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bonjourdaisy

RS485 Common Mode

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My issue comes down to wiring concerns for RS485. I've read quite a few stories where drivers can be fried if the common mode voltage between RS485 devices begins to exceed a certain level (I think it's 7V or something). I guess what I'm confused about is what is the common of an RS485 driver? A lot of devices I see have three wire (SIGN+,SIGN-, and GND) so the commons of my differential signal is pretty much the same between devices (I'm assuming that for these types of devices, the signal GND is my circuit common?) But then I get into issues when there is no GND port for an RS485 device... what then? How I determine the common voltage between devices at this point? Is my source GND at that point my circuit common? But then that adds more questions: if that source is isolated from the others RS485 drivers, then I have zero relationship between commons. I'm just really confused as to how the signal is measured between RS485 devices when there is no common. And more generally, what is the circuit common of RS485 drivers. I've definitely had one experience where I was using an RS232-485 converter and it ended up frying my PLC because there was a signal ground unconnected. So I'm really trying to get a better understanding about this topic. Any insight would be really helpful to me at this stage. Thanks. Edited by bonjourdaisy

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No love for RS485 I guess! Edited by bonjourdaisy

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Three excellent resources for plain-language explanations of RS-485 and the mystery of the "third wire" are: http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/Basics-of-the-RS-485-Standard.aspx http://rs485.com/pfaq.html#RS485Anchor http://robustdc.com/index.php?route=information/information&information_id=10 I cannot recommend enough the work of Lynn Linse, who founded RobustDataComm and who is pretty much the godfather of the industrial Ethernet/Serial server via his work for LanTronix.

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Low levels of common mode noise gets rejected by inversion-subtraction on a differential input: RS-485 features a differential input that rejects common mode noise, within the circuit's limits, as shown below: I like the diagram above because it shows the input signals referenced to a signal ground: When a ground loop exists, the measured voltage is the sum of the signal voltage and the potential difference between the signal source ground and the measurement system ground. Note the added Vcm in this diagram: which can appear as the difference in potential between the grounds. Excessive common mode, shown as Vcm, can increase to a level that either - saturates the circuit making it dysfunctional, but not destroying it, or - damages the components due to excessive current/voltage There are a number of web articles on common mode, like the one here: http://www.embedded.com/print/4169569 Edited by DanW

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