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.
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.
Not all drives have internal termination capability. On older drives, e.g.
the Hawk series, IIRC, the termination is obvious, as it's a pair, or maybe
three, R-packs. This is easily dealt with, since you have to remove them from
the drive unless that physical drive is at one end of the SCSI chain or the
other. It's a good thing to keep in mind that when you have both an internal
and an external SCSI bus, your host adapter generally is in the middle, hence
should have no termination resistors on board.
Once again, the provision of Termpwr and the provision of termination are two
separate issues. Both must be dealt with correctly, else the system is
unlikely to work properly. The device that provides Termpwr does not have to
have its terminations enabled/present, and the devices at the ends of the
cable MUST have terminations enabled/present, though they needn't be the
source of Termpwr.
I've noticed on several occasions, when using DB25-interfaced SCSI devices,
that Termpwr is not present, and/or that termination is not necessarily
available on those devices.
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.
Dick
----- Original Message -----
From: "Doc" <doc(a)mdrconsult.com>
To: <classiccmp(a)classiccmp.org>
Sent: Sunday, February 24, 2002 1:18 AM
Subject: Apple SCSI TermPWR; Was Re: This is funny (ebay)
On Sat, 23 Feb 2002, Fred Cisin (XenoSoft) wrote:
When talking about the DB25 end, it is ALMOST
standardized.
At one time, Trantor (now subsumed by Adaptec) warned about using cables
intended for Apple with their cards. It was an issue of whether
"TermPWR" was connected.
OK, maybe my deal is related to that, maybe not. It's been bugging me
for a while, though.
I have a number of older Seagate Hawk narrow SCSI 1G-2G drives. I have
a couple of LC 475s, the SE/30, and a IIci. I've never gotten one of
the Seagates to work as the internal drive on any of the older Macs
without using a cable terminator. I've tried term power to drive, to
bus, from bus, and IIRC, one of the 1G Hawks allows term power both to
the drive and from the bus. Never works. Terminated cable always
works.
"Somebody 'splain this wonderment to me!"
Doc