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
