Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!uc!cs.umn.edu!kksys!com50!chris
From: chris@com50.c2s.mn.org (Chris Johnson)
Newsgroups: comp.periphs.scsi
Subject: Re: Sun SCSI termination
Keywords: TERMPWR Termination
Message-ID: <1990Aug22.170552.17671@com50.c2s.mn.org>
Date: 22 Aug 90 17:05:52 GMT
References: <1990Aug13.011541.6424@rice.edu> <36040@ppgbms.UUCP>
Organization: Com Squared Systems, Inc.
Lines: 66

In article <36040@ppgbms.UUCP> paul@ppgbms.UUCP (Paul Evan Matz) writes:
>In article <1990Aug13.011541.6424@rice.edu> devil%TECHUNIX.BITNET@cunyvm.cuny.edu (Gil Tene) writes:
>>X-Sun-Spots-Digest: Volume 9, Issue 302, message 9
>>
>>- Which Suns don't provide termination power on the SCSI bus ?
>>  (Does the Sparcstation 1,1+,SLC,IPC ?)
>>
>
>The SparcStation 1 and 1+, Sun3/50 and 3/60, all do not supply
>term power.  And, I think I recall that some 3/60s even have
>their TERMPWR pin (pin 26) wired to ground.  This can cause
>serious smoke if you connect a device that sources term power;
>The 386i seems to be the only Sun (non-VME) that sources term power, 
>as this was required to power the external terminator plug that 
>was supplied with that machine.  Don't know about the SLC or
>IPC, or any of the VME-based machines;  Anyone out there know???
>
>>- If the Sun DOES provide termination power, can setting the terminators
>>  on the drive to use the drive's power HURT anything (two power sources) ?
>>
>
>A device that has termination installed on it locally usually
>does not power it via the TERMPWR lead, but instead via its own
>local +5V supply.  So, another device suppling TERMPWR will not
>cause a problem.  The problem occurs when two devices are strapped 
>to source TERMPWR;  The current practice lately is for devices that
>supply TERMPWR to do it through a diode, so that the two sources of
>+5V don't "fight" with each other, as would normally happen if two
>power supplies were tied together in parallel.  There is a reference
>to this issue in Sun's SCSA (Sun Common SCSI Architecture) documents.
>Their basic position is that any device that supplies TERMPWR must
>do it through a diode.  This may also be a SCSI II requirement.
>
>Because the Sparc doesn't supply TERMPWR, it wasn't immediately
>clear how to use the external terminator, so instead, the last device
>inside the external box was terminated.  This seemed OK since that
>is historically how Sun3 shoeboxes where terminated.  I've always
>wondered about that segment of ribbon, between the last device and
>the SCSI out connector on the shoebox, though.  1-2 Ft. of ribbon
>past the termination does violate the intent of the SCSI spec.
>
>Well, anyway, I guess the SparcStation is more sensitive than a S3,
>(especially considering that creative SS1 SCSI bus implementation)
>because once the termination was moved from the last device to
>the external terminator on the SCSI out port, the error messages
>did stop.  (In this case, it was necessary for one of the
>devices to source TERMPWR;  Both the Viper and WangDAT are strap
>selectable.  The WREN may be, but I don't have the manual, so I
>don't know).
>
>So, folks, consider yourself warned!  Those of you planning to
>use old shoeboxes for disk storage on SS1s should consider buying
>a cache of external terminators.

Uh, well now this is completely contrary to my experience with the 
Sparcstation 1/1+.  They both have 125V/2A FUSES on the the TERMPWR
lead, and provide nominal +5V.  Either that, or my multimeter was lying.
Not that this changes anything else about what Paul Matz says, however.

Incidentally, Wrens have a jumper selectable terminator power source, 
either from TERMPWR or from internal power.  They don't source (provide)
TERMPWR, however.

-- 
   ...Chris Johnson          chris@c2s.mn.org   ..uunet!bungia!com50!chris
 Com Squared Systems, Inc.   St. Paul, MN USA   +1 612 452 9522