SCSI - Computer Busses - page 160

7.6 Message system description

The message system allows the initiator and the target to communicate over the interface
connection. Each message can be one, two, or multiple bytes in length. In a single message
phase, one or more messages can be transmitted, (but a message cannot be split between
multiple message phases). Table 7.6 lists the message format, where the first byte of the mes-
sage determines the format. The initiator ends the message out phase (by negating ATN )
when it sends certain messages identified in Table 7.7.
Single-byte messages consist of a single byte transferred during a message phase. Table
7.7 defines the message type.

Table 7.6 Message format

Value Message format

00h One byte message (command complete)

01h Extended messages

02h-1Fh One-byte messages

20h-2Fh Two-byte messages

30h-7Fh Reserved

80h-FFh One-byte message (identify)

Table 7.7 Message codes

Code Message Direction Description

00h Command complete In Sent from a target to an initiator to indicate that

the execution of an I/O process has completed
and that valid status has been sent to the initia-
tor. After successfully sending this message,
the target s hall g o to the bus free phase by re-
leasing the BSY signal. The target considers
the message transmission to be successful when
it detects the negation of ACK for the com-
mand complete message with the ATN signal
false.

03h Restore pointers In

04h Disconnect In/Out Sent from a target to inform an initiator that the

present connection is going to be broken (t he
target plans to disconnect by releasing the BSY
signal), but that a later reconnect will be re-
quired in order to complete the current I/O
process. This message shall not cause the initia-
tor to save the data pointer. After successfully
sending this message, the target shal l go to the
bus free phase by releasing the BSY signal.
The target shall consider the message transmis-
sion to be s ucce ssful when it detects the nega-
tion of the AC K si gnal for the disconnect mes-
sage with the ATN signal false.