SCSI is a separate bus that is attached to a host system via a Host Adapter. Once the host sets a process in motion, the SCSI devices can "talk amongst themselves" and complete a task or tasks (que) without further reference to the host system--something not as yet possible with IDE. This extra intelligence is what adds a lot of the extra cost of SCSI devices, not to mention that SCSI is aimed at a more "mission critical" business type user and are built to deliver the reliability they require (and are willing to PAY for).
SCSI=5MB/sec - max. txfer rate - narrow (8-bit parallel data)
SCSI2=10MB/sec - "
UltraSCSI=20MB/sec - "
UltraSCSI2 (or SCSI3)=40MB/sec - "
UltraSCSI3=80MB/sec - "
There is a 160MB/sec standard, but I don't know if that is 16-bit or 32-bit or what, or its name.
Wide (16-bit parallel data) versions of above=double the data transfer rate (10, 20, 40, 80, 160).
LVD=Low Voltage Differential - two wires for each data line plus ground- cancels noise and allows longer cables w/o loss at high data rates at standard 5v.
HVD=High Voltage Diff - similar to above, but used higher than 5v for signals - not used much anymore.
Fiber Channel - Uses fiber-optic cables for data transfer instead of copper. Obvious benefits re noise and max possible data rate. Probably only used in mini and mainframe envionments.
Narrow normally uses a 50-pin version of the connector you are used to on IDE drives.
Wide uses a 68-pin connector that looks kind of like the 25-pin D serial connector but much higher density.
SCA is an 80-pin connection standard for disk arrays where drives are usually mounted in special trays and plugged into a backplane. This connector also looks like the IDE connector but pins are closer together. You can use SCA devices on standard cables with adapters which are different for normal or LVD.
SCA is used on all types of devices--narrow, wide, SE and LVD and combinations thereof-- which can make determining cabling and termination "interesting".
Differential chains also use separate terminators which are either built into the end of the cable or plugged into the last connector. SE devices often have terminators built into them which can be activated by jumper(s) or switches. "Active Termination" is preferred for all systems. If at least one of your drives can't provide active termination internally, use a separate terminator plugged into the end position. Fussbudgets will always use a separate active terminator. Only one termination per chain end.
Much of SCSI is backward compatible (LVD drives can often be jumpered to SE), but if you put a SE drive on a LVD channel, for instance, the whole channel will be limited to SE speed. There are host adapters that can handle both LVD and SE chains where both will work optimally.
Newbies should probably check out
Hypermicro's web site for products and advice, and
Gary Field's SCSI Info web site for more technical info.
.bh.