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SCSI Interface Product Manual, Rev. B
5.0
Miscellaneous SCSI bus characteristics
Asynchronous conditions can occur on the SCSI bus that cause a SCSI device to perform certain actions that
may alter the phase sequence of an I/O process. Furthermore, SCSI devices may not all be powered on at the
same time. This publication does not address power sequencing issues. However, each SCSI device, as it is
powered on, should perform appropriate internal reset operations and internal test operations. Following a
power on to selection time after powering on, SCSI targets should be able to respond with appropriate status
and sense data to the TEST UNIT READY, INQUIRY, and REQUEST SENSE commands. Sections 5.1, 5.3,
and 5.4 describe some asynchronous conditions that could arise during operations on a SCSI bus. Section 5.5
describes SCSI pointers, a subject related to the first three. Section 4.0 describes the SCSI message system,
which also enters into the overall SCSI operating picture.
5.1
Attention condition
The attention condition allows a SCSI initiator port to inform a SCSI target port that the initiator has a message
ready. The target shall honor all valid attention conditions by performing a MESSAGE OUT phase.
The initiator may create an attention condition during the SELECTION phase and during all information transfer
phases.
To create an attention condition during the SELECTION phase following normal arbitration, the initiator shall
assert the ATN signal at least two system deskew delays before releasing the BSY signal.
To create an attention condition during the SELECTION phase following a QAS, the initiator shall assert the
ATN signal at least two system deskew delays before asserting the target's ID in the bus.
To create an attention condition during an information transfer phase, the initiator shall assert the ATN signal at
least an attention setup time before negating the ACK signal. To reestablish an attention condition during a
multi-byte Message Out retry, the initiator shall assert the ATN signal two system deskew delays before assert-
ing the ACK signal on the first message byte. To clear an attention condition during an information transfer
phase, the initiator shall negate the ATN signal at least two system deskew delays before asserting the ACK
signal. The initiator shall not negate the ATN signal while the ACK signal is asserted during a MESSAGE OUT
phase.
The initiator shall create the attention condition on or before the last information transfer in a bus phase or infor-
mation unit, for the attention condition to be honored before transition to a new bus phase or information unit. If
the initiator does not meet the attention condition setup time, the target may not honor the attention condition
until a later bus phase or information unit, possibly resulting in an unexpected action. The initiator shall keep
the ATN signal asserted until the target responds to the attention condition.
Once the target has responded to the attention condition by going to MESSAGE OUT phase, the initiator shall
keep the attention condition set if more than one message byte is to be transferred. The initiator shall clear the
attention condition on the last message byte to be sent. The initiator shall clear the attention condition while
transferring the last byte of the messages indicated with a Yes in Tables 26, 39, and 44. If the target detects
that the initiator failed to meet this requirement, then the target shall go to BUS FREE phase (see Section 3.1).
a SCSI target port shall respond to an attention condition with MESSAGE OUT phase as follows:
a. If an attention condition is created during a COMMAND phase, the target shall enter MESSAGE OUT
phase after transferring part or all of the command descriptor block.
b. If an attention condition is created during a DATA phase, the target shall enter MESSAGE OUT phase at the
target's earliest convenience (for example, on a logical block boundary). The initiator shall continue REQ/
ACK handshakes until it detects the phase change.
c. If an attention condition is created during a STATUS phase, the target shall enter MESSAGE OUT phase
after the status byte has been acknowledged by the initiator.
d. If an attention condition is created during a MESSAGE IN phase, the target shall enter MESSAGE OUT
phase before it sends another message. This permits a MESSAGE PARITY ERROR message from the ini-
tiator to be associated with the appropriate message.
e. If an attention condition is created during a SELECTION
phase after that SELECTION phase.
phase,
the target shall enter MESSAGE OUT
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