A fluid-operated logic device suitable for operating an annunciator system, or the like, which device has a bore with a piston valve member slidable therein. The piston has a transverse alert crossover to permit fluid flow between a constant pressure conduit and an alert conduit leading to an alert indicator. The piston is also provided with a crossover which permits communication between a conduit in which pressure increases in response to the detection of trouble and a monitor conduit leading to a monitor alarm. A conduit branches from the trouble conduit and is connected to the casing to act against the head end of the piston valve member. Upon an increase in pressure in the trouble circuit, the piston will be moved to the alert position where the alert alarm will be given until this condition has been acknowledged, the kinetic energy of the moving piston being employed. The piston is then returned to its normal position by an acknowledge arrangement, but should the trouble condition persist, the piston will return to and remain in its monitor position to indicate that the trouble condition has not been removed. When the trouble disappears, fluid pressure will move the piston to its normal position.