🚀 go-pugleaf

Paul Durrant <pdurrant@durrant.demon.co.uk> wrote:

> In article <1d3sfvs.1plnbhi1qowoiyN@p31.nas1.is2.u-net.net>,
> real-address-in-sig@flurbletigibbet.rhubarb.foobar (Rebecca and wrote:
> 
> > Sean Sharkey <sean@g0oanint.demon.co.uk> wrote:
> > 
> > > To cut a very long story short the SCSI spec states that the
> > > host device (Computer) must provide a +5v line on the SCSI
> > > connections to supply the terminator resistors. 
> > 
> > Hang on - that can't be right.  Termination resistors don't need any
> > power at all.  They connect from the signal lines to a 0V line.
> 
> I don't have my reference books to hand (on a train), but IIRC termination
> works by connecting the signals lines to *both* 0V and 5V through some
> resistors. Termination power is certainly needed for even passive
> terminators to work correctly

This is the case for the SCSI standard - I'm not sure why.  I've done
termination myself by just connecting a resistor to ground, and it seems
to work quite happily in other situations.

> > > Guess what, the Mac plus doesn't supply this voltage
> > > so even if you use plug terminators your SCSI chain
> > > still can't figure out that anything is connected to it.
> >
> > I'm using a plug terminator on the end of my Mac Plus's SCSI chain and
> > it works perfectly happily.
> 
> Many external SCSI devices (especially older ones) supply termination power
> to their external SCSI connectors. This lack of termination power on Mac
> PLuses is one reason for the old advice to always turn on external devices
> before turning on the Mac. I remember having terrible problems with one Mac
> Plus/external HD setup, until we found that the internal cable onthe HD had
> the term power line not connected.

I see - thanks.

Rowland.

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