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ULTRA2 LVD SCSI HOST ADAPTER
3. Before You Install: Getting Started
3.1 The Adapter and the SCSI Bus
Your Ultra2 LVD SCSI Host Adapter performs busmastering and works with the
host computer to provide a means of control for the SCSI bus. By daisychaining
peripheral devices together, as many as fifteen devices can be linked to the Adapter
on the SCSI bus. The Adapter does not have to be placed in any particular physical
position on the bus, but because it doesn't have two external connectors, it has to
be at the end of any all-external bus.
SCSI devices communicate on the bus using bus arbitration, which gives each
device an opportunity to use the bus based on the device's priority (which is
determined by the device's ID, not its physical location). The Adapter is factory-
preset to ID 7, the ideal ID for it on a single-adapter bus (see the next section).
3.2 SCSI ID Numbers
Each internal or external device attached to the Ultra2 LVD SCSI Host Adapter, as
well as the Adapter itself, must have a unique SCSI ID number. This SCSI ID
determines priority when two or more devices are trying to use the SCSI bus at the
same time. No two devices can have the same ID; the SCSI ID uniquely defines the
device to the SCSI bus.
The Ultra2 LVD SCSI Host Adapter is factory-preset to use SCSI ID 7, which
gives it the highest priority on the SCSI bus; this ID number should not be
changed. IDs are usually assigned to each peripheral device at the factory, but you
will be able to set them to whatever you need them to be. This is usually done with
jumpers, DIP switches, or other physical controls; refer to each peripheral's
documentation to determine how to set its ID. The key thing to remember is that
no two devices on the same SCSI bus can have the same ID; each ID must uniquely
identify a single device. Also keep in mind that a device's SCSI ID does not
necessarily have any relationship to its physical location or its position on the bus.
For example, you can have an internal SCSI peripheral with ID 0, and an external
SCSI peripheral with ID 6. (Gaps in the number sequence don't matter either.)
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