Hello David, I have the same experience. In 1996 I didn't know what is EMC, ESD, Surge ,... When I used RS485 transceivers ESD protected up to 15kV I supposed I can't do anything more. More than 100 installations worked with no problems, but one was on the hill. Twice a year (spring and autumn) we had problems.
Piotr ----- Original Message ----- From: "Garnier, David S (MED)" <[email protected]> To: "'Piotr'" <[email protected]> Cc: <[email protected]> Sent: Friday, February 06, 2004 8:22 PM Subject: RE: RS485 and CM choke > 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 > > > > > -----Original Message----- > 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 > > ----- Original Message ----- > 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] > > > > > > > > > > > > > > > ------------------------------------------- > > > This message is from the IEEE EMC Society Product Safety > > > Technical Committee emc-pstc discussion list. > > > > > > Visit our web site at: http://www.ieee-pses.org/ > > > > > > To cancel your subscription, send mail to: > > > [email protected] > > > with the single line: > > > unsubscribe emc-pstc > > > > > > For help, send mail to the list administrators: > > > Ron Pickard: [email protected] > > > Dave Heald: [email protected] > > > > > > For policy questions, send mail to: > > > Richard Nute: [email protected] > > > Jim Bacher: [email protected] > > > > > > All emc-pstc postings are archived and searchable on the web at: > > > http://www.ieeecommunities.org/emc-pstc > > > > > > > ------------------------------------------- > > This message is from the IEEE EMC Society Product Safety > > Technical Committee emc-pstc discussion list. > > > > Visit our web site at: http://www.ieee-pses.org/ > > > > To cancel your subscription, send mail to: > > [email protected] > > with the single line: > > unsubscribe emc-pstc > > > > For help, send mail to the list administrators: > > Ron Pickard: [email protected] > > Dave Heald: [email protected] > > > > For policy questions, send mail to: > > Richard Nute: [email protected] > > Jim Bacher: [email protected] > > > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ieee-pses.org/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
