Path: utzoo!utgpu!watserv1!watmath!att!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!julius.cs.uiuc.edu!apple!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.periphs.scsi
Subject: Re: synchronous negot. and number of disks
Message-ID: <35374@cup.portal.com>
Date: 29 Oct 90 23:23:16 GMT
References: <62.UUL1.3#1@vortex.COM> <49699@olivea.atc.olivetti.com>
Organization: The Portal System (TM)
Lines: 24

<Synchonous negotiation is the message passing sequence for two devices 
<to agree on the rate at which synchronous xfers can take place.  Basically, 
<one side might say: "I can do 5 MB/SEC" and the other might answer: 
<"I can do 3 MB/SEC".  The final rate will always be the lower of the two.

Actually, it might have to go a few rounds before it gets resolved.
Some devices only implement a subset of the possible synchronous
periods and offsets.

For example, supposed the first device in the above example can do
2, 4, and 5 MB/SEC.  When it gets the response "3 MB/SEC" from the
second device, it will have to let the other device know that it
has not accepted this, and then try to negotiate 2 MB/SEC.

If you are writing host software, be careful.  Many devices screw
up message handling.  For example, I have a case where the Sony
40 meg drive that Apple sells blatently violates the SCSI protocol
during message handling.  I don't remember the exact details, but
basically the drive gets confused, asks the host for a message when
the host doesn't have one, so the host sends a NOP, and the drive
then just disconnects from the bus!  This case can show up during
synchronous negotiation with this drive.  This is very annoying!

						Tim Smith