Hello,
Here is something else to consider, electrical storms - lightning.
True store. MR was having trouble with hospital sites that
had remote operators consoles (that were remotely located
>from the MR room, like the next hospital building over.) Every
time an electrical storm would come and a nearby lightning
strike would occur the console interface electronics would
get fried (because of the lightning induced voltage differentials
between buildings.) The solution was an fiber optics interface.
dave garnier
David Garnier
e GE Medical Systems
___________________________________________
David S. Garnier
Senior Technician
Functional & CT Engineering
3000 N. Grandview Ave - M/S W-1250
Waukesha, Wi. 53188
Tel: 262.312.7246
Cel: 414.915.6529
From: [email protected]
[mailto:[email protected]]On Behalf Of Piotr
Sent: Friday, February 06, 2004 12:13 PM
To: EMC-PSTC
Subject: Re: RS485 and CM choke
Michael,
I posted exactly the same text. The 1 km is there, but not at the
beginning.
An year ago I thought it is important. Now I think no (from EMC point
of view). The tests are made not on the 1km cable, but on 1m cable.
The problems made by 1 km are simulated with disturbances inserted
in that 1m. So for me the disturbance levels (set in standards) are what
I
am
analysing to be sure my device will pass them.
I agree in real live it is difference if I have 100m and 1km.
I plan to allow 1km but only in one building. As RS485 must be connected
in one line it is possible to have 1km in one building.
Transformers.
RS485 has the low frequency elements and even DC. You can use RS485
transceivers to communicate using transformers (I think) but with some
curry
signal modulated with your data and not the standard UARTs in
microprocessors.
With enough big trafos (my speed is 115kb/s, and there can be 9 zeros or
ones
in serie) you can probably transmit standard RS485 data, but the problem
begins
when the frame is ended, there is (can be) DC (3V) on line, than next
frame
begins.
I suppose the first byte in each frame will be corrupted because 0 on
trafo
output corresponds to +3V on input. -3 on input gives -6 on output.
After
some
time the output becomes to be correct. It is if everything is close to
each
other.
Then analyse 1km cable.
Using small trafos - you must detect slopes and not states - it becomes
to
be not
RS485.
Regards
Piotr
From: "Nagel, Michael" <[email protected]>
To: "EMC-PSTC" <[email protected]>
Sent: Friday, February 06, 2004 1:44 PM
Subject: RE: RS485 and CM choke
>
> Pjotr,
>
> In your original post in sci.engr.electrical.compliance you talked of
a
> cable length of up to 1km. This is not negligible and should also be
> mentioned here.
>
> You have to deal with burst, surge and potential differences (earth is
> not the same potential everywhere) as well.
>
> When you are in control of both ends of the communication link, you
> can decide on how you do it.
>
> Wanting to avoid the higher cost of optoisolation - did you think of
> using transformers? I do not have to deal with RS-485 very often, so
> I leave the maths up to you, but this is comparably cheap.
>
> DSL - transformers should be OK for this, maybe E1/T1.
> Using the center tap for filtering gives you a good common mode
rejection.
>
> I haven't seen this solution for RS-485 yet, but maybe there is
someone
> else who did ... On the other hand I see no reason why this should not
> work but I am ready to learn if there is one ...
>
> Best regards,
> Michael
>
> Michael Nagel - Senior EMC Engineer
>
> FORCE COMPUTERS GmbH Lilienthalstrasse 15
> A Solectron Company D-85579
Neubiberg/Muenchen -
> Germany
> Tel: +49-89-60814-0 Fax: +49-89-60814-376
>
> e-mail: [email protected] WWW:
http://www.forcecomputers.com
>
>
>
>
>
> > -----Original Message-----
> > From: Piotr [mailto:[email protected]]
> > Sent: Friday, February 06, 2004 11:40 AM
> > To: EMC-PSTC
> > Subject: RS485 and CM choke
> >
> >
> >
> > Hello,
> >
> > I posted my question at sci.engr.electrical.compliance and
> > was suggested to
> > post it here.
> > So here it is.
> >
> > I'm trying to find if it is possible to do RS485 without
> > optoisolation.
> > EN61000-6-2 and EN50130-4 needs interface to work with common mode
> > disturbance in 150kHz to 100MHz range of 10V (80% AM
> > modulation) and 150 Ohm
> > source impedance. This gives me 18V top signal level. I've
> > read that it is
> > good practice to add extra 6dB (measurements inaccuracy and element
> > distortions).
> > This way I have 36V.
> > I wont to communicate with 115kb/s (I plan to use IC-s with limited
> > slew-rate), so even high impedance common mode choke should
> > make no problem.
> > I assume up to 32 devices and up to 1km line.
> > I see two solutions:
> > Solution 1.
> > CM choke 2x2mH + 2.2nF capacitor at each line to ground. This
> > reduces 36V to
> > acceptable values but gives me 1.1nF differential mode.
> > Can this 1n1 be a problem ?
> > I can increase L and decrease C. But the higher L the lower
> > its resonant
> > frequency and I'll have more problems with attenuating higher
> > frequencies.
> > Solution 2.
> > CM choke 3x5mH with no capacitors. RS485 with third wire
> > connecting grounds
> > of communicating devices. I've read somewhere that only 3
> > wire RS485 is the
> > right solution. In this third wire I can place 100 Ohm to avoid
ground
> > difference currents. This solution seems to be very good. One
> > coil senses
> > the CM voltage and subtracts it from signal lines. If it is a
> > good solution
> > and RS485 is popular than CM choke manufacturers should have
> > a large offer
> > with 3 coil chokes of several mH inductance but they don't.
> > CM chokes with 3
> > (and more) wires are widely offered but for higher
> > frequencies. From that I
> > think that I am doing somewhere a mistake. But where ?
> >
> > Connected with that subject is the question: Can I connect my
> > circuit ground
> > with Earth ground via 100 Ohm, and than directly connect
> > circuit grounds of
> > communicating devices with this third wire. May be the choke
> > will be not
> > needed at all (probably if I reduce my needs from 36V to 18V).
> > My devices are powered from external 12V power supply
> > selected by installer
> > (not me).
> > I don't know where the problems can arise from in this situation ?
> >
> > Thanks in advance for any comments.
> >
> > Piotr Galka [email protected]
> >
> >
> >
> >
> > -------------------------------------------
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