Quantum 10K II - Manual (Page 340)

Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–19
Initiator-to-Drive Transfer Procedure
1. The drive asserts the REQ signal.
2. The initiator drives the DB (7–0, P) signals to their desired values.
3. The initiator delays at least one Deskew delay plus a cable skew delay.
4. The initiator asserts the ACK signal.
5. When ACK is true at the drive, drive reads the DB (7–0, P) signals.
6. The drive negates the REQ signal.
7. The initiator can change or release the DB (7–0, P) signals.
8. The initiator negates the ACK signal.
9. The drive can continue the transfer by asserting the REQ signal (step 1).
B.5.5.2 Synchronous Data Transfer
Synchronous Data Transfer is optional and is used only in DATA phases and only if
a synchronous data transfer agreement is established. The REQ/ACK offset specifies
the maximum number of REQ pulses that can be sent by the drive in advance of the
number of ACK pulses received from the initiator, establishing a pacing mechanism.
If the number of REQ pulses exceeds the number of ACK pulses set by the
REQ/ACK offset, the drive does not assert the REQ signal until after the leading
edge of the next ACK pulse is received. For successful completion of the data phase,
the number of ACK and REQ pulses must be equal.
The initiator sends one ACK signal pulse for each REQ pulse received. The ACK
signal can be asserted as soon as the leading edge of the corresponding REQ pulse
has been received.
Drive-to-Initiator Transfer Procedure
1. The drive sets the DB (7–0, P) signals to the desired values.
2. The drive delays at least one Deskew delay plus a Cable Skew delay before
asserting REQ.
3. The DB (7–0, P) signals are held valid for a minimum of one Deskew delay plus
one Cable Skew delay plus one Hold Period after REQ is asserted.
4. The drive asserts the REQ signal for a minimum of one Assertion period after
which the drive can negate the REQ signal.
5. After a Skew plus a Deskew plus a Hold Period delay, the drive can change or
release the DB (7–0, P) signals.
6. The initiator reads the DB(7–0, P) signals within one Hold Period of the
transition of the REQ signal to true.
7. The initiator indicates its acceptance of the data by asserting an ACK pulse.
8. The drive waits at least the greater of the following periods before asserting REQ
again: a.) A transfer period from the last transition of the REQ signal to true.
b.) A negation period from the last transition of the REQ signal to false.