Technical data

Chapter 5: System Configuration and Operation

SCSI Conﬁguration Requirements

All SGI 2400 and 2800 rackmount systems are conﬁgured with one or two internal SCSI

drive bays using single-attachment connector (SCA) drives (see Figure 5-2). These drives

must all be single-ended, Ultra SCSI. These Ultra SCSI drives are capable of transmitting

up to 40 MB/sec. In addition, the rackmount can house from one to four optional drive

box enclosures (Origin Vault) to support other types of drives such as differential or

single-ended SCSI drives (see Chapter 2, “Chassis Tour,” for additional information).

Note: The difference between single-ended and differential SCSI channels is deﬁned as

follows: a single-ended SCSI channel pairs each signal line with a ground line.

Differential SCSI channels pair each signal line with a second signal that is the balanced

inverse of the ﬁrst. This conﬁguration makes differential SCSI less susceptible to signal

degradation due to noise and more suitable for remote (longer) cabling.

The maximum allowable length for single-ended (standard) SCSI cabling is 19.6 feet

(6 meters). This length reﬂects the combined lengths of both the internal and the external

cables. The maximum allowable length for differential SCSI is 80 feet (25 meters). As with

the single-ended SCSI, this length is the sum of both the internal and the external cables.

(A wide SCSI bus supports a maximum of sixteen devices, including the M SCSI

controller board. A narrow SCSI bus supports a maximum of eight SCSI devices,

including the controller.)

Note: The most common reasons for SCSI device failure are insufﬁcient noise margins

due to exceeding the maximum cable length, cable impedance mismatches, or a

combination of both. If you are having trouble with certain devices, particularly external

devices, be sure that you have not exceeded the maximum SCSI cable length.

Always use the shortest cable possible. Route external cables away from the potential

damage of foot trafﬁc, cleaning, and so on.

If you have additional questions about SCSI connections and cable lengths, contact your

SGI representative.

Note: To operate multiple external SCSI devices, you must order SCSI XIO boards that

plug into the XIO cardcage.