HP Surestore 80fx Optical Drive and Library SCSI-2 Command Reference - Page 26

SCSI Command Overview SCSI Command Overview Drive Control Commands The transfer period is the minimum time allowed between leading edges of successive REQ pulses and of successive ACK pulses. The REQ/ACK offset is the maximum number of REQ pulses allowed to be outstanding before the leading edge of its corresponding ACK pulse is received at the target. This value prevents overflow conditions in the device's reception buffer and offset counter. A REQ/ACK offset value of zero indicates asynchronous mode. The initiator sets its values according to the rules above that permit it to receive data successfully. If the drive can also receive data successfully with these values (or smaller transfer period or larger REQ/ACK offset or both), it returns the same values in its SDTR message. Synchronous Negotiation Started by the Initiator If the initiator recognizes that negotiation is required, it asserts the ATN signal and sends a SDTR message to begin the negotiating process. After successfully completing the MESSAGE OUT phase, the drive will respond with the proper SDTR message. If an abnormal condition prevents the drive from returning an appropriate response, both devices will go to asynchronous mode for data transfers between the two devices. The drive responds to each initiator requested transfer period as shown in Table 1-3 on page 1-8. Chapter 1 1-7