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In <33797DC9.5A54@mailhost.km-mhm.comp.philips.com> "J.P.M. Theuws" <theuwsj@mailhost.km-mhm.comp.philips.com> writes:

>Does anyone know the exact differance between a "passive" and a "active"
>terminator ?

A traditional SCSI terminator has a series of resistor dividers made from
220 and 330 ohm resistive elements.  For a single signal line, the circuit
is like this: 

+5v -------+--------
           *
           *
data ------+---------
           *
           *
gnd -------+---------

Each vertical pair of asterisks represents a single passive resistor. I
can't remember if the 220 ohm or the 330 ohm sits between +5v and the
signal line though. 8-)

Lots of these in a single assembly form a normal 'passive' terminator.
It's easy to see why it's termed a passiver terminator because the voltage
divider never changes it's parameters regardless of the electrical
conditions of the SCSI cable. For longer cables especially, passive
termination can often cause more problems than it solves.

An 'active' terminator is basically an automatically adjusting version of
the passive terminator, and goes a long way to compensating for signal
degradation in longer cables and in cable assemblies that stretch the
electrical specs close to the limits described in the SCSI 'standards'. 

>And does SUN always make use of a "active" terminator on the SCSI-bus...

Older machines like the Sun-3's and early Sparc-based models all used
passive terminators, but I'm not sure about current and recent generation
machines which have SCSI controllers integrated into big ASIC's with other
major hardware 'components'. I suspect active termination is almost the
norm now, especially with SCSI-2 and later interfaces.

Regards,

Craig.

-- 
          Sun Ripened Kernels - http://www.zeta.org.au/~craig/srk.html
                Surplus and pre-loved Sun Microsystems equipment.
                     Based in Sydney, Australia. Operated by
        Craig Dewick - craig@zeta.org.au - http://www.zeta.org.au/~craig