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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. Figure below shows a source-initiated transfer.

Block Diagram and Timing Diagram for Source-Initiated strobe for Data Transfer
The data bus carries the binary information from 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 in the bus. As shown in the timing diagram of Figure above, the source unit first places the data on the data bus. After a brief delay to ensure that the data settle to a steady value, the source activates the strobe pulse. The information on the data bus and the strobe signal remain in the active state for a sufficient time period to allow the destination unit to receive the data. Often, the destination unit uses the falling edge of the strobe pulse to transfer the contents of the data bus into one of its internal registers. The source removes the data from the bus a brief period after it disables its strobe pulse. Actually, the source does not have to change the information in the data bus. The fact that the strobe signal is disabled indicates that the data bus does not contain valid data. New valid data will be available only after the strobe is enabled again. Destination-Initiated strobe for Data Transfer Figure below shows a data transfer initiated by the destination unit. In this case the destination unit activates the strobe pulse, informing the source to provide the data. The source unit responds by placing the requested binary Information on the data bus. The data must be valid and remain in the bus long enough for the destination unit to accept it. The falling edge of the strobe pulse can be used again to trigger a destination register. The destination unit then disables the strobe. The source removes the data from the bus after a predetermined time interval.
Block Diagram and Timing Diagram for Destination-Initiated strobe for Data Transfer
In many computers the strobe pulse is actually controlled by the clock pulses in the CPU. The CPU is always in control of the buses and informs the external units how to transfer data. The disadvantage of the strobe method is that, the source unit initiates the transfer has no way of knowing whether the destination unit has actually received the data item that was places 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 bus. The Handshaking method solves this problem.


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