LaCie DDS4 Drive, Руководство пользователя

Страница: 34 / 40

34
Essentially, the host adapter and drive will negotiate for the best speed that
they both have in common.
What is termination?
SCSI passes signals between devices over SCSI cables that act as transmission
lines. The transmitting device, the cabling and the receiving device must all be
impedance matched in order to accomplish maximum transfer of energy
(signals) from end to end. If not, upon encountering a change of impedance,
part of the signal will be reflected back to the transmitting device. If those
reflected signals arrive at the transmitter after it has switched to the receive
mode, it will think the reflected signal is the signal it is looking for. Not
recognizing the reflected signals will cause that device to issue a SCSI re-send
or perhaps even shut down the bus.
Termination (or terminators) is used to provide a better impedance match,
reducing these reflections to a minimum. Many Single-ended SCSI devices
feature internal termination that works by means of an on/off switch at the
back of the peripheral. Other SE devices may require an external terminator.
LVD peripherals always require external termination. Contact your computer
supply specialist for advice on which type of terminator to use.
Where should I place the terminator?
On external SCSI peripherals, the terminator must be enabled (internal) or
placed (external) on the last device on the bus.
Should I use passive or active terminators?
We recommend that you use active terminators. Passive, single-ended
terminators are okay on a slow SCSI bus (max data throughput of 5Mb/s), but
you are better off using active terminators on all SCSI single-ended
applications.
LVD SCSI devices must have external active terminators.
What are device IDs, priority and arbitration? How do they affect
the performance of the devices in my SCSI chain?
Each SCSI device is addressed on the bus via a specific ID number. For Narrow
SCSI (which allows up to 8 total devices, including the SCSI controller), 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: i.e., 7 is the highest and 0
the lowest. For Wide SCSI, the additional IDs, from 8 to 15, also have the
highest number as the highest priority; but the entire sequence is a 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.