Compaq AA-RHGWB-TE manual Scsi Bus Versus Scsi Bus Segments, Transmission Methods

3.2.1 SCSI Bus Versus SCSI Bus Segments

An UltraSCSI bus may be comprised of multiple UltraSCSI bus segments. Each UltraSCSI bus segment is comprised of electrical conductors that may be in a cable or a backplane, and cable or backplane connectors. Each UltraSCSI bus segment must have a terminator at each end of the bus segment.

Up to two UltraSCSI bus segments may be coupled together with UltraSCSI hubs or signal converters, increasing the total length of the UltraSCSI bus.

3.2.2 Transmission Methods

Two transmission methods can be used in a SCSI bus:

•Single-ended — In a single-ended SCSI bus, one data lead and one ground lead are utilized for the data transmission. A single-ended receiver looks only at the signal wire as the input. The transmitted signal arrives at the receiving end of the bus on the signal wire somewhat distorted by signal reflections. The length and loading of the bus determine the magnitude of this distortion. This transmission method is economical, but is more susceptible to noise than the differential transmission method, and requires short cables. Devices with single-ended SCSI devices include the following:

–BA350, BA356, and UltraSCSI BA356 storage shelves

–Single-ended side of a SCSI signal converter or personality module

•Differential — Differential signal transmission uses two wires to transmit a signal. The two wires are driven by a differential driver that places a signal on one wire (+SIGNAL) and another signal that is 180 degrees out of phase (-SIGNAL) on the other wire. The differential receiver generates a signal output only when the two inputs are different. As signal reflections occur virtually the same on both wires, they are not seen by the receiver, because it only sees differences on the two wires.

This transmission method is less susceptible to noise than single-ended SCSI and enables you to use longer cables. Devices with differential SCSI interfaces include the following:

–KZPBA-CB

–KZPSA-BB

–HSZ40, HSZ50, HSZ70, and HSZ80 controllers

–Differential side of a SCSI signal converter or personality module

You cannot use the two transmission methods in the same SCSI bus segment. For example, a device with a differential SCSI interface must be connected to another device with a differential SCSI interface. If you want to

3–4Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware