Timeline for RS485 network ground pin - when to connect?
Current License: CC BY-SA 3.0
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| May 11, 2016 at 22:04 | comment | added | sekdiy | Because of Ohm's law. A common mode voltage will cause a current through some part, be it something functional or something protective. Once this voltage and/or current becomes too large, the part fails. | |
| May 8, 2016 at 13:41 | comment | added | fhlb | @Michael Karas I burnt a couple of MAX485 transceivers in setups where a common ground wasn't used and I'm not saying that the C connection is meaningless but I'm trying to understand what is going on. I'm asking "why" and not "how". Thanks for your understanding | |
| May 8, 2016 at 13:37 | comment | added | fhlb | @Jasen can you explain more about that diode thing. I sketched a simple two ESD protected pins with a DC source between those two pins. There is no possible path for the current to flow unless A and/or B are referenced with respect to the local receiver ground | |
| May 8, 2016 at 13:15 | comment | added | Michael Karas | (continued from above) This coupling removes or blocks any DC component in the signal path including the common mode differences. Two examples are land line telephone connections and Ethernet networks. If you have a land line phone wire I invite you to connect a scope probe to one of the wires and see how much line frequency potential will be apparent into the 1M ohm input impedance of the scope. | |
| May 8, 2016 at 13:11 | comment | added | Michael Karas | @fhlb - I am providing a generalized answer since it is not possible to iterate every possible condition that would change the possible common mode shift between the driver and receiver nodes. The fact remains that there are things that can make this happen and thus it behooves you to follow standard practice and connect up the C wire. The RS485 bus is a DC coupled interface and can run up to 4000 feet (some manufacturers of receivers claim up to 10K feet distance is possible). There are other interface types that can support long distances over wires but they are AC coupled. (continued) | |
| May 8, 2016 at 13:02 | comment | added | fhlb | This is exactly my question, what are some of these "undue influence" that may mess everything up? If I use this "wrong setup", when will it fail? | |
| May 8, 2016 at 12:57 | comment | added | Michael Karas | @fhlb - Just because you tested it and it appeared to work for you does not mean that you can then simply eliminate the C wire connection. You got lucky in that one instance that there was nothing in the vicinity of the driver and/or receiver that placed undue influence on one or the other device. | |
| May 8, 2016 at 12:53 | comment | added | Jasen Слава Україні | @Fhib, it only woks because of current flowing in the ESD protection diosed of the RS485 transceivers allows the two nodes to a achieve a somewhat matched common voltage. | |
| May 8, 2016 at 12:53 | history | edited | Michael Karas | CC BY-SA 3.0 | added 617 characters in body |
| May 8, 2016 at 12:49 | comment | added | fhlb | maybe large pull-up/pull-down resistance at the receiver end gives (floating) A and B a valid voltage level as referenced to receiver's ground. That is if Vab=5V, A is connected to a pull down, then Va=0, Vb=-5V | |
| May 8, 2016 at 12:46 | comment | added | fhlb | I tested it and it works. using a PTZ controller operated by an isolated wall adaptor to a dome cam operating from a battery with no earthing. | |
| May 8, 2016 at 12:40 | history | answered | Michael Karas | CC BY-SA 3.0 |