prescribed test piece over the complete field and relevant boundaries; follow exact manual conditions.
3. Fault and feedback: use the approved controlled test method, verifying specified diagnostics and restart inhibition. Do not hold contactors or improvise live shorts.
4. Restart: test clear field, held reset, fault recovery and power restoration; no unintended hazardous start is acceptable.
5. Stopping and record: qualified personnel validate the complete stopping function under separately controlled conditions and archive timing, access assessment and settings.

Self-test log (CSV copy)

Item,Test,Expected,Actual,Result,Notes
1,Prescribed test piece over full field,Hazard disabled for detection checks,,,
2,Channel diagnostics per approved procedure,Unsafe mismatch produces specified safe response,,,
3,Final-element feedback test per manuals,Incorrect feedback inhibits required restart,,,
4,Reset and power restoration,No unintended hazardous start,,,
5,Full-chain stopping and access assessment,Recorded worst case and applicable distance accepted,,,
6,Configuration and interface evidence,Approved versions and settings archived,,,

Primary references: SICK deTec4 Core operating instructions (model-specific OSSD and testing example), ISO 13855:2024, and IEC 62046:2026. These do not establish any DAIDISIKE relay's unsupported functions.

6) Common mistakes & how to avoid

7) FAQ