Re: rescan scsi

Tsung-Hsiang Hsueh (
21 Jun 1997 23:50:19 -0500

> >When you remove the terminator with the SCSI bus still active there is a
> >chance that the wave that is travelling on the wire will get reflected.
> >It is similar to turning on a hose, and then suddenly obstruct the output!
> >The terminator is there exactly to match the impedance so that no waves
> >will be reflected back to the transmitters on the SCSI bus.
> Of course you don't want to yank the termination while there is bus
> activity, but I fail to see even if you did this how it could cause
> psychical device damage. (even though your data may be screwed)
> The signal bouncing back has not been amplified in anyway, and a signal
> within tolerances simply can not hurt anything, not matter how stray it is.
Not really, that could not be withing tolerances, supposing the worst
case, when the reflected signal comes back at 180 degrees out of phase,
then you would have high level (positive voltage) from the reflected
signal matching with low level (negative voltage) of the transmitter output,
or vice versa. And that's like shorting the transmitter output. It might
even survive, but it is definitely not healthy. If it is just one cycle,
nothing will happen, but if it happens for too many cycles, you will have
a burned transmitter chip at least. And this is considering that the data
is not mixed up so badly that could cause other transmitters to fire.
(like some data could be confuse with request for data from other
devices, which could start transmitting signals on the bus... !)