disabled until the safety sub 106 has been activated to close an electrical connection in the example of fig1 the cable 102 is run through a winch assembly 110 which is coupled to a depth sensor 112 the depth sensor 112 monitors the rotation of the winch assembly 110 to determine the depth of the perforating tool 104 the data relating to the depth of the tool 104 is communicated to the surface unit 100 in some systems an internal hardware or software drive system can be used to simulate that the tool 104 has descended to a certain depth in the wellbore even though the tool 104 is still at the earth surface the depth sensor 112 can be used by the surface unit to verify that the tool 104 has indeed been lowered into the wellbore to a target depth as a safety precaution the ability to use the output of the internal hardware or drive system to enable activation of the tool 104 is prohibited the perforating tool 104 also includes a number of sensors such as sensors 114 in the safety sub and sensors 116 in the guns 108 although fig1 shows each gun 108 as containing sensors 116 less than all of the guns can be selected to include sensors in other embodiments data from the sensors 114 and 116 are communicated over the cable 102 to a logging module 120 in the surface unit 100 the logging module 120 is capable of performing bi directional communications with the sensors 114 and 116 over.