Re: hot plugs

[The Chazman <devastator.chaz@extern.ucsd.edu>]

> I was plugging a printer into a printer port of my print server(486 with
> linux 2.0.33) This thing is an old packard bell with a riser card. It has
> 2 parallel ports, 1 built in & 1 on a separate isa card. So I was pluging
> a printer into the port on the iSA card and the sytem was on and running.
> I guess I pressed a little to hard and the isa card came out of its slot
> while the box was on. I thought it was curtains for the system and the
> card, but I saw no sparks so I turned it of, took it apart and replugged
> the card back in. THen I plugged the printer back in and it works. Now I
> didn't know ISA was a hot bus. So why didn't something fry?

The most common way to fry something during a hot-plug or hot-unplug is
to drive a signal into a signal input of a chip when the chip doesn't have
power.  When you unplugged the card from the bus, all of the signal lines
on the chip the bus was driving went to float immediately.  However, the
chips were likely still powered for several microseconds more (maybe
milliseconds more) because the bypass capacitors on the board (a few big
ones for the board in general, and one small one for each chip usually)
store enough charge to keep the chips from losing power instantly.  So in
fact, the chips lose signal well before they lose power.

Unfortunately, during a hot plug, it works against you -- the chips are
not fully powered until all the bypass caps get charged up, which takes
several microseconds.  However, the signal lines will become driven
immediately and could promptly fry the unpowered chips.

This is why I'm careful when I hot plug my external Jaz to always plug or
unplug the SCSI connection while the Jaz is powered.  This is not a 100%
guarantee that you'll never damage anything, but I never have so far.  I
once caused a SCSI error that made the kernel reset the SCSI bus, but
that's it.

                               --------Carl