The devices "daisy-chained" onto a SCSI controller are on what is called a bus -- that is there is one set of wires that various devices take turns using. If one device is using the bus, other devices have to wait before they can transmit or receive data. A device which is connected to the SCSI bus but not in use would have no impact on performance. In many circumstances, you would have only a single device active at a time.
As the SCSI standard has developed, the speed of the bus has increased. Slower devices can be plugged into faster SCSI controllers. There is a myth that this causes the SCSI bus to run at the lowest common denominator speed. This is simply not true -- the fast devices will continue to operate at their normal speed.
What if you have more than one device in operation at any one time? What impact would that have on performance? Suppose you have a CD-ROM drive and a hard drive, both connected to the same SCSI controller. You are copying files from a CD to the hard disk. Because the CD-ROM drive is constantly reading and the hard drive is constantly writing, you would imagine that this would be twice as slow as running the CD-ROM and disk drives on separate controllers.
However, the real bottleneck is the mechanical process of reading and writing to the drives. The computer, the SCSI controller and the drives have caches where data are temporarily stored before being written to the slower medium. This reduces the impact of having devices share a bus. Most network file servers using SCSI will have several devices on the one controller.
The biggest potential issue involved in placing several devices on a single SCSI controller is that some of these devices are less than 100% reliable. Scanners, for example, usually come with a dedicated SCSI card. It is often easier to configure the software with this card rather than use a "general" card. Some scanners will warn you that they may not work properly unless the controller is set to a slower bus speed. If you have an unreliable SCSI device, it may cause the SCSI bus to hang. If your CD-ROM or scanner hangs, it is not that great a problem. However, if the hard drive hangs, your computer hangs with it.
In conclusion, add as many devices as you can onto the one SCSI card. If your scanner came with a dedicated SCSI card, use that for the scanner. If the SCSI bus hangs or devices appear to be unreliable, you might consider adding a second SCSI controller.
- Roy Chambers