With the arrival of Ultra2 SCSI, the SCSI world now uses
three different types of electrical signaling: conventional,
single-ended SCSI, high-voltage differential (HVD) SCSI, and low
voltage differential (LVD) SCSI. The host adapter used in a system must be
electrically compatible with the drives that are to be used.
Let's deal with high voltage differential first. This is the "oddball"; it
was never very popular in the market because of its expense and the fact that it is not
compatible at the electrical level with the more popular single-ended devices. To use high
voltage differential drives, you need to use a host adapter specifically designed to
support HVD signaling. Adapters do exist to allow HVD
drives to run on SE SCSI chains, but this is an expensive solution.
Warning: Do not attempt
to connect HVD drives on a SCSI chain with non-HVD devices, or the latter may be damaged.
Since low voltage differential signaling is required for the newest, fastest devices,
host adapters supporting LVD are taking over the market. Host adapters that support only
SE signaling are still available, but these are severely restricted in terms of the
performance and cable lengths they will allow, so they are generally used only for devices
other than hard disks.
LVD also creates an issue for people who want to use both high-speed hard disks and
also slower devices on the same host adapter. LVD and SE are electrically
compatible--assuming that the LVD devices are multimode capable (LVD/SE or LVD/MSE)--so it
isn't the same issue as with mixing SE and HVD. The problem is that LVD only works if all
the devices on the SCSI bus are running in LVD mode. If you put a single-ended drive on a
SCSI chain with an Ultra160 hard disk, the hard disk will run at no greater than Ultra
speeds, knocking down performance. You will also see your maximum cable length reduced
from 12m to either 3m or 1.5m!
To get around this problem, many SCSI host adapters that support LVD modes (Ultra2 SCSI
and higher) generally include separate support for running devices at Ultra or
slower speeds in single-ended mode. This is implemented either through two distinct
segments on the same SCSI channel--using translater chips that allow the SE and LVD
devices to share data without interfering with each other electrically--or separate,
independent SCSI channels within the same card. (See here for
details on multiple segments and channels.) Be aware that if your SCSI host adapter
does not support this feature, you will not be able to run Ultra speed or slower devices
and Ultra2 or faster devices on the same machine. Of course, you can choose to use two
SCSI host adapters, but this causes other complications...
Next: Connectors
