Message Format; Table 4–1 Message Format - Quantum DLT 8000 Product Manual

The SCSI message system allows communication between an initiator and the drive for
interface management and command qualification. Messages can be originated by
either the initiator or the drive. This section contains a detailed description of the
messages supported by the disk drives.
4.1

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 that are identified in Table 4–2.
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, or BUS DEVICE 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 message (for example, COMMAND 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 4–1, determines the format of the
message.
Message Code
02h – 1Fh
20h – 2Fh
40h – 7Fh
80h – FFh
Table 4–1 Message Format
Message
00h One-byte message (COMMAND COMPLETE)
01h Extended message
One-byte message
Two-byte message
Reserved
One-byte message (IDENTIFY)
Quantum DLT 8000 Tape System
Chapter 4
MESSAGES
4-1