Since a bus is basically just a highway between two devices, there is always a directionality implicit in the transfer, that is, there is always a sender and one or more receivers. One component has to initiate the transfer and it usually does this by asserting logic values on the control wires. The receivers pay attention and notice changes in the values. We call the sender the initiator because it initiates or starts the activity. The other device is called the responder because it waits until the activity starts and it receives a start signal.

If more than one component tries to control the bus by setting the control wires to different logic values, then chaos will result. Thus, only one device can be in control at any point of time, that device is called the bus initiator. The device to which the control signals are directed is called the bus responder. These roles flip back and forth, as one device might be the initiator for one transfer and the responder for the next. It is also possible for there to be multiple responders in a single bus operation.