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4.2.3 Bus signal drive conditions
SCSI devices drive signals of the SCSI bus. The types of SCSI devices are
summarized in the following table, showing the signals that they can drive for each
operating phase of the interface.
There are two kinds of signal driving methods, OR tied and NON>OR tied, as shown
in Table 4.2. During an interface operating sequence, the BSY signal could be
driven simultaneously by two or more SCSI units when the data bus is in the
ARBITRATION or RESELECTION phase. This situation also occurs with the RST
signal (Reset). These two signals must be ORtied. For the other signals, either of
the two methods may be used; further more, different drive methods may coexist for
a signal on the bus.
Table 4.2 Bus phases vs. signal drive sources (1/2)
!!!! Signal
Bus phase
BSY SEL I/ O C/ D
MSG
REQ ACK DB7 to 0
DBP
ATN RST
BUS FREE N N N N N N N N A
ARBITRATION A W N N N N ID N A
SELECTION I&T I N N N I I I A
RESELECTION I&T T T T T I T I A
COMMAND T N T T T I I I A
DATA IN T N T T T I T I A
DATA OUT T N T T T I I I A
STATUS T N T T T I T I A
MESSAGE IN T N T T T I T I A
MESSAGE OUT T N T T T I I I A
N: The signal shall be released, since it is not being driven by any SCSI device.
A: The signal shall be driven by all SCSI devices that are actively arbitrating.
I: If driven, this signal shall be driven only the active initiator.
T: If the signal is driven, it shall be driven only by the active target.
W: The signal shall be driven by the one SCSI device that wins arbitration.
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