SCSI NEWS
                                  July 1989

There seems to be a lot of confusion as to what "SCSI-2" is.  Some people 
think that SCSI-2 is the ANSI standard (X3.131-1986) with CCS (common command 
set) added.  Others think that the "-2" means two I/O cables.  At a recent 
SCSI forum the magnitude of the misunderstanding of SCSI-2 was fully realized 
by some SCSI committee members.  This newsletter is the result of such a 
realization.

WHAT IS SCSI-2?

Firstly, it is the second SCSI standard, now completing development.  It 
consists of the basic SCSI standard with some additions and some deletions.  
The SCSI standard (X3.131-1986) permitted an almost unlimited choice of 
options, features, and selections in the implementation of a SCSI host adapter 
or controller.  There is a high probability that this implementation did not 
"plug and play" with another SCSI vendor's implementation.  SCSI-2 removes 
some of the implementation options that are a carry-over from SASI.  SCSI-2 
will make mandatory the use of parity on the SCSI bus, arbitration, extended 
sense data bytes (short sense will no longer be permitted), better define who 
supplies terminator power, etc. These characteristics will make plug and play 
more feasible at the physical level.
  
Message support is better defined in SCSI-2.  And the level of mandatory 
support has been expanded.  Message protocol for Synchronous Mode has been 
clarified as users have begun to implement this feature.  

At the command level, SCSI-2 adds the Common Command Set for Direct Access 
Devices, caching commands and cache management bits, additional sense codes 
(for error recovery), commands for CD-ROMs, scanners, medium changers, and 
communications.  The MODE SELECT and MODE SENSE pages are expanded from CCS 
and also are being defined for all device types.   The INQUIRY command has 
been expanded to better provide capability for self-configuring systems.  READ 
BUFFER and WRITE BUFFER commands provide the capability of testing the host-
controller interface not normally testable with the controller self-test.  LOG 
SELECT and LOG SENSE commands have been added to extract error and performance 
characteristics of devices.  

The format of the SCSI-2 standard will be somewhat different than SCSI.  The 
commands will be arranged in alphabetical order, not in command code order.  
Error conditions and their corresponding sense codes will be more closely tied 
to the commands which caused them.  

Some other items in SCSI-2 are:

 -  optional 16- and 32-bit data paths utilizing a second cable.  Eight bit 
    backward compatibility is still provided.  
 -  A protocol, called "fast SCSI" increases the synchronous data transfer 
    rate up to 10 Mega-transfers per second using differential drivers.
 -  Queuing of multiple commands (up to 256) to each logical unit from each 
    initiator.
 -  Asynchronous Event Notification allows a device to notify a host of some 
    "event" when there is no command pending.  Such events include not-ready 
    to ready transitions, deferred errors, and operator actions.

SCSI-2 is currently a draft proposed American National Standard. In June 1989, 
The X3T9.2 Task Group of X3T9 voted to forward SCSI-2 Revision 10 to X3T9.  
X3T9 then voted to send it on to the X3 committee for its first public review 
period.  (Later, the X3T9 chairman realized that this vote failed due to the 
small number of voting members present at the meeting -- this should be 
corrected in August.)  

The formal four-month public comment period should begin in September/October 
1989.  Since SCSI-2 is so large (nearly 600 pages), it is likely that some 
public comments will be received during this period.  If X3T9.2 agrees to make 
substantive changes to SCSI-2, then it will go through this process again (and 
again) until no more substantive changes are made.  After the first public 
comment period, the review periods are reduced to two months.

After X3T9.2 has now begun working on (what else?) SCSI-3.  X3 has authorized 
the SCSI-3 project to enhance SCSI-2 even further.  A few features being 
considered include:

 -  A single-cable 16-bit option.
 -  Scatter write and gather read capability.
 -  More than 8 devices per bus.
 -  Longer cable lengths.
 -  File-level command sets.
 -  Autoconfiguration of device addresses.
 -  Operation on other physical layers (such as fiber optics).

The original SCSI NEWS edition was prepared by Ralph Schultz.  This version 
was revised by John Lohmeyer.  Please leave comments, corrections, and/or 
additions in the Message section of the SCSI BBS.

File: SCSI2NWS.WS last updated on 7/31/89