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Browse Prior Art Database

Continuous SCSI Bus-Free Monitor

IP.com Disclosure Number: IPCOM000120082D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 47K

Publishing Venue

IBM

Related People

Keener, DS: AUTHOR [+3]

Abstract

This article describes a technique for eliminating bus dead time on a small computer system interface (SCSI) bus.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 91% of the total text.

Continuous SCSI Bus-Free Monitor

      This article describes a technique for eliminating bus
dead time on a small computer system interface (SCSI) bus.

      Typically, SCSI protocol chips begin monitoring the bus-free
condition after the selection/reselection function is programmed into
the chip.  This results in a 1.2 us dead time before arbitration
whether or not the bus has been free for 1.2 us.  If a device other
than the SCSI protocol chip sets the bus busy during this dead time,
the SCSI protocol chip is locked out of this arbitration.

      The figure shows the SCSI arbitration phase timings. All
devices on the SCSI bus can drop BSY to begin arbitration for the
bus.  T1 and T2 are the mandatory wait periods before BSY can be
dropped to begin arbitration. Once BSY is set active by a device,
SCSI protocol chips which have not met the mandatory T1 and T2
periods cannot participate in this arbitration.  These devices must
wait for the next bus-free condition (SEL and BSY inactive) and then
wait for the T1 and T2 delays before arbitrating.

      The SCSI protocol chip continuously monitors the bus-free
condition and places the arbitration state machine in an all
right-to-arbitrate condition once the T1 and T2 times are met.  If
the all right- to-arbitrate state is active, the chip immediately
begins arbitration on the SCSI bus by dropping BSY when the selection
or reselection function is programmed into the chip.  The additional
1.2 us dead time...