378
Networking Fundamentals
study of the table in Figure 9-7 will help you understand the evolution of the SCSI
technology. To learn more about SCSI technology, visit the Adaptec Web page at
www.adaptec.com. There you will fi nd in-depth knowledge about SCSI systems
and other technologies.
A SCSI hardware system consists of a host adapter or controller card, a
fl at ribbon cable to connect SCSI devices, and SCSI devices such as disk drives,
CD-ROMs, and tape drives. SCSI cables come in a variety to match the many
different classifi cations of SCSI. The cables come as DB-25, 50-pin, 68-pin, and
80-pin styles.
Look at Figure 9-9 to see the way a typical SCSI host adapter, cable, and
devices might appear. The host adapter and devices can connect to any part of
the chain. The chain of SCSI devices must be terminated. Termination is typically
completed at the last device with a termination block.
SCSI ID numbers
Each device, including the host adapter card, must have a unique ID number.
The numbers start at zero and end at seven for an eight-device SCSI chain and
end at fi fteen for a sixteen-device SCSI chain. The host adapter typically is
assigned the highest number.
SCSI ID numbers are assigned through pins and jumpers on the SCSI drive.
The pins are arranged in pairs. When a jumper is applied across the pins, an
Figure 9-9.
Typical SCSI drive
arrangement.
The last SCSI
drive in the chain
is terminated
SCSI drive
ID 6
SCSI drive
ID 2
SCSI drive
ID 1
SCSI drive
ID 0
Host adapter
ID 7
PCI slot
SCSI drives
ID 3–ID 5