# What does "never actively probed" mean for our controllers?

It means the sensor never sends a packet to a device classified as OT: no discovery scan, no port scan, no protocol query, no login attempt. It only reads a copy of the traffic your own systems already exchange, taken at a SPAN port or a network TAP. Active tools work differently: they ask each device questions, and some older PLCs, RTUs and HMIs react badly to unexpected requests, from slow responses to crashes. For a plant manager, the practical meaning is that switching on NetSenX adds no traffic to the control network and cannot change a controller's state, so nothing on the controllers needs a maintenance window. The trade-off is that a passive sensor only knows what devices say: a controller that never communicates on a mirrored segment is not seen. OT monitoring is available from the Pro plan.

FAQ: [OT and IoT](https://netsenx.com/en/faq/ot) · https://netsenx.com/en/faq/what-does-never-actively-probed-mean-for-our-controllers

## Read next

- https://netsenx.com/en/ot-network-monitoring
- https://netsenx.com/en/compare

## Related questions

- [Do we need to change anything on our PLCs to monitor them?](https://netsenx.com/en/faq/do-we-need-to-change-anything-on-our-plcs-to-monitor-them)
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