X3T9.2/87-99

To: John Lohmeyer                                       June 30, 1987                                     

From: Ralph Schultz

Subject: Terminator Power

Several years ago, I brought to the committee a proposal which was born out 
of the confusion in the implementation of SCSI Terminator Power.  This 
proposal was discussed, and resulted in the present wording in SCSI-2.  At 
a recent working group meeting, the subject again raised its ugly head as a 
host adapter vendor objected to the mandatory requirement that the 
initiator supply terminator power to the TERMPWR pin.  

Controller manufacturers apparently have not taken advantage of the wording 
in SCSI-2.  Many of the controllers still have terminator packs mounted on 
the controller board.  Some vendors still provide a jumper to permit the 
target to power the TERMPWR pin.  And still others do not power the 
terminators from the TERMPWR pin, but rather connect the packs to the 
controller +5v supply.

In light of the persistence of the problem and the apparent lack of 
implementation to the agreed-to revision, I should like to re-propose the 
original implementation of Terminator Power for SCSI-2.  It seems to me 
that the best implementation is to require that both initiators and targets 
be capable of supplying terminator power to the TERMPWR pin.  The intended 
implementation of this would be that both initiators and targets have 
provision (i.e. jumper) to supply +5v to the TERMPWR pin through a diode 
(or similar device).  The controller power requirements would be XX ma. @ 
5v if the jumper were not installed and (XX + 800) Ma if the jumper were 
connected.  (Incidentally, the jumper could take the form of installing a 
diode into the PC board).  Likewise, the power requirements for the 
initiator would be ZZ Ma if supplying TERMPWR and (ZZ - 800) Ma if the 
jumper were not connected.  With this scheme, the TERMPWR configuration 
for a particular system would be connected at installation by the system 
integrator or recommended to the user by the supplier.  

If the spirit of this proposal were adopted, then subjects like excessive 
current limiting would be moot.  Likewise, problems with maximum current 
from the +5v backboard pin would be compensated by powering from someplace 
else.  Issues like system integrity with some device powered down would 
also be moot, as the integrity is defined by how the system is configured.  

To implement this proposal, I would like to recommend the following changes 
to Section 4.4.3 (page 4-12) of SCSI-2 Rev. 1:

     Delete the first and third sentence of the first paragraph.

     Add the following first sentence:

"All SCSI devices shall have the capability of supplying terminator power 
to the TERMPWR and TERMPWRB pin(s).  This capability shall be individually 
configurable on each device."

     Change the IMPLEMENTORS NOTE to read:

"IMPLEMENTORS NOTE:  The selection of which SCSI devices actually supply 
terminator power is determined when the system utilizing these devices is 
configured.  The configuration may take the form of connecting a jumper on 
the initiator or target, although the standard does not specify the means 
of implementation.  It is recommended that the initiator(s) be configured 
to supply power in order to preserve powered terminators in the event a 
target device is powered down.  The configuration should insure that the 
maximum current available on the TERMPWR and TERMPWRB pins is limited to 
less than 5 amps."