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0,P_CRCA) or DB(15-0,P_CRCA,P1) signals and shall negate the REQ signal. After the REQ signal is false,
the initiator shall then negate the ACK signal. After the ACK signal is false, the target may continue the transfer
by driving the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals and asserting the REQ signal as described
above.
If the I/O signal is false (i.e., transfer to the SCSI target port), the target shall request information by asserting
the REQ signal. The SCSI initiator port shall drive the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals to
their values, delay at least one system deskew delay plus one cable skew, and assert the ACK signal. The initi-
ator shall continue to drive the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals until the REQ signal is false.
When the ACK signal becomes true at the target, the target shall read the DB(7-0,P_CRCA) or DB(15-
0,P_CRCA,P1) signals then negate the REQ signal. When the REQ signal becomes false at the initiator, the
initiator may change or release the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals and shall negate the
ACK signal. After the ACK signal is false, the target may continue the transfer by asserting the REQ signal as
described above.
3.5.2
Synchronous transfer
Synchronous transfer is optional and is only used in DATA phases. It shall be used in a DATA phase if a syn-
chronous transfer agreement has been established (see section 4.3.16 or 4.3.12). The transfer agreement
specifies the REQ/ACK offset and the transfer period.
When synchronous data transfers are being used, data may be transferred using ST data transfers or, option-
ally, DT data transfers. DT data transfers shall only be used on 16-bit-wide buses that transmit and receive data
using LVD transceivers.
Implementors shall not use this section for timing requirements. For timing requirements, see Section 2.5.
3.5.2.1
ST synchronous data transfer
When a ST data transfer agreement has been established, the SCSI target port shall only use the ST DATA IN
phase and ST DATA OUT phase for data transfers.
The REQ/ACK offset specifies the maximum number of REQ assertions that shall be sent by the target in
advance of the number of ACK assertions received from the initiator, establishing a pacing mechanism. If the
number of REQ assertions exceeds the number of ACK assertions by the REQ/ACK offset, the target shall not
assert the REQ signal until after the next ACK assertion is received. For successful completion of the ST DATA
phase, the number of ACK and REQ assertions shall be equal.
For the timing requirements of the negotiated transfer period see Section 3.5.2.
If the I/O signal is true (i.e., transfer to the SCSI initiator port), the target shall first drive the DB(7-0,P_CRCA)
or DB(15-0,P_CRCA,P1) signals to their values, wait at least one transmit setup time, then assert the REQ sig-
nal. The DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals shall be held valid for a minimum of a transmit
hold time after the assertion of the REQ signal. The target shall assert the REQ signal for a minimum of one
transmit assertion period. The target may then negate the REQ signal and change or release the DB(7-
0,P_CRCA) or DB(15-0,P_CRCA,P1) signals. The initiator shall read the value on the DB(7-0,P_CRCA) or
DB(15-0,P_CRCA,P1) signals within one receive hold time of the transition of the REQ signal to true. The SCSI
initiator port shall then respond with an ACK assertion.
If the I/O signal is false (i.e., transfer to the SCSI target port), the SCSI initiator port, after detecting a REQ
assertion, shall first drive the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals to their values, delay at least
one transmit setup time, then assert the ACK signal. The initiator shall hold the DB(7-0,P_CRCA) or DB(15-
0,P_CRCA,P1) signals valid for at least one transmit hold time after the assertion of the ACK signal. The initia-
tor shall assert the ACK signal for a minimum of one transmit assertion period. The SCSI initiator port may then
negate the ACK signal and may change or release the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals.
The target shall read the value of the DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals within one receive
hold time of the transition of the ACK signal to true.
3.5.2.2
DT synchronous transfer
When a DT data transfer agreement has been established the target shall only use the DT DATA IN phase and
DT DATA OUT phase for data transfers.
SCSI Interface Product Manual, Rev. B
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