Discovering connection of passive cables

In one example in accordance with the present disclosure, a subsystem is provided. The subsystem includes a signal driver/receiver capable of sending and receiving data and signals over a passive cable. The subsystem includes a connection discovery engine to access a low-level enable or disable control of the signal driver/receiver and a low-level loss-of-signal (LOS) control of the signal driver/receiver. The connection discovery engine modulates the enable or disable control to send a local unique ID of the signal driver/receiver over the passive cable. The connection discovery engine monitors timings of transitions on the LOS control to receive, over the passive cable, a remote unique ID of a signal driver/receiver in a second subsystem connected by the passive cable.