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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 10: The Small Computer System Interface, SCSI

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Ultra320 SCSI
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Logical Unit Numbers (LUNs)
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SCSI IDs

The original SCSI specification allowed up to eight SCSI devices, including the controller, to connect to a single 8-bit SCSI bus. Wide (16-bit) SCSI bus-ses support up to fifteen devices and one controller per bus. All SCSI devices, including the controller, must be assigned a unique numeric identi-fier. These SCSI Ids can be any number from 0 - 7 or, for Wide SCSI devices, 0 - 15. Most SCSI controllers have their ID set to 7 by default.

SCSI ID Priorities

In 8-bit SCSI systems, the device with ID 7 has the highest priority and ID 0 has the lowest priority when arbitrating for use of the SCSI bus.

In a Wide SCSI system, IDs have the following priority (highest to lowest): 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8


The IDs 8 - 15 always have a lower priority during arbitration than 8-bit de-vices; this allows 8-bit devices, which do not recognize IDs 8 - 15 to coexist on a SCSI bus with Wide SCSI devices, which may be assigned those IDs. When controlling 8-bit devices, a Wide SCSI controller cannot be assigned an ID greater than 7 because 8-bit devices would not be able to recognize the controller.

Most narrow SCSI 8-bit devices have 3 jumpers for setting the SCSI ID, while wide ones have 4 jumpers. The MSB is the highest value bit; the LSB is the lowest value bit. The most significant bit (MSB) may start on the left or the right; Table 22 shows them from LSB to MSB order going left to right. The jumpers may also be numbered differently entirely (i.e. not 0, 1, 2, 3). In the table, a 0 or 1 denotes jumper off or on, respectively.


Table 22: SCSI Jumper Settings

SCSI ID

Jumper 0
(LSB)

Jumper 1

Jumper 2

Jumper 3
(MSB)

Notes

 

 

 

 

 

 

0

0

0

0

0

All jumpers off (boot drive)

1

1

0

0

0

 

2

0

1

0

0

 

3

1

1

0

0

 

4

0

0

1

0

 

5

1

0

1

0

 

6

0

1

1

0

 

7

1

1

1

0

Usually reserved for host adapter

8

0

0

0

1

 

9

1

0

0

1

 

10

0

1

0

1

 

11

1

1

0

1

 

12

0

1

0

1

 

13

1

0

1

1

 

14

0

1

1

1

 

15

1

1

1

1

All jumpers on


Previous Topic/Section
Ultra320 SCSI
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Logical Unit Numbers (LUNs)
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