Re: rescan scsi

Dave Cinege (
Sun, 22 Jun 97 00:57:38 -0500

On Sat, 21 Jun 1997 23:35:19 -0500 (EST), Tsung-Hsiang Hsueh wrote:

>> >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 you will get a float.....

>And that's like shorting the transmitter output. It might

No it is not.
#1 It is a pulse(s) not a solid connection
#2 It is a digital low, not true ground. It has ground potential, but is
not a raw electron pool.

>even survive, but it is definitely not healthy. If it is just one cycle,

It will be unhappy, but not unhealthily. Take some TTL and tie a high output
to a low output, and what do you get? About 1.5 volts, a float, with an
indeterminate interpretation by any logic on the other end.

Leave them connected for awhile and maybe they will fry. But from out of
phase pulses? Nothing will happen.

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