This paper presents a time division multiple access medium access control protocol for wireless sensor / actuator networks implemented with a contention-free message scheduler. A message scheduler is used to determine which message has access to the medium at any time. A set of messages is contention-free if only one message is ready at a time. Otherwise, some other criterion such as priority must be used to resolve contention. In this case the message scheduler at each node in the network must schedule all messages in order to resolve contention. If the schedule is contention-free however, then each node only schedules the messages that interest it. The key contribution of our contention-free scheduler protocol is scalability. Large wireless sensor / actuator networks may contain hundreds of nodes exchanging thousands of messages. Due to resource constraints it is infeasible that each node schedule every message. Our protocol scales by optimizing each node to schedule only the messages it is interested in.