SCSI Conditions

SCSI bus has three asynchronous conditions
1) the Attention condition
2) the Reset condition and
3) the Contingent Allegiance condition


1)
The Attention condition allows an I to inform a T that the I has a message ready.

1. I asserts ATN to indicate pending message for T. Next phase change will be to Message Out phase.
2. T shall respond with a MESSAGE OUT phase as follows

• If ATN condition occurs during DATA phase
• If ATN condition occurs during COMMAND phase
• If ATN condition occurs during STATUS phase
• If ATN condition occurs during MESSAGE IN phase
• If ATN condition occurs during SELECTION phase
• If ATN condition occurs during RESELECTION phase

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2)
Response to
a) LOGICAL UNIT RESET task management request
b) any logical unit reset event (ex: target hard reset)



1) Abort all tasks in the task set.
2) ACA(NACA=1 i.e SCSI-3 )  or Contingent Allegiance(NACA=0 i.e SCSI-2) -- Clear
3) Disk Reservation -- Clear
4) Data Xfer mode  -- default async
5) Mode parameters -- restore last saved values
6) Unit Attention -- UA condition shall be set for all other devices.

The only mode parameters that are influenced by a reset are the current values.
Every device has a story to tell and each device can change its story, within some limits, at the request of another device, usually an initiator. Each device has the ability to remember each story change for any initiator that has requested a change.

I asserts RESET for 25 usec.
Devices currently on the SCSI bus will immediately release
All devices are reset
All outstanding commands are cleared
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3)  A Contingent Allegiance Condition is:  a Condition (state) that is Contingent (dependent) upon the Allegiance (connection) between two entities, or components, in a SCSI-based system.
( http://www.santrainingblog.com/2010/04/contingent-allegiance-condition-cac)

 The contingent allegiance condition shall exist following
  - the return of CHECK CONDITION (02h) status or
  - the return of COMMAND TERMINATED (22h) status and
  - may optionally exist following an unexpected  disconnect.

• The CAC shall be preserved for the I_T_x nexus until it is cleared. 

• The CAC condition shall be cleared upon the generation of  

             -  a hard reset condition
             - an ABORT message
             - a BUS DEVICE RESET message
             - any subsequent command for the I_T_x nexus(mostly REQUEST SENSE)

•  While the CAC exists, the target shall preserve the sense data for the initiator

•  While the CAC exists, the T shall respond to any other requests for access to the logical unit or  T routine from another I with a BUSY status.
• Execution of queued commands shall be suspended until the CAC is cleared.