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I have used PIC18F45k40 device for RS485 communication. I have 2 ckts one is isolated and another non isolated RS485 design. I have schematic & placement for reference. I have connected multiple slave device independently isolated and non isolated product , its working well. There is no issue with poll mismatch

Now i am combining isolated and non isolated module in parallel.during this stage, after hour isolated module stop communicating but isolated version product working well.

difference i found in both device i found is voltage level during operation isolated version if measure voltage across A& B w.r.t gnd is 7V-9V respectively i.e for ADM2483(Isolated ground)

where as SN75176 non isolated IC voltage level across A &B w.r.t ground is 2.5V-5V respectively.

i would like to why my communication of isolated version get disturbed when non isolated version is introduced.

which one is better to use to make solve this kind of problems

sn75176

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    \$\begingroup\$ Is the ground reference wire connected between RS485 devices, or only the A and B wires? Where is ADM2483 GND2 connected? From where the products get their power, are those power supplies ground/earth referenced or floating? \$\endgroup\$
    – Justme
    Oct 27, 2020 at 9:41
  • \$\begingroup\$ @justme For ADM2483 i am taking ground reference from transformer. transformer which give 12V/GND1 and 5V/gnd2. i have converted 12VDC to 5V using 7805 regulator IC. Isolated and non Isolated module are two independent product each will get power from Transformer . I am not seeing floating voltage in w.r.t ground. when i tested independent they showing as expected voltage levels. i have tested the unit without ground reference,when i combine. as product both work on A& B \$\endgroup\$
    – Ajit N
    Oct 27, 2020 at 10:29
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    \$\begingroup\$ So they do not have common reference? Then, in addition to connecting A and B between tranceivers, also connect GND2 of isolated tranceiver with the GND of the other tranceiver. See if it fixes the issue. RS485 does not work without common reference between tranceivers. \$\endgroup\$
    – Justme
    Oct 27, 2020 at 10:54
  • \$\begingroup\$ I have read few article. where there is non isolated design we no need to use GND. if we using isolated RS485 ckt we should provide common ground reference. Using isolated gnd ckts are always safer to use. since voltage level are 7V-12V. but my confusion is non isolated version has 5V output across A&B w.r.t gnd . But where as isolated version gives 7-12V w.r.t its ground. why 5V o/p non isolated module affect isolated version \$\endgroup\$
    – Ajit N
    Oct 27, 2020 at 11:09
  • \$\begingroup\$ Either the articles you've read are wrong or you've misunderstood them. You must have a common/ground connection between your RS485 transceivers. In a non-isolated system it is possible (but not guaranteed) that this common ground connection travels through the power supply on each side and through mains earth. \$\endgroup\$
    – brhans
    Oct 27, 2020 at 11:57

1 Answer 1

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Based on the description, there are only the differential data wires A and B connected between the two RS485 tranceivers that do not share a common reference, as one of the tranceivers includes isolation and runs on separate isolated power supply, so it completely floats in regards to the other tranceiver and the voltages float more than the common mode range allows for.

RS485 tranceivers must share a common voltage reference, and this is why there should be a common wire between tranceiver, that connects the ground of isolated RS485 tranceiver to ground of the other tranceiver, so they have a common reference potential.

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  • \$\begingroup\$ thanks for all valuable Input . Let me try making all ground common . weather any variation observed in system.. Meanwhile I would like to know when we should use isolated /non-isolated ground design. performance wise which design is better? \$\endgroup\$
    – Ajit N
    Oct 28, 2020 at 8:30

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