Hi Gert, We (myself ;-) ) know the signals in my RS485. Currently it is 9.6 kb/s, but I plan to use there 57.6 kb/s so the max frequency will be 28.8kHz. I use slew rate controlled RS485 drivers (limited to 200kb/s). Data sheet specifies the "Change in steady-state common mode output voltage" to be less than 100mV. If RS485 is not isolated that becomes the max common-mode signal (but most parts have this much lower). I suppose the DM (3Vpp) to CM conversion gives much lower values here even with untwisted cable.
If someone puts these 2 wire line (without third RS485 wire) to one device direction, and power (ground wire) to the other direction this common mode signal will be measured. But my device B has only one cable with 4 wires (power + RS485). This makes this common mode signal much lower when measured. I read in EN55022:2006 that RS232 (IEEE Standard 1284) is not the telecommunication port. I don't know how the RS232 is specified in that standard but for me RS232 is not symmetric signal with 24Vpp, max speed 115.2kb/s, and cable length up to 15m. Comparing to it my 100mVpp, 57.6kb/s I'd like to be able to assume that 10m (better 20m) cable is not telecommunication port here. Why I need it ? Because with that one device (we expect to sell 5 ..10 pcs/year) I'd like to avoid going to the laboratory (no one has too much money nowadays). That device (7-segment LED display of system time (no RTC inside)) switches internally 10mA LED current with frequency 1Hz. I have used the same design techniques that with all previous devices which showed (when measured) no EMC problems. I'd like to measure myself: - the disturbance at power supply (150kHz to 30MHz), - the power filter characteristic from 150kHz up to 200 MHz, because in my opinion it is responsible for radiated disturbances which likes to happen around 60-100MHz from 12MHz microcontroller clock, and based on it (+ explanation why RS485 is not telecommunication port here) and my experience I'd like to declare that my device disturbance is under the EN55022 limits. I have no ISN. I have made myself AMN (up to 500mA) which gives me the comparable results that I got from laboratory for previously measured devices. Do you (all) think can I do it that way with current EMC directive, or I am not allowed to do it that way ? Best Regards Piotr Galka MicroMade Poland ----- Original Message ----- From: "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> To: "Piotr Galka" <[email protected]>; "EMC-PSTC" <[email protected]> Sent: Saturday, September 12, 2009 7:13 PM Subject: RE: EMC question Hi Piotr, If you take the purpose of the *technical* definition in mind: The telecom qualification of your cable is there to decide if a conducted emissions test is to be carried out. The conducted emission test for telecom apparatuses measures the sum of the equipment common mode current plus a value defined by the differential mode voltage te EUT presents to the ISN, attenuated by the LCL value of the ISN applied. The ISN LCL value of the ISN is to match the cable spec of the manufacturer. Anyway the max frequency tested is 30 Mhz. For the telecom cable to have substantial length for this frequency so be able to radiate, it needs to be 1/4 of a wavelength. at 30 MHz that is 2.5 meters, but to be honest, even smaller lengths do radiate at 30 MHz. So from a spectrum protection point of view a Ethernet cable is a long cable (capable of radiating it's signal contents due to asymmetry) >from approx 1.5 meters off. This calculation assumes that the DM/CM conversion takes place right at the beginning of the cable: that is not true of course. One may discuss about the actual length of twisted pair it takes to create a DM/CM conversion of "LCL" value, but imho a cable longer than 3 meters definitely is "long". Of course, for 10 MHz all values triple, but we don't know what signals the cable will conduct, do we ? Gert Gremmen Ce-test, qualified testing bv Van: [email protected] [mailto:[email protected]] Namens Piotr Galka Verzonden: zaterdag 12 september 2009 14:14 Aan: EMC-PSTC Onderwerp: Re: EMC question I have exactly the same problem - how many meters would be appropriate ? I have device A with two RS485 interfaces. One isolated for connection up to 1200m and it is of course telecommunication port. The second not isolated to connect to the device B. Device B is provided with 3m cable (4 wires: 12V power + RS485). In most cases 3m is enough. Do I have specify the max length for this second connection to make it not being the telecommunication port ? If I have, must it be max 10m, or can it be 20m, 25m ? Best Regards Piotr Galka MicroMade ----- Original Message ----- From: "Steve O'Steen" <[email protected]> To: "John Woodgate" <[email protected]>; <[email protected]> Sent: Thursday, September 10, 2009 6:42 PM Subject: RE: EMC question > What maximum length would be appropriate? Consider if the manufacturer > provided the cable for that particular port but did not limit the cable > length for that port in the Installation Instructions. I would think > this would also satisfy the criteria as long as the provided cable is no > longer than the length identified in the answer to the above question. > > Thanks, > > Steve O'Steen > Director, EMC > Advanced Compliance Solutions, Inc. > [email protected] > 770-831-8048 ext. 210 > www.acstestlab.com > > > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
