Synchronous Negotiation Started By The Initiator - HP 2600fx - Optical Disk Drive Command Reference Manual

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