Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpda!hpcupt1!hprnd!kc
From: kc@hprnd.HP.COM (Kurt Chan)
Newsgroups: comp.periphs
Subject: Re: cheapo SCSI disks -- the continuing saga (LONG)
Message-ID: <3170016@hprnd.HP.COM>
Date: 23 Nov 89 00:53:29 GMT
References: <1087@etnibsd.UUCP>
Organization: HP Roseville Networks Division
Lines: 39

|The box has two female "CHAMP"-style SCSI connectors on the back (like
|a Centronics printer connector).  However, when you look inside the box
|you see that they do NOT form a horseshoe loop with the drive in the
|middle -- the ribbon just runs from the first connector to the second
|connector to the drive.  This means you cannot daisy-chain these
|units!  (You would have to modify the cable to make a horseshoe).
|Since I do not plan to daisy chain, I just connect to the lower CHAMP
|connector, the one at the end of the cable (I'm not a SCSI expert, but
|it's my understanding that you must avoid all "T" connections to the
|SCSI bus -- by connecting to the middle CHAMP, the little strand of
|ribbon cable to the lower connector would constitute a T and signal
|quality could be compromised -- SCSI experts please correct/amplify
|these remarks).

The "T" connections you refer to act as transmission line stubs.  ANSI has
limited the length of the stubs to .1 meters on single-ended systems (and
device capacitance to 25 pF, including the stub).  On maximally configured
systems (6 meters, 5-7 peripherals) the cumulative effects of marginal
transmission line design can cause failure.  Also note that SCSI-2 recommends
that devices should be spaced no closer than 0.3 meters to reduce the stub
effects.

|However, now that I am connecting to another vendor's shoebox, I must
|take precautions.  As a simple, if ugly, fix, I just cut wire 26 of the
|ribbon cable at the Sun end, causing line to float.  (Actually, I made
|a short piece of cable with line 26 cut which I can insert between the
|sun and the cable to the disk drives).  Strangely, when I tested for
|continuity between pin 26 and ground on the 3/60, there was none.
|Still, better to be safe...

Hopefully, the Sun is always at one end of the cable and provides it's own
terminating circuit.  You can test this by powering up the Sun, disconnecting
the SCSI cable, and probing the SCSI signal wires on the bulkhead.  They
should read no less than about 2.5V and, when grounded through an ammeter,
should provide about 40 mA of current (depending on the value of Sun TERMPWR
and the resistance of the ammeter).

Kurt Chan
Hewlett-Packard