The 2019 North American Elevator Safety Code (A17.1-2019 / B44-19; A17.1-2022 / B44-22) defines new and clarifies existing requirements for the means of detecting persons or objects between the doors (2D) or approaching the elevator (3D).
Slack cable switches are used to detect the suspension tension on vertically moving doors. These stop the motorized drive as soon as the door movement is obstructed, e.g. if the suspension cable slackens.