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Computer organization Notes


The handshaking method solves the problem of strobe method by introducing a second control signal that provides a reply to the unit that initiates the transfer. Principle of Handshaking The basic principle of the two-wire handshaking method of data transfer is as follow: One control line is in the same direction as the data flows in the bus from the source to destination. It is used by source unit to inform the destination unit whether there a 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 the data. The sequence of control during the transfer depends on the unit that initiates the transfer.
Source Initiated Transfer using Handshaking
Figure below shows the data transfer procedure when initiated by the source. The two handshaking lines are data valid, which is generated by the source unit, and data accepted, generated by the destination unit. The timing diagram shows the exchange of signals between the two units.

The sequence of events shows four possible states that the system can be at any given time. The source unit initiates the transfer by placing the data on the bus and enabling its data valid signal. The data accepted signal is activated by the destination unit after it accepts the data from the bus. The source unit then disables its data valid signal, which invalidates the data on the bus. The destination unit then disables its data accepted signal and the system goes into its initial state. The source does not send the next data item until after the destination unit shows its readiness to accept new data by disabling its data accepted signal. This scheme allows arbitrary delays from one state to the next and permits each unit to respond at its own data transfer rate. The rate of transfer is determined by the slowest unit.
Destination Initiated Transfer using Handshaking
The name of the signal generated by the destination unit has been changed to ready for data to reflect its new meaning. The source unit in this case does not place data on the bus until after it receives the ready for data signal from the destination unit. From there on, the handshaking procedure follows the same pattern as in the source initiated case if we consider the ready for data signal as the complement of data accepted. The only difference between the Source Initiated and the Destination Initiated transfer is in their choice of Initial sate.

The handshaking scheme provides a high degree of flexibility and reliability because the successful completion of a data transfer relies on active participation by both units. If one unit is faulty, the data transfer will not be completed. Such an error can be detected by means of a timeout mechanism, which produces an alarm if the data transfer is not completed within a predetermined time. The timeout is implemented by means of an internal clock that starts counting time when the unit enables one of its handshaking control signals. If the return handshake signal does not respond within a given time period, the unit assumes that an error has occurred. The timeout signal can be used to interrupt the processor and hence execute a service routine that takes appropriate error recovery action.

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