Black Box SCSI-2 LVD manual SCSI-Bus Length, SCSI-Bus Termination

ULTRA2 LVD SCSI HOST ADAPTER

3.3 SCSI-Bus Length

Because of its inherent electrical vulnerability as a high-speed parallel interface, the SCSI bus has definite distance limitations. With earlier versions of the SCSI interface, the maximum length of the bus—the total distance from the terminated device at one end to the terminated device at the other end—were normally inversely proportional to the data rate, as shown below:

Max. Length	Standard
6 m (19.7 ft.)	SCSI-1
6 m (19.7 ft.)	Fast SCSI (SCSI-2)
3 m (9.8 ft.)	Ultra SCSI (SCSI-3), up to 4 devices
1.5 m (4.9 ft.)	Ultra SCSI (SCSI-3), up to 7 devices (narrow) or 15 devices (wide)

Ultra2 SCSI, however, can be extended to distances as great as 12 m (39.4 ft.) with the proper cabling, as long as all of the devices are Ultra2 compliant, or even as far as 25 m (82 ft.) to a single Ultra2 device. (If any attached devices are not Ultra2 compliant, the bus speed drops to that of the slowest SCSI interface represented by a device on the bus, and the maximum distance drops to that of the most length- restricted SCSI interface represented by a device on the bus.) Remember that if you attach both internal and external devices to the Adapter, the bus goes from the last internal device to the last external device. To make sure you can run the greatest possible distance, make sure to use cable with impedance of at least

120 ohms (do not use 90-ohm cable unless all of the devices you’re attaching are non-Ultra2 LVD type); use high-quality twisted-pair cable with high-quality shielded connectors for your external cable.

3.4 SCSI-Bus Termination

To ensure reliable communication, the SCSI bus must be properly “terminated.” That is, the bus must be bracketed with networks of electrical resistors called “terminators” that absorb the remains of data signals and prevent them from echoing and interfering with fresh signals. Unless they are actually built into the cables, terminators must be placed—or enabled, if the last devices have them built in—at the two extreme ends of the SCSI bus. All devices that lie between the ends must have their terminators removed or disabled. SCSI cabling is designed to be connected in daisychain fashion, with no device on the bus being directly connected to more than two others. No branching is permitted on the SCSI bus. Refer to the example setups in Figure 3-1 on the next page.

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