In its simplest form, a host adapter provides support for a single SCSI chain: that is,
a single set of devices that are all connected together on the same SCSI bus. This is the
way that many older and low-end SCSI host adapters work. They are fine for simple
implementations, but are too limiting for complex SCSI setups. Especially with modern
systems that need to use both LVD and single-ended devices, an adapter with support for
just a single segment is insufficient for maximum performance. To expand capabilities,
host adapter manufacturers make cards that support multiple segments, multiple channels,
or both.
A segment is an electrically-isolated "piece" of a SCSI bus; a
single bus can be made up of one or more segments. Cards that implement multiple segments
allow for more flexibility because the segments are electrically separate. Each segment
can have a cable as long as the normal maximum cable length allowed for that particular
type of SCSI, for example. One segment can use an internal cable within the PC and another
an external cable. It's important to remember though that two segments on a single channel
are logically considered to be part of the same SCSI bus even if they are electrically
separate. This means all devices on all segments must have unique IDs,
and that maximum bandwidth is shared between all devices on
all segments that make up the bus.
The most expensive host adapters go beyond multiple segment support and actually have
multiple channels. These are similar in concept to the way an IDE/ATA controller typically has two channels. Each
channel is completely independent of the other, both electrically and logically.
This means the two run in parallel with each other: you get support for twice as many
devices, and twice as much throughput. In essence, a card with two channels is two host
adapters in the same package. For example, an Ultra160 host adapter with dual channels
will support 30 drives (16 per channel less one each per channel for the host adapter) and
theoretical throughput of up to 320 MB/s (160 MB/s per channel). Note that each channel
can itself have more than one electrical segment.
Host adapters that support multiple channels are not really needed for most
applications, especially if already using high-performance SCSI like Ultra160; they are more common in servers than desktop PCs.
Multiple segments, however, are commonly found even in desktop SCSI cards. One common use
for multiple segments is to allow independent use of LVD and SE devices on the same host adapter without causing the LVD
devices to degrade to SE operation. See this page for more on this
important feature of Ultra2 and faster host adapters. Some cards use multiple channels
to isolate LVD and SE, which is probably even better (though it may be more expensive.)
Next: RAID Support
