fq070515-08 - SCSI ID Assignment and Priority

BiTMICRO Knowledge Base

fq070515-08 - SCSI ID Assignment and Priority

OVERVIEW:

Each device on a parallel SCSI bus including the HBA (initiator) must contain a unique SCSI ID. The SCSI ID is used to identify which SCSI device is communicating with the initiator. SCSI ID must be unique on each SCSI bus.

DISCUSSION:

The narrow SCSI bus ID ranges from 0 to 7 while a wide SCSI bus ID ranges from 0 to 15 having ID# 7 as the highest priority. It can be set either via physical jumpers or control signals depending on the device or enclosure.

Provided below is the SCSI ID priority list for Narrow and Wide SCSI (highest to lowest).

Bus	SCSI ID Priority
Narrow	7, 6, 5, 4, 3, 2, 1, 0
Wide	7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8

The Host Bust Adaptor (HBA) usually has the highest SCSI ID (ID# 7).

SCSI ID priority is used during bus arbitration. Briefly, arbitration is a technique used to determine which SCSI device (or ID) can use the bus at a given time. If more than one device is trying to use the bus, the device with the higher SCSI ID priority prevails. Although unlikely, there are cases where there are several types of SCSI devices connected (e.g. fast SCSI drives, SCSI CD-Writer, SCSI Tape drive) on a single bus and the faster SCSI devices have the higher priority ID sets. These devices will most likely hog the bus thus denying access to the slower SCSI devices. This problem was somehow corrected by QAS (Quick Arbitration and Selection).

To set the SCSI ID of a 68-pin E-Disk^® SCSI solid state drive (Wide and Narrow), see table below.

Jumper Settings
JF3	JF4	JF5	JF6	SCSI ID
OFF	OFF	OFF	OFF	0
OFF	OFF	OFF	ON	1
OFF	OFF	ON	OFF	2
OFF	OFF	ON	ON	3
OFF	ON	OFF	OFF	4
OFF	ON	OFF	ON	5
OFF	ON	ON	OFF	6
OFF	ON	ON	ON	7
ON	OFF	OFF	OFF	8*
ON	OFF	OFF	ON	9*
ON	OFF	ON	OFF	10*
ON	OFF	ON	ON	11*
ON	ON	OFF	OFF	12*
ON	ON	OFF	ON	13*
ON	ON	ON	OFF	14*
ON	ON	ON	ON	15*
*Addresses in the range of 8-15 are only supported by SCSI device implementing a 16-bit data bus.

RELATED LINK(S):

SCSI Device IDs Page

REFERENCE TO THIRD PARTIES AND THIRD PARTY WEB SITES
There are links in this document that will allow you to connect to third-party websites over which BiTMICRO has no control. These links are provided for your convenience only and your use of them is at your own risk. BiTMICRO makes no representations whatsoever about the content of any of these websites. BiTMICRO does not endorse or accept any responsibility for the content, or use, of any such websites.

Article ID:	fq070515-08
Creation Date:	04 August 2006
Last Reviewed Date:	04 March 2008
Revision No.:	1.0