A downhole tool for incorporation in a drill pipe for selective operation of the tool from surface level when the tool is in a wellbore. The tool comprises a controller electrically powered by a power source separate from surface level, a first sensor of the controller to detect a dynamic variable of the tool in the wellbore and produce an output signal dependent thereon; a second sensor of the controller to detect a mechanical signal transmitted from an operator at surface level; a motor driven by the power source under the control of the controller when said mechanical signal is received; and an actuator driven by the motor to actuate the tool; wherein the controller switches between at least two states in response to changes in said dynamic variable, only in said second state the controller being receptive to said mechanical signal from the operator to drive the motor. The tool may be a disconnect device and/or a circ sub. A disconnect device has axially displaceable die retention sleeve preventing radial movement of dies that lock between the dies and a mandrel. When the retention sleeve is moved so that the dies can move, the disconnect is actuated. A circsub has a body having a throughbore receiving a piston movable between open and closed positions to control ports in the body selectively connecting the throughbore with the wellbore, said motor driving said actuator to enable or disable movement of the piston to said open position.