  We present a new and powerful class of automata which are explicitly
concurrent and allow a very simple definition of composition. The novelty of
these automata is their time-synchronous message-asynchronous communication
mechanism. Time synchrony is obtained by using global clock. Message asynchrony
is obtained by requiring the automata to react to every input. Explicit
concurrency is obtained by marking each transition with a set of input and
output messages. We compare these automata with a history based approach which
uses the same communication mechanism and show that they are equivalent.
