From: Hans Ridder <ridder@zso.dec.com>
Subject: Re: SCSI signal termination question
Originator: ridder@zowie.zso.dec.com
Organization: Digital Equipment Corporation - DECwest Engineering
Date: Wed, 15 Apr 1992 23:42:56 GMT

In article <40971@shamash.cdc.com> app@shamash.cdc.com ( GSD Advanced Packing/Processes) writes:
>Should there be voltage dividers on each end of the SCSI cable (I
>only have one drive)?

There bus should be terminated at each physical/electrical "end" (i.e.
exactly two terminators.)  Very short buses will *usually* work with
only one terminator, and sometimes you can get away with short
(un)terminated "stubs" (or "branches.)  The resistor type terminators
are usually of the 220/330 ohm network you mentioned.

>I was doing some probing with my scope and it looks like when the
>controller on the interface end is sending a high level signal, the
>voltage goes to approx.  3 volts.  From this I would guess that the
>drivers are open collector outputs and the high level is determined
>by the resistor network.

Yes, usually the drivers are open collector...

>The confusing thing is, when the hard drive is driving the SCSI
>signals, the high level output voltage goes all the way to 5 volts.
>Is there some kind of incopatability here or what?

...however, the standard mentions that you can "drive" the signals high
for possible better performance, rather than relying on the pull-up
network.  I would only expect to see this on high end SCSI devices.

>I know that 3 volts and 5 volts would be seen as high, but I've seen
>strange things happen with logic when an input signal level exceeds
>the supply voltage of the chip.  (The external hard drive has it's
>own power supply, while my interface is being powered by my A500.

This shouldn't be a problem unless, as you say, the signal exceeds the
supply voltage.  The 3V level is just the resistor pull-up level, not
the supply voltage of the chips.

>Another strange thing is that when I turn off power to my A500, but
>leave the power on on the hard drive, the A500 stays powered up!
>
>Pin 26 of the SCSI cable is tied directly to the 5 volt supply of my
>A500.  This signal is referred to as Terminating power of the SCSI
>cable.  My guess is that this voltage should be used by the drive
>as power for the voltage dividers and should be seperate from the
>drive's power source.  When I cut this trace my A500 does not stay
>powered up when I leave the drive on.

You're diagnosis is right on.  Your problem is due to an improperly
designed SCSI host adapter (or "controller" as most people incorrectly
refer to it)  Complain to the manufacturer, there's no excuse for stuff
like this.

The TERMPWR line should be driven through a reverse current diode to
prevent shenanigans like you're seeing.  A plain 1N914/1N4148 will do,
but a low forward drop Schottkey(?) power diode (1N5817?-1N5822?)  would
be better, as the higher the termination voltage, the better.  It would
also be smart to fuse the TERMPWR line at 1.5A for protection.
(Commodore might want to take those last two suggestions too!)

Also note that TERMPWR is usually only used by external terminators, not
usually the internal SIP resistors.  However this depends on the SCSI
device.

>Any insights to this would be greatley appreciated.

Hope I helped.

>					Mike Lundberg

-hans
-- 
  Hans-Gabriel Ridder				Digital DECwest Engineering
  ridder@rust.zso.dec.com			Bellevue, Washington, USA
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