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Figure 7.5 BUS FREE phase
Transition to a BUS FREE phase occurs when the TARG stops the BSY signal in one of the
following events:
• When the RESET condition has been detected
• When the TARG has received ABORT message
• When the TARG has received a BUS DEVICE RESET message
• When the TARG has sent a DISCONNECT message normally
• When the TARG has sent a COMMAND COMPLETE message normally
If a SELECTION or RESELECTION phase is terminated unsuccessfully, the SCSI bus enters a
BUS FREE phase. This BUS FREE phase is generated when the SEL signal becomes false.
In cases other than the above, if the TARG negates the BSY signal to enter a BUS FREE phase,
the TARG informs the INIT that it has detected an ERROR condition on the SCSI bus. The
TARG can enter a BUS FREE phase forcedly regardless of the ATN signal status; the INIT must
treat that phase transition as indicating abnormal end of a command. The TARG clears all
retained data or status and terminates the command being executed. It can then create sense data
indicating the detailed error condition. If the INIT detects a BUS FREE phase when it is not
expected, it should issue a REQUEST SENSE command to read the sense data.
7.5.2 ARBITRATION phase
The ARBITRATION phase allows one SCSI device to gain control of the SCSI bus so that an
INIT starts the SELECTION phase or a TARG starts the RESELECTION phase.