24 Feb
2002
24 Feb
'02
1:09 a.m.
On Sun, 24 Feb 2002, Richard Erlacher wrote:
Is it possible you're mixing two things up? Most
of the later SCSI drives
from SEAGATE permitted TERMPWR to come from any device. The standard suggests
that any/all devices can be providers of Termpwr, since only one will have the
highest voltage, hence be the source, as it supposed to be diode-isolated from
the SCSI bus. To implement this option, they provided two jumpers, typically,
which allowed for four options. (1) Termpwr provided by the drive, and to its
terminators from its own power supply, (2) Termpwr provided by the drive, and
to the bus, but its terminators powered fom the SCSI bus, (3) Termpwr provided
by the bus to its internal terminators, and (4) isolation from Termpwr, but
its terminations powered from its own supply. This did not affect whether or
not internal termination was enabled or disabled. That was a separate choice,
handled by a separate option.
If I understand correctly, you're asking if I know (and implemented)
the diffference between Term Power and Termination. Yes. Typically, I
supply termPWR from the terminating device, if possible. Easier for a
feeble mind to track.
Since the Apple SCSI doesn''t provide Termpwr,
and since Termpwr is supposed
to be diode-isolated (source can drive the bus, but bus can't drive the
source) from the bus, any or all devices are permitted to drive Termpwr. Only
one will actually drive it until the voltage is pulled down to where another
might do so. It's not risky to have more than one device drive the Termpwr
line, though it's safest to know which is driving it and make sure it's
enabled there. If your system consists of a computer and a varying set of
external devices, it's easiest just to enable Termpwr at the host adapter and
be done with it. Since that's not an option with the Apple devices, you must
have it supplied by some device that's usually attached to the SCSI bus. Just
pick one that's normally there, and be prepared to enable it on some other
device if it isn't provided otherwise.
Right. If you look at the list of computers I've had trouble with,
they're all Apples with a single internal device. I don't know if it
makes any difference, but all the Hawk drives I've got came from Sun.
All have internal terminators, not R-packs. I have three or four
around. The one that's in the IIci is an ST31200N. It works just fine
if I power termination from the drive and use a terminated cable. The
Mac can't see it at all if I power termination from the drive and enable
the drive's terminators.
In general, it's easy simply to stick an external
terminator on the ends of
the SCSI chain. Termination has no effect on addressing or on Termpwr, so the
only consideration is its location. If you seldom use more than one external
device, or if you ALWAYS use a specific one, it's reasonable to install/enable
the terminations in that device and install other SCSI devices between the
host adapter and that device, which then saves you the
installation/moving/easter-egg-hunt-for the external terminator.
I'm quite familiar with the easter-egg-hunt.... Whenever I spend more
than 5 minutes looking for a terminator, external or internal, I buy one
next time I see one. At GoodWill, if possible. That means I have a
bunch of spares, stashed in suitably surprising locations.
I also have to admit I'm real lazy about this. If I use cable
termination. I don't have to remember the jumper layout of all the SCSI
drive models I have. I just use a terminated cable if possible and plug
stuff in.
Doc