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I am using the circuit provided in the datasheet of MAX485. But is getting burnt after some time of working. I don't understand what is the problem. I also tried getting new ICs from a trusted supplier, but the problem still persists.

It is working fine for some time, after that some small amount of smoke is coming from it and its input side is getting latched to 5V.

I connected it with a PIC Microcontroller. Everything else in the circuit is working fine including ethernet communication using ENC28J60 IC.

enter image description here

Edit :

So I understand that I need to connect common ground for RS485.

But I need to connect with devices directly working on AC mains. It only provides 2 lines of RS485 (A and B). How can I communicate with these type of devices without causing these problems ?

enter image description here

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    \$\begingroup\$ This is not a schematic. Show how it is integrated on your board. And you have to connect the ground in case the two systems are isolated or having different grounds.. \$\endgroup\$
    – Eugene Sh.
    Jan 12, 2017 at 15:32
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    \$\begingroup\$ Again, what are you connecting to what with this communication? Two systems have to share the common ground. \$\endgroup\$
    – Eugene Sh.
    Jan 12, 2017 at 15:36
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    \$\begingroup\$ Your schematic does not show how the chips are powered/wired. Not connecting ground? You need to be clear about this. \$\endgroup\$
    – Andy aka
    Jan 12, 2017 at 15:37
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    \$\begingroup\$ Your understanding is incorrect. RS485 does require a common ground. \$\endgroup\$
    – brhans
    Jan 12, 2017 at 15:38
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    \$\begingroup\$ Of course it does. What do you think isolated 485 drivers and receivers are used for? \$\endgroup\$
    – Andy aka
    Jan 12, 2017 at 15:38

3 Answers 3

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as per my understanding MAX485 is a differential bus, it doesn't require common ground

There's your problem. Here's the fix: -

enter image description here

I don't speak Norwegian but I do recognize ground continuity in this circuit: -

enter image description here

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  • \$\begingroup\$ Thank you very much. I understand my mistake. Can you suggest me an isolated RS485 driver IC ?? I cannot use common ground in my project. \$\endgroup\$ Jan 12, 2017 at 16:09
  • \$\begingroup\$ One of my device is a RS485 Thermostat which is directly working on AC mains... So I cannot provide a common ground there. \$\endgroup\$ Jan 12, 2017 at 16:10
  • \$\begingroup\$ @LigoGeorge I am pretty sure you can. Do you have the interface pinout? \$\endgroup\$
    – Eugene Sh.
    Jan 12, 2017 at 16:18
  • \$\begingroup\$ Hi, Please check the above updated post.... \$\endgroup\$ Jan 12, 2017 at 16:22
  • \$\begingroup\$ How I am supposed to connect with that thermostat ? \$\endgroup\$ Jan 12, 2017 at 16:32
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You need to use optocouplers. You probably know it already, this thread is very old. Or use a transformer to have a floating power supply. Then you can connect both grounds.

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Here is how I solved it. It was causing basically due to three issues.

  1. Not making common ground for RS485 nodes
  2. External noises/transient voltages captured by RS485 bus
  3. Not earthing if using Class 1 SMPS

For solving the second problem I have used TVS diode as described in the TI application note.

Doing above measures solved my problem.

If you cannot make common ground for different RS485 nodes, you may use isolated RS485 drivers. It think we can also use a back to back diode connected across both grounds to ensure that there won't be any big potential differences between them.

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  • \$\begingroup\$ You've isolated the RS485? Not left the gnd unconnected, right? \$\endgroup\$
    – Jeroen3
    Jun 4, 2018 at 6:40
  • \$\begingroup\$ I didn't isolated...... In my application I made the ground common because isolated drivers or making it was adding extra cost which was undesirable. \$\endgroup\$ Jun 4, 2018 at 7:11

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