Officially known as Small Computer System
this type of interface has been around for over a decade.
It is used to connect a system (such as
a P.C.) to a disk drive, a scanner, or a high-speed printer.
Most of its growth had slowed down and
in the early 1990's, but within the last two years
it has grown again and again, so much so
that I was confused with all the revisions.
Here is a chart and some notes I put
to try to demistify all the terms.
SCSI bus is 8 to 32 bits wide.
16-bit and 32-bit variations are called
wide-SCSI and call for an additional cable.
Wide-SCSI is a SCSI-2 feature.
Commands, messages, and status are sent
across the bus exclusively
using asynchronous transfers. Max
Data is sent usually synchronously nowadays.
SCSI-2 allows up to 10MHz.
Wide SCSI requires an additional cable (or
additional wires in the current cable). Its name
applies to either 16-bit or 32-bit transfers.
||16 (or 32, but no implementations yet)
||68 or 80-pin SCA
SCA = Single Connector Attachment.
An 80-pin connector popular in Unix workstations where the SCSI pins
combined with power and ground pins into one keyed connector.
Single-ended: Each SCSI data pin either
a signal or is a ground pin. Popular with PCs.
LVD: Low-voltage differential.
Became very popular with Sun servers and newer RAID servers.
Differential: Each SCSI data pin either
is +signal or -signal. Popular with Unix servers. Allows
distances. Think large server rooms or
The normal voltage is 5V, but I don't know
what voltage the low-voltage is. Probably 3.3V.
trend is to hang the bytes-per-second on to the word Ultra, i.e.
Ultra-160 or Ultra-320. These terms
are LVD SCSI.
HVD: Originally there were only single-ended and differential
SCSI. Since LVD was created differential
SCSI has been nicknamed HVD. If you see a connector or terminator
that only says "differential SCSI" or
has the universal differential SCSI symbol with the horizontal dash
sticking outside the diamond, that is HVD.
You can use the same cables for single-ended and differential SCSI, but
the terminators are different.
Single-ended terminators won't work properly to terminate differential
busses. I have read that "Ultra2 SCSI"
was available in both HVD and LVD, but no longer in single-ended
configuration. There were a few confusing
years there. Check
1, for an accurate chart.
I've had problems terminating a Sun HVD bus with a non-Sun HVD terminator. After unsuccessful attempts to use a LVD terminator, a forced-perfect terminator, and an HP HVD terminator I finally threw in the towel and EBay'ed a Sun HVD terminator. Those things are expensive and kind of rare ($20 in 2006), but if you need one you don't have much choice.
Last update: 12-Feb-2007