Have common clock synchronous communication, the sequence of all the devices on the bus according to the unified uniform transmission cycle of information transmission, communication both sides a good timing to contact by the agreement,

Asynchronous communication without a common clock, no fixed transmission cycles, using responder communication, specific contact have not interlock, half interlocking, interlocking, all not interlock without mutual restriction relation between two communication parties, half interlock relations have simple communication both sides, the interlock not completely constraint relationship,