Bus Signals
18 SL500: Interface Reference Manual • July 2008 Revision: F • 96122
Bus Signals
Communication between two devices on the SCSI bus occurs any time after they
establish connection using the bus phases. When two devices communicate, one
device acts as an initiator and the other device acts as a target.
FIGURE 3-2 shows the source and direction of the SCSI bus signals.
TABLE 3-2 Bus Signals
BSY Busy is a control signal that indicates the SCSI bus is in use.
SEL Select is a control signal that selects a target or initiator.
C/D Control and Data signals are driven by the target to indicate whether
control or data information is being transferred.
I/O Input and Output are control signals driven by the target to control the
direction data travels on the bus (with respect to the initiator).
MSG Message is a control signal driven by the target.
REQ Request is a control signal driven by the target to indicate a request for a
REQ/ACK data transfer handshake.
ACK Acknowledge is a control signal driven by an initiator to indicate an
acknowledgment for a REQ/ACK data transfer handshake.
ATN Attention is a control signal driven by an initiator to indicate an Attention
condition.
RST Reset is a control signal that generates a Reset condition when asserted.
DB(0-n) Data bus signals (0-n) transfer data where n indicates the number of data
bus signals.
DB(P,P1) Data bus parity signals (P,P1) are undefined during the Arbitration phase
and are defined as odd parity during data transfer.
FIGURE 3-2 Signal Source and Direction
TARGETINITIATOR
BSY
SEL
C/D
I/O
MSG
REQ
ACK
ATN
RST
DB (0-n) DB (P,P1)
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