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A troubleshooting guide for RS-485

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Mike Fahrion
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re: A troubleshooting guide for RS-485
Mike Fahrion   7/24/2012 12:55:34 PM
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I agree in part and have seen many of each of the above scenarios in practice (along with the worst case of #1 but no signal ground cable). Here are two things I would consider. - the installer often has no information on whether the equipment at each node is galvanically insulated or not and doesn't always have the tools or knowledge to test for it. - In scenario 2 where all nodes are isolated, you're lacking a signal return path. While 485 is differential, it's not balanced, at the driver both lines are positive with respect to the local ground. I'm a big fan of solution 3 in applications where 485 may be linking between power systems that may have impedance between them. I'll usually put in some surge limiting at that main ground location as well. Feedback?

mac_droz
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re: A troubleshooting guide for RS-485
mac_droz   7/23/2012 1:23:40 PM
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I did design few systems with RS485 and every now and then I find articles claiming that you always need grounding cable (2-wire ports need in fact 3 wires). That is simply not true. There are three possible scenarios for 2-wire system: 1. RS485 transceivers not insulated from power rails - this would call for additional ground cable (in many applications connected to the local ground through 100R resistors to lower ground loop currents). 2. RS485 transceivers galvanically insulated from the power rails and data lines pulled to the transceiver's rails by resistors (important) - you don't need a ground cable. No current loops. Make sure differential potential between local grounds is less than the insulation of power blocks/opto-couplers. 3. Same as pt.2 but with the additional grounding cable connecting insulated grounds of the transceivers and globally connected to one main ground (in one place). This is an overkill option but might be useful especially if shielded wires are used in some harsh environment. Solution 1 is by far the worst because it closes ground loops - bad idea as those currents can change rapidly and mess up with your data (think high power motors and big contactors). Solution 2 is the most practical but requires insulating power blocks and opto-couplers. Solution 3 is for extreme environments. Same applies for 4-wire version.

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