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Access to the SCSI bus
is handled through arbitration.
The
arbitrating SCSI device with the highest bus address is given
priority.
The Initiator selects a Target.
The Target then
manages all further communications, requesting commands from the
Initiator,
transferring the required data,
and transmitting
ending status.
A Target may reselect an Initiator to complete a
disconnected operation.
Data transfers on the bus are asynchronous and follow adef i ned
REQUEST/ACKNOWLEDGE protocol.
One eight-bit byte of information
may be transferred with each handshake.
4.1.1
BUS SIGNALS
The SCSI bus consists of
9
control signals and
9
data signals.
These are described below:
4.1.1.1
BUSY (BSY)
BSY is an "or-tied" signal which indicates that the bus is in
use.
4.1.1.2
SELECT (SEL)
SEL is an "or-tied" signal used by an Initiator to select a
Target or by a Target to reselect an Initiator.
4.1.1.3
CONTROL/DATA (C/O)
C/D is a Target-driven signal to indicate whether CONTROL or DATA
information is on the data bus.
Assertion indicates CONTROL.
Outbound control information is a command while inbound control
information is status.
4.1.1.4
INPUT/OUTPUT (I/O)
I/O is a Target-driven signal which controls the direction of
data movement on the data bus
relative
to
an
Initiator.
A~sertion
indicates INPUT to the Initiator.
4.1.1.5
MESSAGE (MSG)
MSG is a Target-driven signal indicating the MESSAGE phase.
4.1.1.6
REQUEST (REQ)
REQ is a Target-driven signal indicating a request for a REQ/ACK
data transfer handshake.
4.1.1.7
ACKNOWLEDGE (ACK)
ACK is an Initiator-driven signal, in response to
a
target REQ,
indicating the transfer of a byte to or from the ACB-5500
500504-00
4-2
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