In a wireless sensor and actuator network (WSAN), a group of sensor nodes, actuators, and actuation devices are geographically distributed and linked by wireless networks. Sensor nodes gather information for an event occurring in the physical world and send them to actuators. Actuators perform appropriate actions on actuation devices by making a decision on receipt of sensed values from sensor nodes. Sensor nodes are low cost, low powered devices with limited energy, computation, and wireless communication capabilities. Messages may be lost due to collision and noise and sensor nodes may be faulty. Here, nodes are required to reliably communicate with each other in realtime manner. In order to achieve the reliability, we discuss the multi-actuator/multi-sensor (MAMS) model where each sensor node sends sensed values to multiple actuators and each actuator receives sensed values from multiple sensor nodes in an event area. Even if messages are lost and sensor nodes are faulty, actuators can surely receive sensed values. An actuator makes a decision on what actions to be performed in what order. We discuss a semi-passive coordination (SPC) protocol for multiple actuators to communicate with multiple sensor nodes and perform actions. We evaluate the SPC protocol in terms of the number of messages exchanged among actuators.