🚀 go-pugleaf

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

> > 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.

Paul Durrant