Seestern said:
Is it allowed to remove or connect a cable to
the LAN Connector (port) (like USB) on the PC
during operation?
Or do I have to turn the PC and other devices off?
Thank you for your advise!
Seestern
The RJ-45 cabling you use to connect your computer to
router or broadband modem, is galvanically isolated and
reasonably low amplitude. There are no power sequencing issues.
I've been hot plugging them for years, without consequence.
Galvanically isolated, means each Ethernet chip has a set
of transformers. The transformers break the DC path (ground
path). There is no "ground signal" on an Ethernet cable.
A capacitor may be present for bypassing AC currents or ESD
transients, but any DC should be prevented from flowing. This is
why we don't have to worry too much, about the ground potential
of each connected Ethernet device. The actual Ethernet signals
are differential and AC signals. The transformer can pass the
AC signals, where the Ethernet chip can sense them. Common mode
potentials on the wire pair (induced noise) are largely removed
by the common mode rejection of the transformer. It's one of
the nicest designs you can find in electronics, in terms of
how elegant it is compared to the previous generation of
Ethernet interconnect (coax).
Now, there is a difference between a home computing environment,
versus what your IT department faces at work. Someone decided it
would be cool, to carry power down the Ethernet cable, to power
remote devices. POE may not be too common at home, but may see
more usage in a business/industrial setting.
http://en.wikipedia.org/wiki/Power_over_Ethernet
I don't know the details, and there is no mention of whether hot
plugging is safe or not. Notice that there is a protocol, so when the
cable is first inserted, the full voltage is not present on the pins.
If you inserted an open cable into a POE source, the resistive sense
operation would detect whether a POE compatible device was present.
So it looks like power is only delivered, when the (intelligent)
power source detects a compatible device has been connected. That
sounds pretty safe.
The article also mentions the possibility of using "dumb" power
supplies. Such a situation might be appropriate for a surveillance
system with IP video cameras connected to a central device. There would
be a cost saving, by not having to run power cabling to each video
camera, instead using the Ethernet cable for power. The intention
would be, that the cabling not be disturbed once installed. A dumb
device might leave power on the cable at all times. It's probably
still safe for the equipment in that case to be hot plugged - the
difference would be, the possibility of the operator receiving a
shock if touching the contacts on the end of the cable. I'd expect
though, there'd be a significant spark as the contacts touched,
which could possibly remove the plating from the contacts after
a number of cycles. The usage of a dumb power source, really
sounds safest, if the dumb power is turned off before repairing
the equipment, rather than hot plugging it.
So before that POE nonsense came along, I'd have been able to
say the Ethernet cabling was totally benign. Now, not so much.
It's more caveat emptor now, and the context of your situation
may help predict how much care is required. If you're working
on surveillance IP cameras, perhaps more care is required. It
might depend on whether you suspected there were dumb POE
solutions attached to the wiring or not. POE has unnecessarily
complicated a very nice and simple design.
I know my home has no POE solutions in it (like say a VOIP
telephone with no AC adapter being present). So at least
here, I plug and unplug the RJ-45 without giving it a
second thought.
Paul
