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SCSI BUS
7-18 C156-E228-02EN
7.6 Bus Phases
The SCSI bus must be in one of the following eight phases:
• BUS FREE phase
• ARBITRATION phase
• SELECTION phase
• RESELECTION phase
• COMMAND phase
• DATA phase INFORMATION TRANSFER phase
• STATUS phase
• MESSAGE phase
The SCSI bus can never be in more than one phase at any given time.
Note: In the following bus phase definition, signals are false unless
otherwise defined. Signals on the timing charts are assumed to be
positive logic.
7.6.1 BUS FREE phase
No SCSI device uses the bus during a BUS FREE phase. SCSI devices shall
detect the BUS FREE phase after SEL and BSY signals are both false for at least
Bus Settle Delay.
SCSI devices which have detected the BUS FREE phase shall release all bus
signals within Bus Clear Delay after BSY and SEL become false for a Bus Settle
Delay. If an SCSI device requires more than Bus Settle Delay to detect the BUS
FREE phase, it shall release all bus signals within the following period (t):
t = (Bus Clear Delay) – (Period required for BUS FREE phase detection)
+ (Bus Settle Delay)
The maximum time allowed for releasing the bus after both SEL and BSY
becomes false is 1.2 µs.
Figure 7.9 shows the BUS FREE phase.