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Small Computer System Interface Synchronous Negotiation Initialization And Management Method

IP.com Disclosure Number: IPCOM000102080D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 5 page(s) / 173K

Publishing Venue

IBM

Related People

McNeill, AB: AUTHOR [+2]

Abstract

A technique is described whereby computer systems equipped with small computer system interface (SCSI) adapters are provided with synchronous negotiation initialization and management to ensure that a synchronous data transfer is always concurred between the target and the initiator.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 47% of the total text.

Small Computer System Interface Synchronous Negotiation Initialization And Management Method

       A technique is described whereby computer systems
equipped with small computer system interface (SCSI) adapters are
provided with synchronous negotiation initialization and management
to ensure that a synchronous data transfer is always concurred
between the target and the initiator.

      The concept described herein provides a means whereby the
system will always renegotiate on the next command following the
receipt of a check condition in the status byte from the target
device.  Data TRANSFER transfer operations can occur in two modes
using the American National Standard Institute (ANSI) SCSI
specification, synchronous and asynchronous modes [*].  Synchronous
mode has various data rates and REQuest/ACKnowledge (REQ/ACK) offsets
by which the data can be transferred.  The concept provides a method
of keeping both the device transferring the data and the device
receiving the data updated with the correct mode, transfer rate and
REQ/ACK offset.

      Data is sent asynchronously or synchronously.  In asynchronous
mode, the control signals, REQ/ACK perform a handshake sequence to
transfer each byte of data.  Fig. 1 illustrates the timing for a SCSI
asynchronous operation.  In synchronous mode, the REQ/ACK control
signals are independent, no handshake occurs.  Fig. 2 illustrates the
timing for a SCSI synchronous operation. The maximum rate at which
the REQ/ACK pulses may be sent back to back in the synchronous mode
is known as the maximum synchronous data rate.  There is a limit to
the number of requests which can get ahead of the acknowledges.  This
limit is called the synchronous REQ/ACK offset.

      Before transferring in synchronous mode, synchronous
negotiations must be completed to tell each device how fast (the
synchronous period) and what REQ/ACK offset the transfers will use.
This is accomplished by exchanging five byte messages.  The target or
the initiator may begin negotiations by sending the first message.
The two key bytes of the five-byte block are the transfer period and
the REQ/ACK offset, as described above.  Once complete, the
negotiation is good until a reset condition occurs.  This occurs at
power on, if a reset is given to the target device (bus device reset
message), if the reset line on the SCSI bus is pulled active, or if,
for some reason, the target device resets, such as in a media change,
hardware error, etc.

      It might seem appropriate that the initiator, the one beginning
the communication, should send the first message to begin the
negotiation at power on.  Then, following power on, if the initiator
gets reset, the initiator will send the first message to renegotiate
and if the target gets reset, the target will send the first message.
The problem with this method is that most targets will not begin
renegotiations and will merely go back into asynchronous mode and do
nothing...