Subject: Re: select vs select with ATN (scsi bus management)
To: Eric S. Hvozda <>
From: Leo Weppelman <>
List: tech-kern
Date: 09/14/1995 21:30:43
> This is kinda of topic, but since the question developed while writing 
> my scsi driver, we'll try it here...
> Apparently the FAS408 and ESP406 can do select, select with ATN, and 
> select with ATN and stop.
> select selects a target; this I know.  So what's the difference between
> select and select with ATN?  Why would I want to do one over the other?
When select+ATN is used to request the target to enter the message-out
phase after the selection. This way, an identify message can be exchanged.
You'll need this when you want to implement disconnect/reselect because
the selected target needs to know who's talking to him. But as the
identify-message is optional, there are cases in which ATN is not needed.
The same is true when you don't intend to support re/disconnect ;-)

For more detailed info, I guess you should read the SCSI spec. It's available
for ftp. I don't know the exact address, but it should be in the SCSI-FAQ