Seagate Ultra 160 - Manual (Page 170)

156 Parallel SCSI Interface Product Manual, Rev. A
For each new run, the Sequence ID is set to zero for the first word transferred, set to one for the second word
transferred, set to two for the third word transferred, and set to three for the fourth word transferred. The
Sequence ID then cycles back to being set to zero for the fifth word transferred, and so forth until the run is
complete. At the beginning of the next run, the Sequence ID is set to zero again.
The Sequence ID provides detection of errors that occur when an information transfer is missed or double
clocked. A Sequence ID error causes a protection code error. If a protection code error is detected, then the
information transfer is invalid. The method for recovery from these errors is the same as the method for parity
error recovery.