Browse Prior Art Database

Rotection of Standalone SCSI Subsystem Against Spurious Reset

IP.com Disclosure Number: IPCOM000036755D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Cameron, AR: AUTHOR

Abstract

The SCSI interface, referred to in this disclosure, was built to conform with ANSI Small Computer Systems Interface (SCSI) specification X3T9.2/82-2 rev 17B of December 16, 1985.

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Rotection of Standalone SCSI Subsystem Against Spurious Reset

The SCSI interface, referred to in this disclosure, was built to conform with ANSI Small Computer Systems Interface (SCSI) specification X3T9.2/82-2 rev 17B of December 16, 1985.

A standalone disk file subsystem with a SCSI interface should be able to perform diagnostic and maintenance actions while the initiator is powered down. Problems experienced when the controller is not attached to any interface cable or to a cable without a powered initiator are overcome by this solution. It proposes converting the initiator "terminator power" line to a logic signal and ANDing it with the RST signal. Thus only RST signals sent by a powered-up initiator can cause a reset. This facilitates certain standalone maintenance and diagnostic operations.

Spurious Reset: The SCSI implementation consists of a maximum of 8 devices, some of which are initiators and some of which are targets. The disk controller referred to here is a target, attached using the differential electrical version of SCSI. The architected interface has a dedicated signal called RST (reset) which may be asserted optionally by any device. Also, there is a line called terminator power, which supplies power to the terminating networks and may be powered optionally by any device. The terminator power options are a subject of architectural confusion, so in this application only initiators provide terminator power, and the target only receives SCSI RST. This is most cost- effective.

The controller (target) has both power supply and control independent of the initiator and is required to support standalone maintenance and diagnostic operations, whatever the state of the interface. However, problems are experienced when the controller is not attached to any interface cable or to an interface cable with no powered initiator. The nature of the receivers (National Semiconductor DS3695) is such that when unbiased, either a logic high or logic low signal can be detected and fed to the controller. This is satisfactory in most SCSI applications where a target has no function when not attached to an initiator, so that its reset state is not relevant, or when a common power supply is shared. This is not appropriate to a subsystem where standal...