Q Is it possible to replace the SCSI controller that came with my HP scanner with an Adaptec card that will enable me to add a CD-ROM writer? The current card is an ISA Symbios 53c416. I would like to change to a PCI controller.

A SCSI controllers are designed to allow several devices, such as scanners, hard drives, CD-ROM drives, tape drives, etc, to be chained off the one controller. All these devices are said to be on the SCSI bus. As the SCSI standard has evolved over the years the speed of the SCSI bus has increased. To allow maximum compatibility, the SCSI standard allows faster devices to be plugged into slower controllers and slower devices to be plugged into faster controllers. Well, this is the theory anyway.

When you have a fast device and a slow device plugged into a fast controller, both devices should be able to operate at their own speeds. However, I have seen in the documentation for several different scanners a recommendation that the SCSI controller that came with the scanner should be used. Furthermore, they suggest that if you use any other controller, the scanner may not work unless you set the controller to run at a slower speed suitable for the scanner.

You could purchase any brand of SCSI controller and have the scanner work with it, if you are lucky. You will get the best results if you buy a quality SCSI controller and if you use the drivers that come with the card rather than those that come with Windows 95. The preferred approach may be to continue to use the dedicated SCSI controller with the scanner and merely add a second SCSI controller.

Scanners are not always the most reliable of devices. One problematic device on the SCSI bus could cause the entire SCSI bus to hang. This is not such an issue for a CD-ROM writer. But should you have a SCSI hard drive on the same SCSI bus then the whole computer would hang.