Each SCSI device is addressed on the bus via a specific number. For narrow SCSI (which
allows up to 8 total devices), these are numbered 0 through 7; for wide SCSI (16 devices)
the numbering is 0 through 15. The priority that a device has on the SCSI bus is based on
its ID number. For the first 8 IDs, higher numbers have higher priority, so 7 is the
highest and 0 the lowest. For Wide SCSI, the additional IDs from 8 to 15 again have the
highest number as the highest priority, but the entire sequence is lower priority than the
numbers from 0 to 7. So the overall priority sequence for wide SCSI is 7, 6, 5, 4, 3, 2, 1
, 0, 15, 14, 13, 12, 11, 10, 9, 8.
The priority levels are used to guide the arbitration process. I describe arbitration in some detail here, but in a nutshell, it
is the process by which different devices decide which one can have control of the bus. If
more than one device wants control at the same time, the higher-priority device will
"win", while the lower-priority device will have to wait for its turn. Since
devices are addressed specifically by other devices, the ID setting on the bus doesn't
matter a great deal on low-traffic buses--all that is essential is that no two devices be
set to the same ID, or obviously great confusion will ensue. In high-traffic settings, you
will typically want to set the slower devices (scanners, tape drives) to the
higher-priority IDs, to ensure that they are not crowded off the bus by the faster devices
like hard disks.
Another consideration is that if you have any devices that absolutely cannot tolerate
delays in receiving their stream of data--such as a CD recording drive or a video
encoder--they should be given top priority on the bus. Many people also like to make the
host adapter the highest-priority device on the bus, which is why host adapters will often
default to a SCSI ID of 7. It should be noted that some older host adapters can be finicky
about device IDs. Some will only boot a hard disk if it is set to device ID 0. (This is
inflexible and has been basically done away with in newer hardware.)
Warning: Some host
adapters support multiple bus segments on the same SCSI
bus. Device IDs must be unique across all segments that are on the same bus. You can't put
two devices both with ID #7 for example on two different segments of the same SCSI chain.
The method of actually configuring the ID depends on the specific device. Many devices
use hardware jumpers or switches, or even a rotary dial to set the device ID, on the back
of the device enclosure. More sophisticated devices use software utilities--this is most
common with more modern SCSI host adapters. If the drive uses jumpers, be sure to check
the configuration settings carefully; different drives use different jumper combinations
to set particular numbers.
Finally, the use of Plug and Play SCSI allows
for automatic assignment of device IDs on the bus, to eliminate devices trying to use the
same IDs simultaneously, for systems that support the feature (and when it works). Single Connector Attachment drives also have their
IDs set by the host system, to allow for automatic configuration and hot swapping.
Next: BIOS Issues
