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4.1.2.1
BUS FREE PHASE
The BUS FREE phase, indicating that the bus is available for use,
is entered by the deassertion and passive release of all bus
signals.
Once the BUS FREE phase is detected, the active initia-
tor must deassert and passively release all bus signals (within a
BUS CLEAR DELAY) after deassertion of BSY.
Devices sense BUS FREE when SEL and BSY are not asserted
(simultaneously within a DESKEW DELAY of 45 nsec) and the RESET
condition is not active.
4.1.2.2
ARBITRATION PHASE
The ARBITRATION phase allows one SCSI device to gain control of
the bus.
Once a device that wants to arbitrate for
the bus
detects the BUS FREE phase, it waits a BUS FREE DELAY, 800 nsec,
and then asserts BSY with its own
1.0.
bit on the Data Bus.
(Data
parity is not guaranteed valid during ARBITRATION.)
After an ARBITRATION DELAY, the device examines the data bus.
If
a higher priority
1.0.
is on the bus or a select from another
device is present,
the device clears itself from arbitration by
releasing the BSY and
1.0.
signals.
If the device determines
that its own
1.0.
is the highest priority, it leaves BSY asserted
and asserts SELECT.
ARBITRATION is then complete.
4.1.2.3
SELECTION PHASE
During the SELECTION phase,
the I/O signal is deasserted so that
this phase can be distinguished from the RESELECTION phase.
The
ACB-5500 operates in both arbitrating and non-arbitrating sys-
tems.
In non-arbitrating systems, there may be only one Initia-
tor.
It may raise select with the Target
1.0.
asserted whenever
the BUS FREE Phase is
p~esent.
In arbitrating systems, the SELECTION phase is entered with both
BSY and SEL asserted;
the Initiator then waits a minimum of a
Bus C I ear
0
e 1 a y p 1 usa Bus Set tIe
0
e 1 a y be for e d r i v i n g the- D A T A
bus with
the Target
1.0.
and
its own
1.0.
After
two DESKEW
DELAYS, the Initiator then releases BSY.
If only one Initiator
is installed, and no disconnection is supported, the Initiator
need not provide its own
1.0.
On detecting
the condition that Select and
its own
1.0.
are
asserted, and BSY and I/O are not asserted, the selected Target
examines the DATA bus for the Initiator
1.0.
and responds by
asserting BSY.
500504-00
4-4
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