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Appendix - SCSI Q & A
page 30
LaCie d2 SCSI Hard Drive
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).
• Termination: This is the biggest issue with mixing devices. Here is the problem: when you have a Wide SCSI bus and you
connect Wide (16-bit) devices to Narrow (8-bit) devices, there are an extra 8 data bits present on the bus. If you just
connect the Narrow data signals and leave the Wide signals "hanging," then that part of the bus will not be terminated!
The same is true for connecting Wide devices to Narrow buses: many Wide devices, when connected to a Narrow bus,
won't work correctly if the high bits are not dealt with. As a result, the Wide data signals must be terminated when bus
widths are mixed. Some adapters that are used to mix devices can terminate these Wide data signals. Connectors that
automatically terminate the extra "Wide" signals include high byte termination. Ask your computer supply specialist for the
adapters that meet your specific needs.
Can I mix Single-ended (SE) devices and Low-Voltage Differential (LVD) devices on the same SCSI chain?
This is possible, because LVD devices are backwards compatible, meaning they can be used with SE buses. The LVD devices
will simply be reduced to the maximum speed and cable length limitations of the SE bus.