Parallel data transfer maybe of two type :-

1. Synchronous: - This type of data transfer is used when device which sends data and devices which receives data are synchronised with the same clock. Works when IO devices and the CPU works with the same speed. IN/OUT instructions are used to transfer data from IO devices to memory and vice versa. Generally used in IO mapped IO scheme, can also be used with memory mapped IO scheme with proper memory read/write instruction. Data is transferred as soon as CPU gives instruction to do so. There is no need to check if the device is ready or not. 

2. Asynchronous: - It means at “regular intervals”. This type of data transfer scheme is used when speed of the IO device does not match with that of CPU. There is no predictability of timing characteristics. The microprocessor always pings the other device to check whether it’s ready or not. During initiation the CPU checks whether device is ready to transfer data, before the actual transfer of data the memory keeps sending signal to IO device. This is called handshaking. The CPU sends initialising signal to device during start and after actual data transfer.