Computer Organization: Asynchronous Data Transfer Methods

Asynchronous Data Transfer

• Asynchronous data transfer takes place in two ways: Strobe Control and Hand Shaking.

Strobe Control

• Uses a single control line to time each transfer.
• The strobe can be activated by the source or the destination unit.
• Source Initiated Data Transfer:
  • The data bus carries information from source to destination.
  • The strobe signal informs the destination unit when a data word is available in the bus.
  • The strobe signal is given after a brief delay, after placing the data on the data bus.
  • A brief period after the strobe pulse is disabled, the source stops sending the data.
• Destination Initiated Data Transfer:
  • The destination unit activates the strobe pulse, informing the source to send data.
  • The source places the data on the data bus.
  • The transmission is stopped briefly after the strobe pulse is removed.
• Disadvantages of strobe control:
  • The source unit has no way of knowing whether the destination unit has received the data or not.
  • The destination unit has no way of knowing whether the source unit has placed data on the bus or not.

Hand Shaking Method of Data Transfer

• Solves the difficulties of strobe control by using a second control signal that provides a reply to the unit that initiates the transfer.
• Source Initiated Transfer Using Hand Shaking:
  • The Fig. 4.4 shows the data transfer procedure when initiated by the source.

Comparison of Synchronous and Asynchronous Data Transfer

• Synchronous Transmission:
  • Transmission starts with a block header which holds a sequence of bits.
  • Data is sent in the form of blocks or frames. +Present with the same clock pulse.
  • Advantages: Fast, efficient use of transmission line, and no gap between data.
  • Disadvantage: Costly.
• Asynchronous Transmission:
  • Uses start and stop bits preceding and following a character respectively.
  • Data is sent in the form of byte or character.
  • Absent with the same clock pulse.
  • Advantages: Economical, and transmission line remains empty during gap in character transmission.
  • Disadvantages: Slow, and there is a gap between data.