Section 4
SCSI Interface
4.3.3.1
Signal Values
Signals may assume true
or
false values.
If
any driver is asserted, the signal
is
true. Whenever the
tenn
negated is used in this manual, it means that the signal is actively driven false.
4.3.3.2 OR-Tied Signals
The BSY and
RST
signals are OR-tied only. During ordinary operation
of
the bus, these signals are
simultaneously driven true by several drivers. No signals other than
BSY, RST, and DB(P) are
simultaneously driven by two
or
more drivers, and any signal other than BSY and
RST
may employ OR-
tied
or
non-OR-tied drivers. DB(P) shall not be driven false during the ARBITRATION phase. There is no
operational problem in mixing
OR-tied and non-OR-tied drivers
on
signals other than
BSY
and RST.
4.3.3.3 Signal Sources
Table 4-1 indicates which type
of
SCSI device may source each signal. No attempt is made to show
if
the
source is driving asserted, driving negated,
or
is passive. All SCSI device drivers that are not active sources
must be in the passive state. Note that the
RST signal may be sourced by any SCSI device
at
any time.
TABLE
4-1
SIGNAL
SOURCES
Signal
CIO
1/0
MSG
ACK
Bus Phase BSY
SEL
REQ
ATN
DB(7-0,P)
BUS
FREE
None
None
None
None
None
ARBITRATION
All
Winner None
None
SCSIID
SELECI10N
I&T
Initiator
None
Initiator
Initiator
RES
ELECTION
I&T
Target
Target
Initiator
Target
COMMAND Target
None
Target
Initiator Initiator
DATA
IN
Target
None
Target
Initiator
Target
DATA
OUT
Target
None
Target
Initiator
Initiator
STATUS
Target
None
Target
Initiator
Target
MESSAGE
IN
Target
None
Target
Initiator
Target
MESSAGE
OUT
Target
None
Target
Initiator Initiator
Notes/or Table 4-1:
All:
The
signal is driven by all SCSI devices that are actively arbitrating.
SCSI
ID:
A unique data bit (the SCSI
10)
is driven by any SCSI device that is actively arbitrating;
the other seven data bits are released (not driven) by this
SCSI device. The parity bit
(DP(P» may be undriven
or
driven to the true state, but must never be driven to the false
state during the arbitration phase.
I&T:
The
signal is driven by the initiator, target,
or
both, as specified in the SELECTION phase
and
RESELECfION
phase.
Initiator:
If
this signal is driven, it is driven only by the active initiator.
None:
The
signal must be released, i.e., not be driven by any SCSI device. The bias circuitry
of
the
bus tenninators pulls the signal to the false state.
Winner:
The
signal is driven by the SCSI device
that
wins arbitration.
Target:
If
the signal is driven,
it
is driven only by the active target.
4 - 4 SCSI Interface
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