Exabyte EXB-8505 8mm manual Requirements for Connecting a Scsi Cable, Impedance, Cable Length

4 Requirements for Use

Requirements for Connecting a SCSI Cable

The cable for connecting the tape drive to the SCSI bus is not provided with the tape drive. You must provide a cable that complies with the appropriate safety and regulatory agency requirements. To comply with FCC, Canadian DOC, and VDE limits, the tape drive requires shielded cables when the cables are external to the mounting enclosure.

Impedance

Ideally, to match the cable terminators, the cable should have a characteristic impedance of 122 ohms (differential) or 132 ohms (single-ended). However, since cables with this high of a characteristic impedance are not generally available, somewhat lower impedances are acceptable. A characteristic impedance of 100 ohms ± 10% is recommended for unshielded flat or twisted-pair ribbon cable. A characteristic impedance greater than 90 ohms is recommended for shielded cables.

#Important To minimize discontinuities and signal reflections, Exabyte recommends that cables used on the same bus have the same impedances.

Cable Length

The maximum length of the SCSI cable depends on whether you are using a single-ended or differential SCSI configuration, as follows:

For single-ended SCSI configurations, the maximum allowable bus length is 6 meters (19.7 feet). A stub length of no more than

0.1meters (4 inches) is allowed off the mainline interconnection within any connected equipment.

For differential SCSI configurations, the maximum allowable bus length is 25 meters (82 feet). A stub length of no more than

0.2meters (8 inches) is allowed off the mainline interconnection within any connected equipment.

May 1994	EXB-8205 and EXB-8505	4-7
	(Standard and XL)