Browse Prior Art Database

Small Computer System Interface Arbitration Mandate

IP.com Disclosure Number: IPCOM000099417D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-14
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

McNeill, AB: AUTHOR [+2]

Abstract

This article describes a method for use in a small computer system interface (SCSI) multitasking environment which allows a device to keep control of the SCSI bus at a time when normally a bus-free phase would occur.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 96% of the total text.

Small Computer System Interface Arbitration Mandate

       This article describes a method for use in a small
computer system interface (SCSI) multitasking environment which
allows a device to keep control of the SCSI bus at a time when
normally a bus-free phase would occur.

      The ANSI SCSI specification has no method of insuring that an
initiator will acquire the bus following a disconnection or an end of
command for a given device.

      On the SCSI controller card the system can send commands to up
to fifteen SCSI devices for simultaneous processing.  Performance is
increased by getting the commands to the devices as quickly as
possible so that seeks and other delays within the devices can
overlap.  In this way reconnection occurs as much as possible during
the latency in the other disconnected devices.

      The SCSI bus-free condition occurs when busy and select go
inactive for 1.2 us.  No device can connect or reconnect to the bus
until the bus is free.  Once connected to the bus, the target device
drives busy.

      Fig. 1 shows the arbitration mandate timings.  The method of
mandating the bus for the initiator is to drive the busy line active
along with the target during a disconnection message (target is going
bus free) or during an end command message (target has completed the
command). When the target releases busy the initiator still has busy
active and he can start an arbitration without losing the bus to the
devices.

     ...