synchronous bus is a bus used to interconnect devices that comprise a computer system where the timing of transactions between devices is under the control of a synchronizing clock signal.
 A device connected to a synchronous bus must guarantee to respond to a command within a period set by the frequency of the clock signal or a transmission error will occur. Such buses are usually employed in closely controlled processor backplane environments where device characteristics and interdevice signal delays are known.
It incude a clock in the control line.
It has a fixed protocol for communication relative to its clock.
Every device on the bus must run with the same speed.