🚀 go-pugleaf

Sean Sharkey <sean@g0oanint.demon.co.uk> wrote:

> Rebecca and Rowland <real-address-in-sig@flurbletigibbet.rhubarb.foobar>
> wrote:
[snip]
> Pulling the logic line low without the resistors can be done but in 
> the event you ever get a short to ground anywhere on your system
> your SCSI interface is going to start drawing large emounts of
> current, subbenly your Mac and external gear starts getting very
> smelly, very burnt and very dead, time for a new Mac.

Erm, yes, but I was suggesting that the resistors should be wired up to
the 0V line, thus pulling the logic lines low.  This doesn't need a
power supply.  I don't have the SCSI spec to hand, but it seems that the
220ohms to 5V and 330ohms to 0V arrangement would pull the logic lines
high(ish) - 3V - with an effective resistance of 132ohms to ground as
far as the signal's concerned.

> > > 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.
> 
> I'll take a bet that one of your external SCSI devices has been
> modifyied internally to supply the +5v line.

As far as I know, the external SCSI device is unmodified.  It's an
original Apple 20SC HDD, developed to run off the Mac Plus, so it'll
supply any termination power needed itself.  I'm using a normal modern
plug-in terminator.

Rowland.

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