What is the Safety Interlock Feature On My NI PXIe-4135/4136/4137?

Updated Feb 22, 2019

Reported In


  • Connector Kit for SMUs
  • Safety Interlock Cable
  • PXIe-4135
  • PXIe-4136
  • PXIe-4137

Issue Details

  • I have a NI PXIe-4135/4136/4137 and I do not know how I am supposed to connect the Safety Interlock terminals on the front panel. What are these terminals for and how are they used?
  • Do the safety interlock connector and cable have to be connected for me to use my PXIe-4135/4136/4137?


The NI PXIe-4135/4136/4137 is capable of generating voltages that can be hazardous to users (± 200 VDC). It is the responsibility of the system designer, integrator, installer, maintenance personnel, and service personnel to make sure that the module is used safely. The safety interlock circuitry provides a means by which the output of the SMU can be automatically placed in a safe state regardless of the programmed state of the device. It is required to be wired in order to use these devices.

Safety interlocks are common in household appliances. For example, a safety interlock mechanism is responsible for shutting off a microwave when the door is opened to prevent harm to the user. Similarly, washing machines halt their cycle when the lid is opened due to a safety interlock system.

The NI 4135/4136/4137 safety interlock connector has 4 pins on it (see Figure 1). If there is a completed circuit between pin 1 and pin 2, the module will be able to output its full voltage range (± 200 VDC). If there is no electrical connection between pin 1 and pin 2, the module is limited to outputting ±40 V. Pins 3 and 4 are used for daisy-chaining the safety interlocks of multiple NI 4136/4137 modules together.
Figure 1. Safety Interlock Wired to Test Fixture Enclosure

Do not short the interlock pins directly at the connector under any circumstances. If you are using the NI 4135/4136/4137 to output voltages > 40 V, it is best to use an enclosure to cover the outputs, components, or conductors. Relays should be installed in the door(s) of the enclosure so that the safety interlock circuit is complete only when the doors are closed. This way if anyone opens the doors to access the module, its high voltage capabilities are temporarily disabled until the doors are closed again. Users should confirm on a regular basis that the safety interlock is functioning by performing a safety interlock test. All system operators should receive proper training so that they understand the potential hazards and how to protect themselves.

Pins 3 and 4 allow the safety interlock circuitry to be daisy-chained across multiple modules so that every high-voltage module in the system limits itself to ±40 V when the enclosure door is opened. 
Figure 2. Daisy-Chained System-Level Safety Interlock Connection

Do not apply voltage to the safety interlock connector inputs. The interlock connector is designed to accept passive normally open contact closure connections only.