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7.2 Interface Signal Definition
C156-E228-02EN 7-5
(3) SEL (SELECT)
This signal is used by an INIT to select a TARG (SELECTION phase) or by a
TARG to reselect an INIT (RESELECTION phase).
(4) C/D (CONTROL/DATA)
This signal is driven by a TARG to identify the type of information transferred on
the DATA BUS in combination of I/O and MSG signals. (See Table 7.1)
(5) I/O (INPUT/OUTPUT)
This signal is driven by a TARG to specify the information transfer direction on
the DATA BUS or to identify a SELECTION or RESELECTION phase. (See
Table 7.1)
(6) MSG (MESSAGE)
This signal is driven by a TARG to indicate that a message is being transferred on
the DATA BUS. (See Table 7.1)
(7) REQ (REQUEST)
This signal is driven by a TARG to indicate a transfer request to an INIT in
INFORMATION TRANSFER phase.
(8) ACK (ACKNOWLEDGE)
This signal is driven by an INIT to indicate a response for REQ signal to a TARG
in the INFORMATION TRANSFER phase.
(9) ATN (ATTENTION)
This signal is driven by an INIT to indicate that the INIT has a message to be
transferred to the TARG and is used to generate an ATTENTION condition.
(10) RST (RESET)
This signal indicates the RESET condition which is used to clear all SCSI devices
on the bus.
Table 7.1 INFORMATION TRANSFER phase identification
C/D I/O MSG DB7-0, P Direction Phase
0 0 0 Data
INIT → TARG
DATA OUT
0 1 0 Data
INIT ← TARG
DATA IN
1 0 0 Command (CDB)
INIT → TARG
COMMAND
1 1 0 Status
INIT ← TARG
STATUS
0 0 1 – – not used
0 1 1 – – not used
1 0 1 Message
INIT → TARG
MESSAGE OUT
1 1 1 Message
INIT ← TARG
MESSAGE IN