This paper is concerned with the event-based control for networked control systems via dynamic output feedback controllers (DOFCs). The output measurement signals of the physical plant are sampled periodically. An output-based discrete event-triggering mechanism is introduced to choose those only necessary sampled-data packets to be transmitted through a communication network for controller design. Under this event-triggering mechanism, the resultant closed-loop system is first modeled as a linear system with an interval time-varying delay. Then Lyapunov-Krasovskii functional approach is employed to establish a novel stability criterion. Based on this stability criterion, some new sufficient conditions are established to co-design both the desired DOFCs and the event-triggering parameters. Finally, a satellite control system is taken to show the effectiveness of the proposed method.