Recall that the purpose of a bus protocol is to enable two brainless devices to communicate with one another by spelling out every step of the process.
The bus protocol makes sure that devices know each others' state of affair when they take the next step.
In an asynchronous protocol, the communicating devices signal each other using special control signals.
The device that the master device wants to communicate with is called the Slave Device
In a read operation (where the CPU requests that the memory sends it some data), the CPU is the master device and the memory is the slave device.
Here is an example of a completion of the asynchronous read protocol:
This diagram contains timing for 3 different operations: read, write and a read operation to a slow memory device so that the M68000 CPU has to wait (see the w above)