11.3 Peripheral Devices
11.3.3 SCSI interface issues
SCSI drives require a separate controller from the IDE controller. This controller operates with ROM BIOS under DOS and Windows. The ROM BIOS contains management, surface verification, and low-level format applications. SCSI BIOS is accessed during the boot up process by pressing the setup key combination.

The SCSI bus operation and the transfer rate are controlled by adapter settings. If there is a transfer speed mismatch amongst the SCSI controllers, the drive might have I/O errors. I/O errors are fatal to the computer. As a rule, attach the SCSI controller set to the last SCSI ID, which is SCSI ID #7. Input/Output errors might result if the SCSI ID is not set to 7. Also, to ensure optimal performance, enable parity checking and host adapter termination.

SCSI devices must have unique SCSI IDs. SCSI devices cannot share ID numbers. The location of the SCSI ID on the bus is not important. The SCSI IDs do not need to be placed sequentially on the bus. Be sure that devices do not share the same ID numbers. If SCSI device share IDs, errors will occur.

An important consideration with SCSI IDs is, the higher the SCSI ID, the higher the priority. The actual ID priority is 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8. The SCSI host controller should be set to SCSI ID 7. This will give the SCSI host controller the highest priority. Hard-drive should be set with lower IDs. Hard-drives have the potential to consume a large amount of the bus speed. Setting the hard-drives with a lower priority will allow time sensitive information to have a higher priority. Use IDs 6, 5, and 4 for CD-RWs and other streaming media drives.

SCSI controllers are configured with the following parameters:

  • Sync negotiation
  • Transfer Rate
  • BIOS SCAN
  • Send Start Unit Command

SCSI fixed disk I/O errors will occur if there is a wrong configuration with the SCSI parameters. SCSI controllers allow for drive configuration for larger SCSI drives. If these parameters are not set correctly, the drive might be inaccessible. This error can be fixed if the configuration parameters are correctly set for the SCSI controllers.

A SCSI drive can have built-in or external termination that can be enabled or disabled. The SCSI bus must be terminated at both ends of the controller and must have two termination points, one at the beginning and one at the end of the SCSI bus. The ending termination point must also be within four inches of the ending points of the bus. The drives on the SCSI bus must also run sequentially, drive A to B, B to C, C to D, and so on. Also, the SCSI bus must not contain any "Y" shaped drive configuration.

SCSI Interface Levels
Most personal computers today have SCSI ports built into them. SCSI ports are supported by all major operating systems. There are several levels of SCSI, including SCSI-1 that evolved into SCSI-2 and SCSI-3 (all three mostly outdated), Ultra-SCSI (widely implemented SCSI standard), and Ultra-3 which is the latest SCSI standard. Although not all devices support all levels of SCSI, the evolving SCSI standards are generally backwards compatible. All the SCSI standards have different speeds. When installing SCSI drives and other devices it is important to consider the cabling. Do not make the SCSI bus longer than the speed of the cable being used. The cable length specifications for the various SCSI types are summarized in Figure . A common source of malfunction of SCSI devices is when these specifications are ignored when installing and configuring these devices.