Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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3.5.1
General message protocol
A message may be one, two, or multiple bytes in length. One or more messages may be sent during a single
MESSAGE phase, but a message may not be split over MESSAGE phases. The initiator is required to end
the MESSAGE OUT phase (by negating ATN) when it sends certain messages identified in Table 3.5.2-1.
One-byte, Two-byte, and extended message formats are defined. The first byte of the message determines
the format as follows:
Value
Message Format
00h
One-Byte Message (Command Complete)
01h
Extended Messages
02h - 1 Fh
One-Byte Messages
20h - 2Fh
Two-Byte Messages
30h - 7Fh
Reserved
80h - FFh
One-Byte Message (Identify)
One-Byte messages consist of a single byte transferred during a MESSAGE phase. The value of the byte
determines the message to be performed as defined in Table 3.5.2-1.
Two-byte messages consist of two consecutive bytes transferred during a MESSAGE phase. The value of
the first byte determines the message to be performed as defined in Table 3.5.2-1. The second byte is a
parameter byte that is used as defined in the message descriptions in Section 3.5.2.
See section 3.5.3.1 for details on the one and two byte messages.
Extended messages consist of from three to 7 bytes transferred during the MESSAGE phase. The extended
message includes a three byte header and up to 4 extended message argument bytes. See Section 3.5.3.2
for details of the extended messages.
3.5.2
Messages - General
The messages supported by the various disc drives are listed in a table in the drive’s Product Manual. Table
3.5.2-1 lists Messages that are defined by the SCSI protocol. The message code values are given a direction
specification (in-0ut). Detailed descriptions follow the table. Messages other than those supported by a disc
drive are answered by the drive with a Message Reject message.
