LaCie DDS4 Drive, Руководство пользователя

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at both ends of the bus. In this case, termination is relatively straightforward
because all of the devices have the same width.
Sometimes, however, you may need to mix Narrow and Wide devices on a
single SCSI channel. This is becoming increasingly common as the newest
hard drives are Wide only, but many other kinds of devices are still produced
for the Narrow interface.
The best solution for mixing Wide and Narrow devices is to use a host adapter
that has built-in support for separate segments or channels for Wide and
Narrow devices. Using this sort of host adapter will enable you to set up the
Wide and Narrow devices separately, using a Narrow cable and terminator for
the Narrow segment (or channel), and Wide hardware for the Wide segment
(or channel).
When configured properly, the host adapter handles the mixing of the devices.
This sort of adapter is preferred because Narrow devices generally use single-
ended signaling, while modern Wide devices require LVD for maximum
performance. The two cannot be mixed on the same bus segment without the
LVD devices dropping down to single-ended mode. Thus, this type of host
adapter enables you to use the LVD devices to their full potential.
If you do not have a host adapter with separate support for Narrow and Wide
devices, you will have to mix them on the same SCSI chain. This more
complicated way of mixing introduces several issues to be taken into account:
• Adapters: You will have to use either a Wide or Narrow cable, depending on
whether the host adapter you are using is Wide or Narrow. Then, whichever
drives are the opposite width, will need an adapter so they fit onto the cable.
For example, if you have a Wide cable connected to a Wide host adapter and
several Wide devices, to add a Narrow device to this chain, you will need an
adapter for the Narrow device to let it plug into the Wide cable. Narrow
devices generally use
25-pin or 50-pin connectors, whereas Wide devices generally use 68-pin
connectors.
• Performance: If you put a Wide device on a Narrow SCSI channel, you will
cut its potential maximum performance, because it will only be able to send
data at the Narrow SCSI channel rate. Conversely, putting Narrow devices on
a Wide channel will not double the Narrow channels performance.
• Device Addressing: Narrow devices cannot see or access device IDs over 7. If
you are going to use Narrow devices on a Wide host adapter, set the host
adapter's device ID to something between 0 and 7, or the device won't work
(we recommend that you leave the host adapter’s ID at 7).