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Automatic Target Mode for the Small Computer System Interface

IP.com Disclosure Number: IPCOM000102024D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 4 page(s) / 206K

Publishing Venue

IBM

Related People

McNeill, AB: AUTHOR [+2]

Abstract

This article describes a technique which allows a target system on a small computer system interface (SCSI) network to set up its SCSI controller such that the controller will accept data from an initiator system and then not return the command status to that system until its own target system has processed the data and set-up the target for the next cycle. In this way, the initiator system never gets a "not ready indication."

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 32% of the total text.

Automatic Target Mode for the Small Computer System Interface

       This article describes a technique which allows a target
system on a small computer system interface (SCSI) network to set up
its SCSI controller such that the controller will accept data from an
initiator system and then not return the command status to that
system until its own target system has processed the data and set-up
the target for the next cycle.  In this way, the initiator system
never gets a "not ready indication."

      The SCSI protocol requires significant interaction between the
device sending a command (initiator) and the device receiving a
command (target).  This interaction includes the Unit Attention
condition, the target Not Ready condition, handling of abort and
reset messages and support for the Inquiry, Request Sense, Test Unit
Ready and Send Diagnostic commands.  The initiator requires feedback
as to the outcome of the command it sent to a target.  Often the
target host system does not want to take the time away from the
system processor to take care of these tasks and the initiator host
system, which may be communicating with several devices, needs to get
the information to the target as quickly as possible.  The method
disclosed herein provides for the host system to communicate as a
target without significant host system overhead.

      There are typically six phases to a SCSI command: the selection
phase, the message out phase, the command phase, the data phase, the
status phase and the message in phase. The target SCSI adapter may
interrupt the target host at any time to either give information or
receive information. There may be several initiators attached to the
SCSI bus waiting to interact with the target at any given time.  The
selection phase tells the target which initiator is trying to
communicate (1 of up to 7 physical units).  The message out phase
tells the target which logical unit within the target physical unit
is being selected (1 of up to 8 logical units within each physical
unit).  Within this message is a control bit which lets the target
know whether the initiator accommodates disconnection.  Disconnection
is common for SCSI devices such as fixed disks and optical devices
which disconnect from the bus during a seek to free the SCSI bus for
other devices to communicate.  The target must allocate a buffer for
the command bytes from the given initiator without overwriting
commands from initiators which are active but have not completed (in
the case of an initiator which sends a command and is disconnected).
The final byte of the SCSI command is a control byte.  If the link
bit is on, the target may send a Linked Command Complete message
during the final message in phase and then go directly back into the
command phase for the next command from this initiator, thus skipping
the selection and message out phases on this subsequent command.
Following the command phase, the target must decide which directio...