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d. When switching the Data Bus or DB(P_CRCA) signal direction from out (SCSI initiator port driving) to in
(SCSI target port driving), the target shall delay driving the Data Bus, DB(P_CRCA), and/or DB(P1) by at
least one data release delay plus one bus settle delay after asserting the I/O signal, and the initiator shall
release the Data Bus, DB(P_CRCA), and/or DB(P1) no later than one data release delay after the transition
of the I/O signal to true. When switching the Data Bus, DB(P_CRCA), and/or DB(P1) direction from in (tar-
get driving) to out (initiator driving), the target shall release the Data Bus, DB(P_CRCA), and/or DB(P1) no
later than one system deskew delay after negating the I/O signal. The initiator shall assert the Data Bus,
DB(P_CRCA), and/or DB(P1) no sooner than one system deskew delay after the detection of the negation
of the I/O signal.
e. The DB(P_CRCA) signal direction may switch direction while the Data Bus and/or DB(P1) does not (e.g.,
changing from COMMAND phase to DT DATA OUT phase). When switching the DB(P_CRCA) signal direc-
tion from out (initiator driving) to in (target driving), the target shall delay driving the DB(P_CRCA) by at
least one data release delay plus one bus settle delay after negating the C/D signal and the initiator shall
release the DB(P_CRCA) signal no later than one data release delay after the transition of the C/D signal to
false. When switching the DB(P_CRCA) signal direction from in (target driving) to out (initiator driving), the
target shall release the DB(P_CRCA) signal no later than one system deskew delay after asserting the C/D
signal. The initiator shall negate the DB(P_CRCA) signal no sooner than one system deskew delay after the
detection of the assertion of the C/D signal.
f. The ATN and RST signals may change as defined under the descriptions for the attention condition (see
Section 5.1) and hard reset (see Section 5.3).
3.11
SCSI bus phase sequences
3.11.1
SCSI bus phase sequences overview
The order in which phases are used on the SCSI bus follows a prescribed sequence.
During DT DATA phases, the target shall not change phases except at data group boundaries or SPI informa-
tion unit boundaries. If a SCSI initiator port detects a phase change within a data group or information unit, it
shall consider any data transferred for that data group or information unit to have been transferred incorrectly.
The SCSI initiator port shall consider this condition a protocol error and respond accordingly.
A hard reset aborts any phase and is always followed by the BUS FREE phase. Also, any phase may be fol-
lowed by the BUS FREE phase, but many such instances are exception conditions for SCSI initiator ports (see
Section 3.1.1).
3.11.2
Phase sequences for physical reconnection and selection using attention condition with
information unit transfers disabled
The allowable sequences for either physical reconnection or selection using attention condition while an infor-
mation unit transfer agreement is not in effect shall be as shown in Figure 11. The normal progression for
selection using attention condition (see Section 3.2.1.1) is:
1. from the BUS FREE phase to ARBITRATION;
2. from ARBITRATION to SELECTION or RESELECTION; and
3. from SELECTION or RESELECTION to one or more of the information transfer phases (i.e., COMMAND,
DATA, STATUS, or MESSAGE).
78
Parallel SCSI Interface Product Manual, Rev. A
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