PC Hardware in a Nutshell, 3rd Edition

SCSI Interoperability

SCSI host adapters and drives used in PCs are in theory interoperable whatever their age and level of standards compliance. That is, if you have the proper cable, you can connect a new Ultra Wide SCSI hard drive to an old SCSI-1 host adapter and it will work, albeit at only the 5 MB/s transfer rate supported by the old host adapter. Similarly, you can connect an elderly SCSI-1 CD-ROM drive to a U2W host adapter and expect it to work. But just because you can do something doesn’t mean you want to. Keep the following in mind if you mix SCSI device types:

• All devices on a SCSI bus communicate at the speed of the slowest device. For example, if you connect a U2W (80 MB/s) hard disk and a Fast SCSI (20 MB/s) CD-ROM drive to the same bus, the hard disk operates at 20 MB/s, which may significantly degrade hard disk performance. In general, assuming that your hard disks are all of the same type, the best practice is to place all hard drives on one host adapter or channel and put other SCSI devices (such as CD-ROM drives, tape drive, and scanners) on another, slower channel or host adapter.

• Although you can connect both wide and narrow devices to the same channel on a wide host adapter, you must install the wide devices physically closest to the host adapter, and use a cable converter that terminates the wide portion of the cable between the last wide device and the first narrow device.

• The presence of one SE device on the SCSI bus forces all other devices on the bus ...