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  • Writer's pictureChristopher Woller

Machine Safety Basics: Test Pulses vs OSSD

Understanding these types of safety pulses, along with the advantages of various fieldbus and software options, will ensure your machine functions without putting anyone in harm’s way

How do you know a safety circuit on your machine is actually safe? You don’t want a short circuit, crossed or cut wires to result in someone getting hurt. So how does the tech prevent this from happening?

Let me introduce you to test pulses. Basically, test pulses drop signal voltage in a quick repeating pattern that can be identified by the device producing them. This enables greater diagnostic coverage – i.e., the measure of a system’s ability to detect failures. Designers provide a diagnostic coverage level commensurate with the risk being addressed. The greater the intended risk reduction to be provided, the higher the diagnostic coverage must be. 

How do safety pulses function?

EL1918 TwinSAFE Terminal from Beckhoff
The EL1918 TwinSAFE Terminal produces test pulses that check for continuity across dry contacts.

The inputs of safety controllers and devices leverage some type of test pulse from a clocked output. Appropriate passive mechanical devices, such as emergency-stop buttons, limit switches, door sensors, or any others with dry contacts, are connected to a safety interface module. This includes basic safety relays up to more advanced integrated, programmable safety solutions.

Safety input cards like the EL1904 or EL1918 TwinSAFE Terminals from Beckhoff produce pulses checking for continuity across dry contacts. To provide standard safety functionality, each input is paired with a clocked output supplying a pulsed +24V. The clocked output sends out test pulses in a particular pattern, they go through the E-stop or other device, and then return to the input in the same pattern.

If there are any discrepancies, including wire breaks, crossed wires, or a short to power, the safety input or input card will fault and go into a safe state.  Then you need to check the wiring for the cause of the issues. These should be easy to identify based on the diagnostic information received – e.g. lights, messages over the network, etc.

How do OSSDs work?

Electro-sensitive protective equipment (ESPE) is a self-monitoring safeguard or presence-sensing device that uses electricity rather than physical means to detect parts of the human body. Examples include safety light curtains, laser scanners, vision-based protective devices, RFID sensors and radar.

Beckhoff TwinSAFE integrated functional safety terminals
By understanding if your safety device requires test pulses or OSSD, you can select and properly parameterize the correct functional safety terminals.

The Output Signal Switching Device (OSSD) is the component of the ESPE connected to the control system in pairs. When the sensing device is activated during normal operation, the OSSD pair responds by going to the “off” state. Solid-state OSSDs provide fault monitoring. In short, it is the actual output of the light curtain, for example. The responsibility for testing if the device is working and properly wired shifts from the safety card to the sensor itself.

So then, how do OSSDs, or safety-rated outputs, really work? Instead of being generated by the input card, the safety-sensing device itself creates the test pulses. These devices have an internal safety processor that evaluates if the OSSD pulsed off when it was intended to. If wires are crossed or shorted, the ESPE will fault, and both OSSDs will be set to off.

It’s important to note that the safety-rated input at the other end does not generally evaluate the OSSD pulses because they’re filtered out. So it’s critical to know how to set up these devices in your safety editor software.


A graphic shows the differences between test pulses and OSSD

Regardless of what device generated the test pulse, they’re ultimately looking for the same things: faults in the wiring or other issues that could create unsafe working conditions. With an integrated safety approach, this functionality should be directly built into the standard automation software platform, simplifying communication across the various sections of a machine and cell.

For dry contacts, some terminals have fixed parameters while others are variable. When working with self-testing sensors, parameterizing is slightly more involved but follows the same general principles.

When working with self-testing sensors, parameterizing the EL1904 and similar safety devices requires just a couple more steps. Since the OSSD will generate the pulses, you should be able to turn off the safety card’s test pulses with the appropriate safety parameter for the channels you intend to use.

Ready to enhance your systems with integrated safety technology? Contact your local Beckhoff sales engineer today. 


Christopher Woller, Beckhoff USA Safety Product Manager

Christopher Woller is the Safety Product Manager for Beckhoff Automation LLC.



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