# Re: How to build a db25 terminator

John M. Mills (jmills@jmills.gtri.gatech.edu)
Tue, 8 Sep 1998 11:09:42 -0400 (EDT)

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I checked an old SCSI reference and found the following:
1) the given schematic is correct for single-ended SCSI termination, but
not for differential. For differential, they suggest:

Vcc (+5.0 nominal)
|
\
/ 330 ohm
/
\
|
-Signal ---+
|
\
/ 150 ohm
/
\
|
+ Signal ---+
|
\
/ 330 ohm
/
\
|
GND (0.0 v. nominal)
2. The signal table seems correct for differential 50-pin, but that was
not the same as single-ended 50 pin for at least some signals.

3. My manual had no information on other configurations.

I will look at the information which came with my SCSI card and my scanner
to see if they confirm and/or extend this. I have no reason to expect the
low-speed SCSI interface at my card or scanner are differential; my
aha1502e card uses the same DB-25 configuration as the Symbios P'n'P which
came with the HP 4p scanner, and the scanner uses a 50-pin configuration.
(My scanner includes a switch for internal termination, so I never worried
about this. Our Suns use submini DB-50 connectors and Sun provides a
terminator to plug in at the end of the chain. I have no descriptions of
these - sorry.)

John M. Mills, Senior Research Engineer -- john.mills@gtri.gatech.edu
Georgia Tech Research Institute, Georgia Tech, Atlanta, GA 30332-0834
Phone contacts: 404.894.0151 (voice), 404.894.6285 (FAX)
"The cardinal virtues of a programmer are Laziness,
Impatience, and Hubris." -- attr. Larry Wall

On Sun, 6 Sep 1998, Bostjan Lemut wrote:

> Rogier Wolff wrote:
>
> > Bostjan Lemut wrote:
> > [Charset iso-8859-2 unsupported, filtering to ASCII...]
> > > Hi!
> > >
> > > I've bought a second-hand UMAX Astra 600S scanner, but it came without
> > > terminator (the scanner won't work without it). Since I can't buy it in
> > > any computer shop in our country (it's very small one), I decided to
> > > make one by myself. The problem is, i can't find a scheme on the WEB.
> > >
> > > I would be very greatful if anyone send me the scheme how to build the
> > > damn terminator.
> >
> > +5V
> > |
> > |
> > |-|
> > | | 220 Ohm
> > |_|
> > |
> > Signal ---------+
> > |
> > |-|
> > | | 330 Ohm.
> > |_|
> > |
> > |
> > GND
> >
>
> I've done some more detailed measurments, and I came up with next (see
> attachments).
> Can anybody confirm all this?
>
> Bostjan
>

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