Asynchronous Data Transfer

Asynchronous Data Transfer:- The internal operations in a digital system are synchronized by means of clock pulses supplied by a common pulse generator. Clock pulses are applied to all registers within a unit and all data transfers among internal registers occur simultaneously during the occurrence of a clock pulse. Two units, such as a CPU and an I/O interface, are designed independently of each other. If the registers in the interface share a common clock with the CPU registers, the transfer between the two units is said to be synchronous. In most cases, the internal timing in each unit is independent of the other in that each uses its own private clock for internal registers. In that case, the two units are said to be asynchronous to each other. This approach is widely used in most computer systems. Here we provided Asynchronous Data Transfer in Computer Architecture.


Asynchronous Data Transfer


Asynchronous data transfer between two independent units requires that control signals be transmitted between the communicating units to indicate the time at which data is being transmitted. One way of achieving this is by means of a strobe pulse supplied by one of the units to indicate to the other unit when the transfer has to occur. Another method commonly used is to accompany each data item being transferred with a control signal that indicates the presence of data in the bus. The unit receiving the data item. responds with another control signal to acknowledge receipt of the data. This type of agreement between two independent units is referred to as handshaking.

The strobe pulse method and the handshaking method of asynchronous data transfer are not restricted to I/O transfers. In fact, they are used extensively on numerous occasions requiring the transfer of data between two independent units. In the general case, we consider the transmitting unit as the source and the receiving unit as the destination. For example, the CPU is the source unit during output or a write transfer and it is the destination unit during input or a read transfer. It is customary to specify the asynchronous transfer between two independent units by means of a timing diagram that shows the timing relationship that must exist between the control signals and the data in the buses. The sequence of control during an asynchronous transfer depends on whether the transfer is initiated by the source or by the destination unit.


Strobe Control

The strobe control method of asynchronous data transfer employs a single control line to time each transfer. The strobe may be activated by either the source or the destination unit. The figure shows a source initiated transfer. The data bus carries the binary information rom source unit to the destination unit. Typically, the bus has multiple lines to transfer an entire byte or word. The strobe is a single line that informs the destination unit when a valid data word is available on the bus.



The disadvantage of the strobe method is that the source unit that initiates the transfer has no way of knowing whether the destination unit has actually received the data item that was placed in the bus. Similarly, a destination unit that initiates the transfer has no way of knowing whether the source unit has actually placed the data on the bus.

The handshake method solves this problem by introducing a second control signal that provides a reply to the unit that initiates the transfer. The basic principle of the two-wire handshaking method of data transfer is as follows. One control line is in the same direction as the data flow in the bus from the source to the destination. It is used by the source unit to inform the destination unit whether there are valid data in the bus. The other control line is in the other direction from the destination to the source. It is used by the destination unit to inform the source whether it can accept data. The sequence of control during the transfer depends on the unit that initiates the transfer.

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