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I/O, C/D, and Message. The I/O, C/D, and Message Status lines define six
of these phases of the SCSI Bus (see table B-l).
Busy. The bus is considered free when the Busy and Select lines are
inactive. An initiator asks for the bus by driving Busy active and driving its
identification (ID) line active on the data lines. If no other device with a
higher priority drives the line, then the initiator signals that it has the bus
by driving Select active. This is called the Arbitration Phase. The parity
bit is not tested in the Arbitration Phase.
Select After the initiator has the bus and drives Select active it then
drives the ID line of the device it is calling (the target) active and releases
Busy. The device being called responds by driving Busy active. The
Inititor at this point releases Select and Select is not used for anything
else. The target device is now in control of the Busy line and is the only
device that can release the bus. At this point the bus is ready to transfer
information. This is called the Select or Reselect phase. Reselect is when
the Target is calling the Initiator. This is accomplished by driving the I/O
line active after the Arbitration phase.
Request and Acknowledge. These are handshake lines which are only
driven by the target These handshakes are used to transfer information
between devices in either direction. The information that is transferred
between devices is enveloped in one of six phases that are determined by
the setting of the three lines: I/O, C/D, and Message (see table B-l).
Attention. Since the target is in control of the bus lines, the Attention line
(Atn) is used by the Initiator to request to send a Message Out.
Reset This is used to reset the bus to a Bus Free State and reset the
devices that are on the bus.
Note
The SCSI standard includes over 50 commands, 7 Status
conditions, and 17 Messages in its instruction set and has room
for more to be added. The bus is defined to be capable of
transferring 5.5 Megabytes per second.
SCSI Overview
B-3
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