Here's our Hardware Tip for.. April 2, 1999
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SCSI Adapters and Drives
Unlike IDE devives, you have to work a bit harder to install a SCSI subsystem. Each SCSI subsystem consists of a SCSI adapter (also called a SCSI host bus adapter), one or more SCSI devices, and a series of cables that connect the adapter and devices in a daisy chain conliguration. There are a fair number of rules, but they are pretty easy to follow:
- SCS1 is configured as a bus, and each device on the bus must be assigned a unique address ranging from 0 through 7. (Wide SCSI buses can support addresses ranging from 0 through 15; consult the documentation tor your SCSI adapter.)
- The SCSI adapter will always be assigned address 7, which is the factory default.
- Most SCSI adapters expect device 0 to be the hard drive that boots the system. Adapters also might expect drive 1 to be a hard drive. If that is the case, don't assign address 0 or 1 to other types of devices.
- Lower-numbered SCSI addresses have higher priorities. If your machine has hard drives and a CD-ROM, assign the hard drives to the lower addresses.
- A system can support up to four SCSI adapters. You must, of course, insure that each adapter is assigned unique settings for interrupts, memory, and so on.
- Some SCSI adapters are equipped with floppy drive connectors. If your floppy drives are attached to other controllers (such as the controllers built into most current motherboards), be sure to disable floppy drive support on the SCSI adapter.
- You add devices to the SCSI bus by daisy chaining cables from device to device.
- The first and last devices in the SCSI daisy chain must be terminated*.
- Devices in the middle of the daisy chain must not be terminated.
- External SCSI devices must be separated by at least .3 meters of cable.
- The total length of the SCSI cable daisy chain cannot exceed 6 meters.
- At least one device must supply power to the SCSI bus. In most cases, this will be the SCSI adapter. All devices are protected by diodes, so it doesn't matter if more than one device is supplyng power to the bus.
SCSI is an attractive technology because one controller can manage seven devices (or more with Wide SCSI). Each device is identified on the SCSI bus by a unique SCSI address. Several precautions were mentioned in the preceding list.
A PC can support four SCSI buses, and devices on separate buses can share the same address. You must, of course, find distinct IRQ and other required settings for each SCSI bus adapter. The addresses on internal devices are usually set with jumpers. Many external devices are equipped with switches that make it easy to set the addresses.
A few new SCSI devices are being equipped with a SCSI Plug and Play feature that enables them to be configured with a SCSI address automatically. Plug and Play devices can coexist on the same bus with standard SCSI devices.
*Note: Whenever you have a SCSI adapter connected to ONLY internal or ONLY external devices, but not BOTH, the adapter itself is considered to be the "end of the line", and therefore it must be terminated.