Connecting Safety Light Curtains to PLCs: Relay Output vs. OSSD Solid-State Output
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Quick answer: Compare relay contacts and OSSD outputs by their failure modes and the receiving safety inputs. Dual OSSD must enter compatible qualified safety logic; a standard PLC receives auxiliary status only. Relay contacts can form part of a validated safety function when their architecture and monitoring are suitable. Use the installation and wiring guides to identify the exact circuit before a legacy retrofit.
Open up the control cabinet of any legacy stamping press or packaging machine, and you will likely see a web of mechanical relays clicking away. For decades, when you wanted to stop a machine, a simple “dry contact” relay (Normally Open or Normally Closed) did the job. When the safety light curtain was blocked, the physical relay contacts inside would snap open, cutting the power circuit.
But when you unbox a modern, high-end Type 4 safety light curtain today, those familiar relay wires are gone. Instead, the wiring diagram points to two mysterious wires labeled OSSD1 and OSSD2.
If you have ever stared at those wires wondering how to correctly connect the light curtain to your PLC without burning something out, you are not alone. Let's strip away the technical jargon and look at why the industry moved to OSSD, and exactly how you should wire it up.
The Problem with Traditional Relays
Mechanical relays are simple, but they have a fatal flaw in high-risk industrial safety: contact welding.
Every time a mechanical relay opens or closes under load, a tiny electrical arc occurs. Over millions of cycles, the metal contacts can literally melt and weld themselves permanently shut. If an operator reaches into a hydraulic press and the light curtain triggers the relay, but the relay contacts are welded shut... the press will not stop. This is a catastrophic, hidden failure.
Enter OSSD: The Modern Solid-State Standard
OSSD stands for Output Signal Switching Device. Instead of using mechanical metal contacts, an OSSD output safety light curtain uses solid-state transistors (typically two redundant channels supplying 24V DC). Because there are no moving parts, an OSSD output has no contact-welding failure mode — the welded-contact risk that affects relay outputs does not apply here.
But the real magic of OSSD is its self-diagnostic capability. It is continuously checking its own health using Test Pulses.
How OSSD Test Pulses Prevent Accidents
Even while the machine is running perfectly, the OSSD outputs rapidly drop the 24V signal down to 0V for a few hundred microseconds, and then back to 24V. This happens so fast that your machine doesn't shut down, but the light curtain's internal brain is actively monitoring the line.
- If a forklift runs over the sensor cable and shorts the OSSD wire to the 24V power supply, a mechanical relay wouldn't notice.
- The OSSD system, however, will realize that during its microsecond “test drop,” the voltage didn't actually drop to zero (because of the short circuit).
- The light curtain immediately detects the fault and locks the machine down into a safe state before anyone gets hurt.
How to Wire OSSD to Your Machine
Wiring an OSSD output requires a bit more care than a dry contact relay. Because of those high-speed diagnostic pulses, you cannot just wire OSSD signals into a standard, cheap PLC input or directly to a heavy motor contactor.
What If My Machine Is Old and Doesn't Support OSSD?
This is the most common roadblock. You want the highest level of Type 4 optical protection, but your 20-year-old stamping press or conveyor belt only understands basic, heavy-duty relay contacts. Do you have to replace the entire control cabinet?
A compatible safety interface may be part of the retrofit, but the existing cabinet and final elements still need assessment; a relay module alone does not make a legacy stop circuit safe.
If the original circuit cannot receive OSSD, choose a qualified safety controller with documented OSSD inputs and the required restart and feedback functions. A DA31 listing is not evidence of that interface: obtain its exact supplied manual before including it in the design.
The B2J0601 is a general-purpose delay module for non-safety process timing. It is not a safety relay and must not be used as the OSSD decision element or stop-path bridge for personnel protection.
Here is how the bridge works:
- The advanced OSSD wires from the light curtain connect to the input of the Safety Relay Module.
- The Safety Relay reads the fast pulses and monitors the health of the light curtain.
- Use the controller's specified safety contacts and final-element arrangement, with redundancy and feedback where required. An auxiliary NC contact is not automatically a protective output, and a legacy control input still needs validation.
Upgrade Your Safety Without the Headache
Transitioning from old-school mechanical sensors to modern solid-state safety doesn't have to mean completely rewiring your factory. By understanding the difference between OSSD and Relay outputs, you can ensure your operators get the safest, most reliable protection possible without causing electrical nightmares for your maintenance team.
Are you trying to retrofit a modern DAIDISIKE safety light curtain onto legacy machinery? Contact our technical support team today. We will help you review your wiring diagrams and supply the controller requirements and commissioning checks for the specific installation.
The GuardShield 450L manual is an example of a device-specific OSSD interface specification. Check load limits and controller compatibility in the manuals for the actual installed devices.