Abstract: A downhole system for multistage fracturing wherein a stopper assembly is installed in a plugging device in manner that allows pressure integrity from above the plugging device, and the stopper assembly is further configured to allow a bore of the plugging device to open with an at least minimal pressure from therebelow in order to remove a stopper from a baffle in order to open a flow path through the plugging device in either direction.

Abstract: For use in a control system containing first and second devices coupled together for inter-device communication, a circuit for and method of distinguishing a failure of the first device from a failure of the inter-device communication and a real-time process control system employing the circuit or the method. In one embodiment, the circuit includes: (1) a device failure signal generator, associated with and separately powered from the first device, that communicates a device failure signal to the second device upon occurrence of a predetermined condition and (2) a device failure signal detector, associated with the second device, that detects the device failure signal. Detection of the device failure signal contraindicates a failure of the inter-device communication.