[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Host Adapters ]

Signaling Type Support (SE, HVD, LVD)

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...

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