On Tue, 29 Jun 1999 bob@kunk.jriver.com wrote:

> I've been really careful about that. One thing does mystify me though...
> The performa has the internal hard drive on the end of the internal
> cable and a CDROM inline on the internal cable. The internal cable ends at the
> connector to the motherboard. In my system, the hard drive is terminated and I
> assume the other end of the termination is on the motherboard. What happens
> though when you plug in an external drive? Does the motherboard automatically
> switch off it's internal termination because the external drive must be
> terminated?

I believe so> Apple has documented this behavior in PowerMac machines.

Also, it could be that the internal chain is short enough given its speed
that it runs fine with one end unterminated.

A rule of thumb I learned in grad school was that the speed of light is
one foot per second. In cable it'll be slower. The worst ringing will
happen from the drive next to the terminator (its signal will travel the 
whole cable, ring off the unterminated end, and travel back).

If we assume the index of refraction is 2 (speed = 1/2 speed in vacuum)
and a one foot cable, the worst case is 2 (trips) * 1 ft / (0.5 ft/ ns) =
4 ns. That's a 250 MHz clock rate. Normal SCSI, which runs at 5 MHz
probably won't notice that. So you could get away with even having the
internal bus only terminated on one end.

Though definitly terminate something external. :-)

Take care,

Bill