Seagate Ultra 320 - Manual (Page 75)

Parallel SCSI Interface Product Manual, Rev. A 61
Synchronous transfers shall only be used for negotiated transfer rates less than or equal to Fast-80.
Paced transfers shall only be used for a negotiated transfer rate of Fast-160.
3.5.1 Asynchronous transfer
The target shall control the direction of information transfer by means of the I/O signal. When the I/O signal is
true, information shall be transferred from the target to the initiator. When the I/O signal is false, information
shall be transferred from the initiator to the target.
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 desired values, delay at least one system deskew delay plus one
cable skew, then assert the REQ signal. The DB(7-0,P_CRCA) or DB(15-0,P_CRCA,P1) signals shall remain
valid until the ACK signal is true at the target. The initiator shall read the DB(7-0,P_CRCA) or DB(15-
0,P_CRCA,P1) signals after the REQ signal is true, then indicate its acceptance of the data by asserting the
ACK signal. When the ACK signal becomes true at the target, the target may change or release the DB(7-
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 ini-
tiator 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