SCSI data is sent using a series REQuests and ACKnowledgements, or REQs and ACKs. When data is sent Asynchronously, each REQ must receive an ACK before the next signal is sent. All commands are sent Asynchronously over the 8 bit bus.
When data is transferred Synchronously, multiple REQs can be issued, up to the maximum offset (number of outstanding REQs), and then wait for the corresponding number of ACKs to be returned. Using Synchronous transfers can greatly improve throughput.
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