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I have a Arduino Pro Mini 3.3V connected to a sim800l chip using a serial connection of 9600bps. When I connect the Arduino to the device, it seems I don't receive anything on the Arduino Rx pin.

I then connected a FTDI that listen the sim800l Tx pin:

enter image description here

The FTDI is able to receive data so this means the sim800l is working well by receiving data from the Arduino and responding accordingly.

So normal logic would say that the Arduino Rx pin is not working but when I connect the Arduino Rx and Tx pin together:

enter image description here

I'm able to receive what I send on Tx on the Rx pin. Why would that be?

I tried with AltSoftSerial and SoftwareSerial. The Arduino is powered by a FTDI connected to my computer via USB. The sim800l is powered by a second FTDI that is also powered by my computer USB. So all component uses the same ground and I verified that with my multi-meter.

Here is the code I use:

AltSoftSerial sim800Serial(8,9); // RX, TX

void setup()
{
  Serial.begin(9600);
  sim800Serial.begin(9600);
  void loop()
{

while(1)
{
  if(sim800Serial.available())
  {
    Serial.write(sim800Serial.read());
  }

  if(Serial.available())
  {    
    sim800Serial.write(Serial.read());
  }
}     
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  • Please complete the circuit with VCC and GND so we understand the complete connection. Commented Jan 24, 2016 at 0:21
  • As well as the code you wrote to receive data from the device Commented Jan 24, 2016 at 0:49
  • 1
    I added more details to my question that answer yours. Let me know if I need more clarifications.
    – Pat
    Commented Jan 24, 2016 at 7:11
  • 1
    with power off, I measured all ground and they are shorted and have a very little resistance
    – Pat
    Commented Jan 24, 2016 at 21:32
  • 1
    I also tried putting all devices on the exact same ground without success.
    – Pat
    Commented Jan 24, 2016 at 21:48

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