Seagate Ultra 160, Ultra 320 manual Miscellaneous Scsi bus characteristics

5.0Miscellaneous 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 describes the SCSI message system, which also enters into the overall SCSI operating picture.

5.1Attention 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 trans- fer 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 information 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 con- dition 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 27, 40, and 45. 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).

Parallel SCSI Interface Product Manual, Rev. A 107