Quantum 10K II - Manual (Page 346)

Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–25
B.7 SCSI MESSAGE FORMAT
A message can be one or more bytes in length. One or more messages can be sent
during a single MESSAGE phase, but a message cannot be split over MESSAGE
phases. The initiator is required to end the MESSAGE OUT phase (by negating
ATN) when it sends certain messages.
A message cannot be split. The drive will not split messages longer than one byte
even if ATN is deasserted.
When a connection to the drive is established (i.e., the drive is selected with ATN
asserted), the first message byte passed by the initiator must be either an
IDENTIFY, ABORT TASK SET, or TARGET RESET message. If not, the drive
discards the message, saves no status information, and goes to the BUS FREE
phase.
If an initiator supplies an unsupported or incorrect messages (for example, TASK
COMPLETE, or a reserved or undefined message code), the drive returns a
MESSAGE REJECT message and continues where it left off (possibly returning to
MESSAGE OUT if ATN is raised).
The first byte of the message, as defined in Table B-10, gives the format of the
message.
Table B–10 SCSI Message Format
Message Code Message
00h One-byte message (TASK COMPLETE)
01h Extended message
02h − 1Fh One-byte message
30h − 7Fh Reserved
80h − FFh One-byte message (IDENTIFY)
The drive supports the messages listed in Table B-11. The message code and the
direction of the message flow is also included in the table.