RS485 communication Noise

rs485+noise

hi to everyone here!
I have query about some noise cancellation technique.
I have ARM kit which is configured to have RS485 communication with an AC drive.
The GND wire between drive and ARM kit is only one, along with 485 cable(approx. long of 1 feet) .
Now when ARM starts communication with DRIVE via 485 , the o/p leds connected to othe port pins get signalled randomly.
This happens sometime and does not sometime.(i.e. randomly.)
How can be tackled with this situation?
Any suggestion will be heartly appriciated!
Thanks & Best regards!

rs-485 noise

My opinion you need biasing lines with pullup line A and pulldown B using resistors.

6n137 opto isolator rs-485 circuit

oh ! Already used.
Can i use optoisolator for different GND for 485 communication?
If yes, then which one is better choice and how shoul i connect it?
Awaiting for the response.
Regards!

what ispicture of smps

Hi,
Opto isolator will help if the noise you are seeing is due to ground coupling. You can use 6N137 type opto and you should use it at the TTL level before RS485, at the output of the Micro. You should then use an isolated power supply to power the 485 ICs and isolated section of the opto. You can derive this power supply from another winding of the mains transformer.
But from your description, it appears to be due to some power disturbance from Mains supply when your AC drive is being controlled, do you have any source of high switching current from SCR/ Capacitor/ Motor etc. that causes severe mains disturbance? You have mentioned that your 485 based controller is only one foot away.
How is your mains ground connected to the total system which includes your Micro Controller, AC Drive system etc.? Is the safety earth connected on the ARM kit side as well as on the AC drive side, if so there is the risk of a prt of the ground return current from AC drive flowing through the signal ground of ARM. If ARM and its logic is getting power from a 2pin type AC plug, this problem will not come. The heavy switching ground current, if any, should be prevented from entering ARM side. That is, connection to safety earth should be made only at the drive side and not at the ARM side.
Is there any chance of arcing in contacors, relays etc in the AC drive side?, if so, you may need ARM to be shielded. One thing you can readily try is to keep ARM a few meters away and connect to drive controls using a longer RS485 twisted pair wire. Also in this case, use a different power socket to connect mains power to ARM circuit to avoid sharing the same power board used for the drives.
Regards,
Laktronics

rs485 high noise!

Hi Lacktronics!
Thanks 4 such a descriptive answer.
My system is powered from single mains socket(3 pin: Phase, Neutral, Earth).
Then it is distributed to AC drive and SMPS.
SMPS feed the power to the ARM kit.
And all gnd(ARM signal GND and AC drive 485 GND) is connected to the EARTH point of the socket.

do you have any source of high switching current from SCR/ Capacitor/ Motor etc

Click to expand...

I think AC drive which controls an 3Φ AC motor itself is the source of high switching current.
That is the picture of the system.

As far as i got from your post, I should remove the EARTH wire from SMPS AC input only and keep intact that for the AC drive.
Is it right?
Regards!

6n137 rs485

Hi,
No, No, No.... do not remove the ground of SMPS, it will require it. If it is not a problem, atleast for testing purposes, move SMPS and ARM kit away by about 3 meters and connect SMPS 3 pin plug to another Mains outlet. Try shielding of the ARM kit.
I think using optoisolators will be required and if you still get mains supply disturbance, you may need to use a stabiliser and mains filtering to supply power to SMPS. Hope your SMPS is adequately rated.
Regards,
Laktronics