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PIC 18F452 , communications problem

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Mesut_yigit

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Hi friends;

I have 30 pic boards connected to a PC. Communication system is based on 4 wire RS 422 system. Biasing of comms line is performed using two 470 ohms to GND and + 5volt lines. Also , lines are terminated at each end. Line drivers are SN75176.

Strange thing is , when I start communications , PIC is resetting , exactly at the same point of communication. Even if I remove the comms cable , problem is still exist. Without the cable plugged, system becoming more sensitive against noise. Even when I try to touch to system , the LED s that are connected to Tx and RX lines (C6 and C7 pins) are lit.

I cancelled watch dog , Stack Underflow /overflow reset , Brownout ETC. But nothings changed. I am suspecting the noise. System is working in an industrial plant. I have used CAT 5 cable as comms. cable.

Could someone help me ?
 

If you don't require 100% isolated system try to connect 1MΩ resistor in parallel with 10-100nF capacitor between your local GND and any nearest EARTH point..
 

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