dentifiers

There are three components in any SCSI system: • Controller • Device • Cable The controller is the heart of SCSI. It serves as the interface between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built right into the motherboard. On the controller is the SCSI BIOS. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the SCSI bus. Usually, each device on the SCSI bus has a built-in SCSI adapter that allows it to interface and communicate with the SCSI bus. For example, an SCSI hard drive will have a small circuit board that combines a controller for the drive mechanism and an adapter for the SCSI bus. Devices with an adapter built in are called embedded SCSI devices. Each SCSI device must have a unique identifier (ID). As you saw in the previous section, an SCSI bus can support eight or 16 devices, depending on the specification. For an eight-device bus, the IDs range from zero to 7, and for a 16-device bus, they range from zero to 15. One of the IDs, typically the highest one, has to be used by the SCSI controller, which leaves you capable of adding seven or 15 other devices. With most SCSI devices, there is a hardware setting to configure the device ID. Some devices allow you to set the ID through software, while most Plug and Play SCSI cards will auto-select an ID based on what's available. This auto-selection is called SCSI Configured Automatically (SCAM). It is very important that each device on an SCSI bus have a unique ID, or you will have problems. All of the variations in the SCSI specifications have added another wrinkle: There are at least seven different SCSI connectors, some of which may not be compatible with a particular version of SCSI. The connectors are:
• DB-25 (SCSI-1)
• 50-pin internal ribbon (SCSI-1, SCSI-2, SCSI-3)
• 50-pin Alternative 2 Centronics (SCSI-1)
• 50-pin Alternative 1 high density (SCSI-2)
• 68-pin B-cable high density (SCSI-2)
• 68-pin Alternative 3 (SCSI-3)
• 80-pin Alternative 4 (SCSI-2, SCSI-3) No matter which version of SCSI you are using, or what type of connector it has, one thing is consistent -- the SCSI bus has to be terminated.