What Is OSSD Output on a Safety Light Curtain?

What do OSSD1 and OSSD2 actually tell the machine?

They carry the light curtain's protective state—not a measurement of distance and not a motor-power supply. OSSD stands for Output Signal Switching Device. On a typical safety light curtain, two monitored semiconductor outputs connect the receiver to a compatible safety control system.

In normal protective operation, an intrusion that the curtain must detect causes the output pair to switch OFF. Downstream safety logic uses that change to initiate the required protective action. The machine still needs the appropriate switching devices or safe drive functions to carry out that action.

The descriptions above assume ordinary protective operation. Approved muting or blanking functions deliberately change what is detected or evaluated; they cannot be inferred from two voltage readings alone.

Why are there two outputs instead of one ordinary PNP signal?

Two separately evaluated channels, together with diagnostics, help prevent a single output or wiring fault from being mistaken for a valid permissive signal. The second output is not a spare wire. Connecting the channels together removes the independence that the intended safety architecture relies on.

PNP describes the electrical behavior; OSSD describes a safety-related interface

A PNP output sources current to its load. That fact alone says nothing about redundancy, self-monitoring or suitability for a safety function. Many light curtains use two PNP OSSDs, but “OSSD” does not universally mean PNP.

For example, OMRON's F3SG-SR/PG specifications list selectable PNP/NPN safety outputs and separately identify auxiliary outputs. This is why a replacement must match the actual output configuration, not just the familiar OSSD label. An auxiliary status output may be useful for an HMI or ordinary PLC, but it is not a substitute for the safety output pair.

The two channels must stay separate through the intended input interface

Conceptual signal path. A safety relay or controller may combine the middle stages. This is not a terminal assignment or a validated circuit.

Use the receiver's exact connection drawing to identify OSSD1, OSSD2, supply and reference conductors. Cable colors and M12 pin counts are not enough to identify a universal pinout.

Why does an OSSD briefly switch OFF when the light curtain is clear?

A short OFF interval can be an intentional test of the output circuit. In many semiconductor OSSD designs, the curtain briefly commands an output OFF and checks the result. A failure to follow that command can expose a fault such as a short to a supply or another output, within the product's documented diagnostic coverage.

A compatible safety input tolerates the specified test behavior without treating each brief pulse as a field interruption. That is different from ignoring a sustained OFF condition. The safe input may filter the pulses while the light curtain itself evaluates its output tests.

Read the maximum pulse width, not just the typical value

Do not confuse an OSSD test pulse with an optical sensing pulse. One tests an electrical output; the other belongs to how the emitter and receiver detect an interruption.

Can you connect OSSD outputs directly to a PLC or safety relay?

Yes, when the receiving interface is explicitly suitable for those OSSDs and the complete safety design supports the connection. A safety PLC requires the appropriate safety input module and configuration. Two ordinary PLC inputs do not become a safety input pair simply because software compares them.

For indication only, an ordinary PLC can use a permitted diagnostic interface or auxiliary output. Keep that status path separate from the path relied on to stop hazardous motion.

Match the input's electrical and timing requirements

• Electrical levels: match sourcing/sinking behavior, supply reference, ON/OFF thresholds and leakage limits. Include voltage drop and the permitted input load.
• Test-pulse handling: compare the curtain's maximum pulse duration and relevant load conditions with the module's accepted behavior.
• Channel evaluation: use the specified two-channel mode, transition tolerance and fault response. Do not join the outputs or silently discard one channel.
• Reaction time: account for any permitted filtering and processing delay in the safety-function response calculation.

Do not copy the settings for a mechanical contact

A dry-contact safety circuit may use test pulses generated by the input module. A self-testing light curtain generates its own OSSD pulses. Beckhoff's TwinSAFE explanation makes this distinction explicitly. The correct input mode therefore matters as much as matching the nominal voltage; follow the receiving device's instructions rather than applying a generic “pulse test” preset.

Also separate pulse filtering from channel discrepancy timing. Filtering governs how brief changes are treated; discrepancy monitoring addresses how long the channels may disagree. Rockwell's Light Curtain instruction documents these as distinct behaviors. Neither is a general-purpose setting to increase until a fault disappears.

Direct control of downstream switching elements is possible only where the manufacturer's documented circuit and the safety assessment permit it, including load and feedback requirements. An OSSD current rating alone does not authorize connecting a contactor coil.

Why are the outputs OFF even though the protective field looks clear?

A clear field is only one condition for a permissive output. A reset interlock, an external-device feedback problem or a device fault can keep operation inhibited. First distinguish an actual OSSD OFF state from a downstream controller that is rejecting or withholding the signal.

Reset and EDM answer different questions

Reset addresses the configured restart interlock: has the required deliberate acknowledgement occurred after the protective event? A reset is not, by itself, a machine-start command. Clearing the light curtain must not create an unintended restart.

External device monitoring (EDM) checks downstream switching-device feedback. In the deTec4 example, an unexpected contactor state can keep the OSSDs OFF. Thus “both outputs switched OFF” and “the final switching devices returned correctly” are separate pieces of evidence.

These functions may be implemented in the curtain or downstream safety logic. Identify which device owns each function before interpreting an indicator.

Does an OSSD connection prove that the machine is safely protected?

No. It is one interface within a complete safety function. IEC 61496-1 addresses the protective equipment and its interface; its scope does not prescribe the hazard-specific position of the detection zone. ISO 13849-1 addresses the design and integration of safety-related control systems, not an automatic performance level granted by connecting two wires.

Before the machine is released, the responsible integrator must verify the required safety performance, final-element behavior, restart prevention and total stopping response, as well as the curtain's positioning and protection against reaching around it. A Type 4 label or an OSSD pair does not independently establish PL e for the entire function.

For a connection review, start with the receiver part number, its output/timing specification, the receiving safety module and configuration, and the actual circuit drawing. For positioning, use the separate safety light curtain installation-distance guide. Keep the distinction clear: the curtain detects, the interface communicates, and the validated system performs the protective action.