> Pitor > > Are you avoiding Optoisolation (and associated DC:DC) due to cost, space > both? > The trade off with other problems may be large as you are seeing in the > discussion. > A great site for application notes on web: > http://www.robustdc.com/?libsect=appnotes > Check out the app note AN005 - Grounding & RS-422/485 on your common third > wire topic. > Note that you will have some issues with the third wire. > There are few cables available that have the 3rd wire integrated without > having to use a 3rd wire off a second data pair for this purpose. > Belden http://bwcecom.belden.com/ does have ONE but then you are limited on > voltage rating and applicaiton specs. > What most industrial RS485 users do is use the shield as the return. > And so have a driven shield. > The shield would be AC referenced to ground at each node (.0047 -.0022 uF Y2 > to ground and 1M bleed off resistor) > I expect this will raise the hair on the backs of the shield purists here > but I challange you all to come up with a solution that would allow the > customer to use a single twisted pair cable with shield that would support > the needs of RS485 and various code application issues. > > Here is a transciever that may help you reduce space and although the IC > cost is significant - the overall circuit design may be where you want it. > http://www.rhopoint.co.uk/components/DI/PDFs/485.pdf > I was tempted to use it but had issue with the failsafe topic (see max part > ref below) > That is a deep subject. if you want some references I will dig them up. > Another approach > http://www.linear.com/ezone/rs485trs.html > > Distance > At 115Kbaud - 1KM is a long distance > You will have to consider the RC loading of the cable (~35pf/M?), loading of > the transient protectors and the loading of the transcievers themselves. > Check out the 1/8 loading transcievers like this one from Maxim > http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1522/ln/en > I am using the MAX3082E in a new design - note guranteed failsafe operation > when line open or shorted > Also note 15KV ESD protection with E suffix . > Another good set of application notes: > http://www.maxim-ic.com/appnotes.cfm/appnote_number/367 > > You will want to specify a low capacitance cable for this distance and limit > the number of nodes on the link. > If you use 1/8 loading transcievers and limit system maximum number to 32 > you may be able to go the distance at this baud rate. > However you have to consider some transient protection. > > EMI > You really want some limiting impedance and transient protection. > About 4 ohms on each data line and 100 ohm on return to terminal helps a > lot. > It will effect your impedance matching but the protection is worth the > compromise. > You would need to use resistors that can handle surge/transient events. > Consider 1-2 W MELF style they hold up well under EMI stress > http://www.irctt.com/pdf_files/CHP.pdf > Between the limiting impedance and the transciever you will need some TVS > protection for CM to DM conversion. > You must use a low capacitance TVS or your baud rate will not work. > Check out biderctional 500/600 joule TVS with built in series diode that > pulls capacitance way down > http://www.microsemi.com/datasheets/MSC1701.PDF > http://www.microsemi.com/datasheets/srlc05.pdf > http://www.microsemi.com/datasheets/sd26a.pdf > You have to dig around on this site to find the best ones - my links are > close but I could not find exactly what I wanted. > Note that you need L:L and L:C protection - there is an SO8 part with bridge > and two TVS that gives all protection needed. > The ESD protection on the transciever will act as your last stage of > protection but note that it does not have the joule rating for IEC1000-4-4 > or -5 tests by itself. ESD event happens in nanoseconds = low joule rating. > > You said you need to deal with CM noise up to 100MHz > You could simply add two ferrite beads on the data lines along with the > series resistance. > Check out http://www.steward.com/ and look for beads to give you ~ 50-90 > ohms at 100MHz. > Opt for current rating that won't saturate quickly (1 Amp). > They do have paired solutions but I suspect that they do not truely act as a > CM choke. > Using separte ferrites will cause some effect on signal but I think it is > managable since the ferrite impedance does not kick in till about 1-10 MHz. > They are lower cost over full blown high frequency CM choke and take up less > room. > > I do remember some app notes on using centertapped telecom transformers > (ISDN?) to isolate RS485 1992 vintage > It was off the site that supplied Arcnet chips www.smsc.com > Try google search on "RS485 transformer isolated" - I found a few > http://www.mucosa.franken.de/arccola/msg00638.html > http://www.arcnet.com/toyo.htm > I built one of these hybrids using an ISDN transformer from Pulse to > experiment and it had some merrit - but I never ironed out the wrinkles. > But even with the transformer you still have issues with EMI. > 100 MHz will walk right through the 20-30 pF Cww of one of these > transformers. > But that could be addressed by the ferrites I spoke of. > Nice issue is that there is no need for return 3rd line! > > Well that is a lot > I expect some reaction to some of these links and tangents. > That's what it is all about - we all have our angles > > Chris Wells > > > ----- Original Message ----- > From: "Piotr" <[email protected]> > To: "EMC-PSTC" <[email protected]> > Sent: Friday, February 06, 2004 5:39 AM > 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
