What is the difference? SCSI vs. scsi diff

[Ratzie]

I already posted about trying to use a regular scsi instead of a fast wide scsi diff.

But, I would like to know the diffence...
What each does.

[Colin Gomes]

well i guess the normal SCSI would support data transmission rates of upto 4 MBps and the fast wide SCSI supports upto 20 MBps.

[Naveej.K.A · Accepted solution]

Hi,

You have the Single Ended SCSI and the Differential SCSI and the main difference is in the type of the signalling used.

In SE a positive voltage is a "one", and a zero voltage (ground) is a "zero" while in Differential SCSI, two lines are used. For a logical "zero", zero voltage is sent on both wires. For a logical "one", the first wire of each signal pair contains a positive voltage, similar to the signal on an SE bus, but not necessarily at the same voltage.

The second wire contains the electrical opposite of the first wire. The circuitry at the receiving device takes the difference between the two signals sent, and thus sees a relatively high voltage for a one, and a zero voltage for a zero.

in short it uses a diffential voltage for signalling.

Regards,
Naveej

[Suraj Singh_1]

Hi,

Have a look at http://www.pctechguide.com/26interfaces_SCSI_evolution.htm.

Regards,
Suraj

[Uwe Zessin]

And watch out as 'differential' is not always the same as 'differential'.

The old 'differential' is called "HVD" (high voltage differential) these days and is completely incompatible with "SE" (single ended) or "LVD" (low voltage differential).

[Thayanidhi]

Hi,
See attached which is related to SCSI.
It includes connector pictures.

Regds
TT

[Pete Randall]

This always used to puzzle me as well, so when I saw this posting from Gregory Fruth, I saved it:

"Quoted from Gregory Fruth:



There are many many types of SCSI. Each type
has a "speed", a "bus width", and a "signal type".

The "speeds" are: (None), Fast, Ultra, Ultra2,
Ultra3/Ultra160, and Ultra320. "(None)" refers
to the speed below "Fast"; it was the only speed
defined at the time so there was no need to give
it a name. For consistency they ought to have
renamed it "Slow" or "Single Speed" or "1x" or
something when "Fast" came out.

The "bus widths" are (None)/Narrow and
Wide/(None). Again, originally SCSI only had
"Narrow", so the term "Narrow" wasn't used at
the time. If you didn't specify "Narrow" or "Wide"
it was assumed to be "Narrow". However, the
newer SCSI types are "Wide" only, so if you
don't say "Narrow" or "Wide" you can't assume
which one it is unless you know it's Ultra3, for
example, in which case it has to be "Wide".

The "signal types" are Single-Ended (SE), Low
Voltage Differential (LVD) and High Voltage
Differential (HVD). These days HVD has almost
totally vanished, and most people are pretty good
about stating whether a device is SE or LVD.

The term "FWD" stands for "Fast Wide Differential".
The "Differential" in that case is HVD (LVD hadn't
been developed yet). The term "UWSE" stands for
Ultra Wide Single Ended."

See also:

http://www.scsita.org/terms/scsiterms.html
http://www.scsita.org/terms/SCSI_Overview.html
and (from Harry Brown):
http://scsifaq.paralan.com/


Pete

[Ratzie]

Excellent reference