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This question already has an answer here:

I find a strange thing that the led is on when power is off. And it's the problem of my serial module. Arduino may get power from my serial module. My serial module use max2322. I don't if it will harm my arduino or has influence to my program. Is it needed to add a resistance?

marked as duplicate by Majenko Nov 29 '17 at 10:50

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  • you have an Arduino ??? powered from ??? and you connected a serial module ??? to rx/tx, ground and ??? – Juraj Nov 29 '17 at 9:52
  • @Juraj Arduino no power。I just connect tx rx and gnd to serial module. And my module is connected to pc by usb – erow Nov 29 '17 at 13:55
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Maybe the serial module has a serial cable attached to it which gives a voltage.

I think it can hurt, since you have two voltage sources, one from the Arduino and one from the Serial cable. Not using the power wire from the serial cable should fix the problem.

A resistor will not help because you still have two power sources in case you have both voltage from the Arduino (adapter/USB) and serial.

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