Technical data

3.2 SCSI Bus Performance
Before you set up a SCSI bus, it is important that you understand a number
of issues that affect the viability of a bus and how the devices that are
connected to it operate. Specifically, bus performance is influenced by the
following factors:
• Transmission method (Section 3.2.2)
• Data path (Section 3.2.3)
• Bus speed (Section 3.2.4)
3.2.1 SCSI Bus Versus SCSI Bus Segments
An UltraSCSI bus may comprise multiple UltraSCSI bus segments. Each
UltraSCSI bus segment comprises 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.
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.
3–4 Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware