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SCSI Drive and Device Configuration 119
SCAM—Automatic ID Setting
Some SCSI hard drives and host adapters support SCAM (SCSI
Configure AutoMagically), which automatically assigns the drive a
unique SCSI ID number. To use SCAM, both the host adapter and
drive must support SCAM, and SCAM must be enabled (usually by
a jumper on the drive).
SCSI ID Setting for External Devices
SCSI drives and devices can be used both internally and externally,
often with the same interface card. For external devices, one of the
following methods will apply for each device in the SCSI daisychain.
Use Table 4.14 as a general reference. Typically, the ID setting
control is at the back of the device, near the SCSI interface cable.
Depending on the device, the device ID can be set by a rotary dial, a
push-button control, or a sliding switch. Not all SCSI ID numbers
are available with every device; many low-cost devices allow a
choice of only two or three numbers. Regardless of the setting
method, each internal and external device on a single SCSI daisychain
of devices must have a unique ID! If you use Adaptec SCSI
interface cards, use the SCSI Interrogator program before you add a
new SCSI device to determine which device IDs you have remaining.
If you are adding a new SCSI device with limited ID choices
(such as the Iomega Zip 100 SCSI drive), you might need to move
an existing device to another ID to make room for the new device.
For high-performance SCSI cards that offer multiple buses, you
should be able to reuse device numbers 0–7 for each separate bus
on the card. If you have problems with duplicate ID numbers on
various buses, the device drivers for either the device or the interface
card might not be up-to-date. Contact the device and card
maker for assistance.
SCSI Termination
SCSI termination is simple. Termination is required at both ends of
the bus; there are no exceptions. If the host adapter is at one end of
the bus, it must have termination enabled. On the other hand, if the
host adapter is in the middle of the bus—and if both internal and
external bus links are present—the host adapter must have its termination
disabled, and the devices at each end of the bus must have
terminators installed. Unfortunately, the majority of problems that I
see with SCSI installations are the result of improper termination.
Terminators can be external or internal (set with a jumper block or
with switches or sliders). Some devices also terminate themselves
automatically.
The pass-through models are required when a device is at the end
of the bus and only one SCSI connector is available.