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Table 4.2 Bus phases vs. signal drive sources (2/2)
ID: A unique data bit (the SCSI ID) shall be driven by each SCSI device that is
actively arbitrating. The other seven data bits shall be released (shall not
driven) by this SCSI device. The parity bit (DB(P)) may be released or
driven to the true state, but shall never be driven to the false state during
this phase.
I&T: The initiator and target drive the signal according to the interface operating
sequence. The RESELECTION phase includes a sequence in which the
initiator and target simultaneously drive the signal.
The signal shall be driven by the initiator, target, or both, as specified in the
SELECTION phase and RESELECTION phase.
Table 4.3 Method of driving the interface signal
OR connection
No signal is driven by any SCSI
device. Signal status is made false by
the termination resistor circuits.
NON>OR connection
The signal is driven false by a certain
SCSI device (initiator or target), or is
not driven by any SCSI device.
False
True A SCSI device drives the signal true.
4.3 Bus Phases
The SCSI architecture includes the following eight distinct phases:
f BUS FREE phase
f ARBITRATION phase
f SELECTION phase
f RESELECTION phase
f COMMAND phase
f DATA phase
f STATUS phase
f MESSAGE phase
INFORMATION TRANSFER phase
The SCSI bus can never be in more than one phase at any given time.
The following diagram shows how each phase transits to another.
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