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SCSI Drive and Device Configuration 117
damage equipment by plugging printers into DB-25 SCSI connectors
or by plugging SCSI devices into DB-25 printer connectors. So,
if you use this type of SCSI connection, be sure it is marked well,
because it’s impossible to tell DB-25 SCSI from DB-25 parallel
printer connectors by looking at them. The DB-25 connector is
shown in Figure 4.11.
13 1
25 14
Figure 4.11 DB-25 SCSI connector.
Again, I recommend you avoid making SCSI connections using this
type of cable or connector. If you must use this type of device, add
it to the end of the daisy-chain. The 25-wire connector can prevent
important signals from reaching devices designed to use 50-pin
connectors, causing them to not be initialized and not function.
SCSI Drive and Device
Configuration
SCSI drives (and other devices) are not too difficult to configure,
but they are more complicated than IDE drives. The SCSI standard
controls the way the drives must be set up. You need to set two
items when you configure a SCSI drive or device:
• SCSI ID setting (0–7 or 0–15)
• Terminating resistors
The number of SCSI IDs available on a host adapter depends on its
design: 0–7 on SCSI adapters with an 8-bit bus; 0–15 on SCSI
adapters with a 16-bit bus; two groups of 0–15 on a 16-bit bus with
a dual-processor host bus adapter.
SCSI Device ID
Up to 7 SCSI devices (plus the adapter, for a total of 8) can be used
on a single narrow SCSI bus (8-bit) or up to 15 devices (plus the
adapter, for a total of 16) on a wide (16-bit) SCSI bus. Now, dualprocessor,
16-bit host adapters are available that can operate up to
30 devices plus the host adapter. In every case, each device must
have a unique SCSI ID address. The host adapter takes one address,
so the rest are free for up to 7 SCSI peripherals (or more as defined
by the host adapter). Most SCSI host adapters are factory-set to ID 7