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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 signs (Reset). These two signals
must be ORtied. For other signals, either of the two methods may be used: further more, different drive methods may
coexist for a signal on the bus.
Table 4-3 Bus phases vs. signal drive sources (1/2)
Signal C/D DB7 to 0
Bus phase BSY SEL '10 MSG REQ ACK DBP ATN RST
BUSFREE 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 l&T T T T T I T I A
COMMAND T N T T T I I I A
DATAIN T N T T T I T I A
DATAOUT 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 be driven only by the active initiation
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.
ID: A unique data bit (the SCSI ID) shall be driven by each SCSI device that is actively arbitrating.
The other seven data bits shall be released (shall not be driven) by this SCSI device. The parity bit
(DB(P)) may be released or driven to the true state, but shall never be driven to the false state
during this phase.
I&T: The initiator and target drive the signal according to the interface operating sequence. The
RESELECTION phase includes a sequence in which the initiator and target simultaneously drive
the signal.
The signal shall be driven by the initiator, target, or both, as specified in the SELECTION phase and RESELECTION
phase.