Re: SCSI mod success!

[dempson@actrix.gen.nz (David Empson)]

Randy Shackelford <shack@onyx.southwind.net> wrote:

> In article <3782AFCF.522A4D10@swbell.net>,
> Rubywand  <rubywand@swbell.net> wrote:
> 
> >> Also I recall David Empson expressing some reservations about have the
> >> card provide the termination -- any reason to get nervous?
> > ....
> >
> >     David's main concern seemed to be possible current demands upon the
> >computer for Termination Power supplied through the SCSI interface.
> >Unless you connect some old dinosaur hard drive which also does not
> >supply Term Power, there should be no problem.

If you're going to do that, then there is no point modifying the SCSI
card to supply termination power as well.  I've always used hard drives
which supply termination power, so I've never had to modify any of my
SCSI cards.

(On the other hand, if I needed to hook my ZIP drive up to the IIgs and
didn't have my normal hard drive along, then it wouldn't work properly.)

> >(And, the small diode recommended for the mod would act like a fuse
> >for any really heavy load.)

Not well enough to protect anything, I expect.  You could probably drag
a few amps through the diode before it burned out, unless you used a
diode that wasn't able to pass sufficient current in the first place.

I examined one of my hard drives closely, and observed that it had a 1
amp fuse on TERMPWR along with a diode.

> The reason why Apple SCSI cards (and Mac Pluses and Powerbooks) don't supply
> terminator power is that it's power supply intensive.

Yep.  If I remember my rough calculations correctly, a correctly
terminated SCSI bus could drag 480 mA out of the TERMPWR signal if
standard passive termination is used (in the worst case).  Active
termination might need more than this.

The Apple II power supply is only supposed to deliver 500 mA on the +5V
rail for all the slots to share.  If the SCSI bus is dragging 480 mA for
termination power, that doesn't leave anything for the SCSI card, let
alone any other cards you might have in there.

Using a heavy duty power supply together with beefed up +5V wiring to
the slots would get around this problem.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand