SCSI termination

Julian Stacey jhs at
Thu Jun 14 15:27:18 UTC 2007

Scott Long wrote:
> docaia wrote:
> > can somebody please explain to me a simple way what is a single ended
> > terminator and what is a differential terminator and what is the damage
> > which may be caused by putting Single Ended devices on a Differential bus?
> > I am a geophysicsit so I don't have a strong electronic backgroud, so
> > consider me as a very beginner, and please explain this to me thoroughly.
> > thank you
> The big issue to watch out for is High Voltage Differential (HVD) vs Low
> Voltage Differential (LVD).  They operate at dangerously different
> voltage levels, and mixing them together will often cause smoke, fires,
> and permanent equipment (and personnel) damage.

To add to Scot's top down look, Here's a bottom up look:
QUESTION: What is a SCSI terminator? Why do I need them?
ANSWER From: Gary Field (scsifaq at
Updated:  May, 1999 
    * Passive terminators consist of pairs of resistors. A 220 Ohm
    pulling each signal up to TERMPWR and a 330 Ohm pulling each
    signal down to GROUND. Passive terminators were considered
    adequate in SCSI-1 when the bus only ran at 5 MHz. In SCSI-2,
    passive terminators were given the name "Alternative 1".

    * Active terminators consist of 110 Ohm resistors connected
    from each signal line to a common 2.85 Volt regulated power
    supply. Active terminators both terminate the bus better (less
    reflection), and supply cleaner pullup current (due to their
    Voltage regulation).  They were first defined in SCSI-2 and
    were given the name "Alternative 2" to distinguish them from
    passive terminators.

I see you ask about "differential terminator" too,
perhaps googling will give a better answer, but:
    "Differential signalling costs more (not just scsi, but way
    back to telegraphy, modems long V24 serial cables etc),
    differential measures difference in voltage between _2_ adjacent
    usually twisted wires.  Works further/ faster/ less errors,
    than comparing a single voltage on a single wire to a reference ground.

PPS I've had 50 wire flat old scsi-1 melt through plastic & hot
wire hurt fingers & droop way out of ribbon threatening more damage,
(I'd got pins offset or wrong way on a board with bare 50 pins &
no socket for alignement... It worked again with new cable 
(was a scsi-1 on a PC532 )

Julian Stacey. Munich Computer Consultant, BSD Unix C Linux.
 HTML mail unseen. Ihr Rauch=mein allergischer Kopfschmerz. Dump cigs 4 snuff.

More information about the freebsd-scsi mailing list