From: jjg@pt.com (John Grana)
Subject: Re: Problems with Fast SCSI-II on Sun Sparc 10 (long)
Organization: Performance Technologies, Incorporated
Date: Thu, 19 Nov 1992 01:04:45 GMT

In article <1992Nov16.130949.1490@devon.co.uk> mcneill@devon.co.uk (Keith McNeill) writes:
>From article <1992Nov15.121008.12717@pt.com>, by jjg@pt.com (John Grana):
>
>Ok...novice question.  What is the difference between active and passive
>termination?  Does the active termination have a power source?
>

Passive termination uses 2 resistors - a 220 ohm and a 330 ohm per signal
hooked up something like this: (aka parallel termination)

         TERMPWR (~4.4 to 5 Volts)
            V
            |
            \
            /  220
            \
            |
            +----------- SCSI SIGNAL (i.e. DATA, MSG,CD,REQ,ACK etc)
            |
            \
            /  330
            \
            |
            -
           GND

Active uses a low noise voltage regulator and 110 ohm series resistors
to terminate each signal:
                                                    
                                                   110 Ohm 
                      --------------           |--/\/\/\-----> SCSI SIGNAL
    TERMPWR >---------|  Regulator |-----------|
                      |            | 2.85V     |--/\/\/\----->
                      --------------           |
                            |                  |--/\/\/\----->
                            -
                           GND

The passive terminators tend to keep the line at a level more determined
by the level of TERMPWR since there is no isolation. In addition, any
noise induced onto the TERMPWR line (its just another wire in the cable)
will show up at the termination point at the resistors.

Active solves this by regulating TERMPWR to 2.85 volts and matches
the ideal line impedance of 110 ohms. Be forewarned... this 110 ohm
impedance is ok for ribbon cable, but in the Sun world we exit the
system on shielded round cable - closer to 90 ohms impedance! So,
ideally, you could hope that the series termination resistors are
in sockets and try to reduce them to 100 or even 90 ohms.

FPT uses a more complex circuit that uses a combination of various
filter caps, a series resistor and a collection of planer/zener diodes
on each line. Sorry, I can't draw that one from memory!

Passive was fine for older SCSI (3-4 MB/sec). For a loaded SCSIbus (5 or
more devices) and/or SCSI Fast (10 MB/sec) it doesn't work. Period.
Active does a much better job for 10 MB devices but starts to
fall after 2 or 3 fast devices and/or long (> 4 feet) cables.
A combination of active (on the data and phase lines) and FPT on the
critical handshake lines seems to do fine for 4 or 5 fast disks and 
a tape or cd-rom. 

Last but not least, you MUST have the same style of termination on
BOTH ends of the SCSIbus to get the benefits. Active at one end and
passive on the other is only as good as the weakest link. I have found
that active only on one end and active/fpt at the other is not near
as robust as active/fpt at both ends.

-- 
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|John Grana, Performance Technologies Incorporated		jjg@pt.com|
|315 Science Parkway, Rochester, New York 14620           uupsi!ptsys1!jjg|
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