Date   : Tue, 06 Jul 2004 23:29:59 +0100
From   : Richard Gellman <splodge@...>
Subject: Re: Acorn Winchester unit

Pete Turnbull wrote:

>On Jul 6, 19:34, Jules Richardson wrote:
>  
>
>>On Tue, 2004-07-06 at 17:58, Pete Turnbull wrote:
>>    
>>
>>>I have the manuals for the ACB4000 and X1410A,
>>>      
>>>
>>Hmm. I've got the Xebec S1410 one.
>>    
>>
>
>Oops, typo alert.  I meant S1410A, not X1410A.
>
>  
>
>>(I've still not found anyone who can say what the difference between
>>SCSI and SASI is - is SASI just SCSI before the days of the common
>>command set, or are there other differences in the protocol?)
>>    
>>
>
>*SCSI* is SCSI before CCS.  CCS was developed about the same time the
>SCSI standard was ratified, but wasn't incorporated into the standard
>until SCSI-2.  The ACB4000 manual says a few things about SASI vs SCSI.
> One difference is the ATN line, not present in SASI.  I don't think
>SASI had a notion of disconnection/reconnection (optional in SCSI-1),
>and I thin SCSI is more strict about implementation of certain commands
>being mandatory.  However, I've never read a genuine SASI spec, and
>wouldn't consider myself an expert on the fine details of SCSI either,
>s I'm not sure what other differences might exist.
>  
>

A brief history:
There once was a guy named Mr. Shugart. He invented the floppy drive 
interface, and made lotsa money under Shugart Associates with the floppy 
interface patent.

His vision was for a bus system to conrtol hard drives from a point of 
view of platform independency, multiple units (i.e. more than the two 
winchester gives you), and not needing to know geometries.

He created the Shugart Associates System Interface (SASI).

The people who deal with these things looked at it and went "Nice 
idea... but what if..."

 From the original SASI spec, a modified version was produced that not 
only dealt with hard drives, but also any other block device you care to 
attach, making it even more flexible.

Thus we get the Small Computer Systems Interface (SCSI). The usage 
differences between SASI and SCSI 1 are actually quite few: SCSI has the 
ATN line which is used by the target (hard drives) to signal the 
initiator (computer) that its ready to resume a disconnected operation - 
that is an operation that was intentionally terminated while it was 
still running so that the SCSI bus can be freed during a long operation. 
Note that some SASI systems (rare) grounded this line and occasionally 
(even rarer) cause problems for SCSI drives. SCSI implements "Start 
Unit". SASI assumes the unit will already be "started" before it is 
used, where as SCSI assumes that a device may be in a dormant state, and 
will warm up on "Unit Start". SCSI implements extended sense data, SASI 
implements a series of status codes.

To clarify that last part: SASI had no status out blocks in its bus 
sequence. A single byte was returned indication one of many error codes, 
and it was left up to the host to transalte those into error messages. 
Since SCSI could (Theoretically) encompass more device types, SCSI 
defined a status out block indicating the presence of extended sense 
data, and a command to retrieve extended sense data so that SCSI hosts 
can make better sense of what went wrong.

 From a simple-use perspective, SASI and SCSI can be seen as basically 
identical. But I wouldn't connect a SASI drive (rare!) to a modern SCSI 
controller and expect it to work, at least not without some interesting 
fiddling.

-- Richard