Canon BJ-W9000, Service Manual

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8) Overview of SCSI bus phases
Data transfer is performed using the following eight phases:
Bus-free phase
Represents the condition where none of the SCSI devices are using the bus. In the
normal operation mode, this Bus-free phase is established when the target transmits
the BSY signal. During the Selection or Re-selection phase, the phase changes to
this Bus-free state when the time-out process is executed and the SEL signal is
transmitted.
Arbitration phase
This is the phase for allowing SCSI devices to get the right to use the bus. When a
SCSI device requests a target, the BSY signal and the SCSI ID for that SCSI device
are propagated if the current phase is Bus-free. If multiple devices request the right
to use the bus, only the device with the highest ID will transmit the SEL signal and
gain the right.
Selection phase
This is the phase where the initiator can select the device that operates as a target.
Re-selection phase
In this phase, a target device, whose connection to the initiator has been temporarily
suspended to limit the use of the bus by the same device (time out), can establish a
connection with the initiator again.
Information-transfer phase
This phase consists of the following four phases during which the control
information or data is transferred.
Data phase (In/Out)
During this phase, information contained in the command such as the type,
direction, and data-length is transferred between the target and initiator.
Specifically, the Data phase is called a Data-in phase when data is transferred from
the target to the initiator, and a Data-out phase when the transfer direction is the
opposite.
Command phase
During this phase, a command is transmitted from the initiator to the target in
response to a request by the target.
Status phase
The information indicating the result of a command execution (return value) is
transmitted from the target to the initiator.
2-11
BJ-W9000 Part 2: Product Specifications
Figure 2-8 SCSI Bus Phases
Information-transfer
phase
• Command phase
• Data phase
• Status phase
• Message phase
Reset
Bus-free phase Arbitration phase
Selection phase
Re-selection phase