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Parallel SCSI Interface Product Manual, Rev. A 83

4.0 Message system specification

SCSI protocol messages allow communication between a SCSI initiator port and a SCSI target port for the pur-

pose of link management. The link management messages used for this purpose are defined within this stan-

dard and their use is confined to this standard. Other SCSI protocol messages allow communication between

the application client and the task manager for the purpose of task management. The task management func

tions are defined in the SCSI Architecture Model-4 standard. Messages that convey the task management

functions are defined by this manual.

4.1 General message protocols and formats

One or more messages may be sent during a single MESSAGE phase, but a message shall not be split

between multiple message phases.

If an information unit agreement is not in effect, the first message sent by the SCSI initiator port after a suc-

cessful SELECTION phase with an attention condition shall be an IDENTIFY, ABORT TASK SET (see Section

4.5.3), or TARGET RESET message. If a SCSI target port receives any other message, it shall cause an unex-

pected bus free by generating a BUS FREE phase (see Section 3.1.1).

If the first message is an IDENTIFY message, it may be followed by other messages, such as the first of a pair

of SYNCHRONOUS DATA TRANSFER REQUEST messages. With tagged queuing, a task attribute shall fol

low the IDENTIFY message, then more messages may follow. The IDENTIFY message establishes a logical

connection between the SCSI initiator port and the specified logical unit within the SCSI target device known

as an I_T_L nexus.

If an information unit agreement is not in effect, after the RESELECTION phase the SCSI target port’s first

message shall be IDENTIFY. This allows the I_T_L nexus to be re-established. Only one logical unit shall be

identified for any physical connection or physical reconnection; if a SCSI target port receives a second IDEN

TIFY message with a different logical unit number during a physical connection or physical reconnection, it

shall cause an unexpected bus free by generating a BUS FREE phase (see Section

3.1.1).

If an information unit agreement is in effect, the SCSI target port enters a DT DATA phase after the RESELEC-

TION phase as illustrated in Figure 13.

All SCSI initiator ports shall implement the mandatory messages tabulated in the “Initiator” column of tables 27,

40, and 45. All SCSI target ports shall implement the mandatory messages tabulated in the “Target” column of

tables 27, 40, and 45.

Whenever an I_T_L nexus is established by a SCSI initiator port that is allowing physical disconnection, the ini-

tiator shall ensure that the active pointers are equal to the saved pointers for that particular logical unit. An

implied restore pointers operation shall occur as a result of a RESELECTION phase or a successful receipt of

a SPI L_Q information unit.

4.2 Message formats

One-byte, two-byte, and extended message formats are defined. The first byte of the message determines the

format as defined in Table

24.

1 2 ... 92 93 94 95 96 97 98 99 100 101 102 ... 185 186

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Seagate Ultra 320 User Manual Page 97