TOP- MOUNTED MUD-TELEMETRY PULSER ASSEMBLY FOR DOWNHOLE COMMUNICATIONS, AND DOWNHOLE VALVE

A mud pulser for generating pulses in a fluid flow in a wellbore. The mud pulser has a valve housing having an uphole opening for receiving fluid flow, and a piston valve situated in a piston chamber is longitudinally moveable between an open and closed position by closing and opening a pilot port downhole of the uphole opening and in fluid communication with the piston chamber. A first channel extends in the sidewall of the valve housing and fluidly connects the uphole opening and the pilot port. One or more fluid egress ports are in communication with the piston chamber. A metering orifice may be inserted in said one or more fluid egress ports for first channel. Opening and closing the downhole pilot port moves the piston valve and thus opens and closes the uphole opening thereby generating pressure pulses in the fluid flow. A downhole valve is further disclosed.